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diff --git a/Documentation/core-api/dma-api.rst b/Documentation/core-api/dma-api.rst new file mode 100644 index 000000000000..ca75b3541679 --- /dev/null +++ b/Documentation/core-api/dma-api.rst @@ -0,0 +1,868 @@ +============================================ +Dynamic DMA mapping using the generic device +============================================ + +:Author: James E.J. Bottomley <James.Bottomley@HansenPartnership.com> + +This document describes the DMA API. For a more gentle introduction +of the API (and actual examples), see Documentation/core-api/dma-api-howto.rst. + +This API is split into two pieces. Part I describes the basic API. +Part II describes extensions for supporting non-coherent memory +machines. Unless you know that your driver absolutely has to support +non-coherent platforms (this is usually only legacy platforms) you +should only use the API described in part I. + +Part I - DMA API +---------------- + +To get the DMA API, you must #include <linux/dma-mapping.h>. This +provides dma_addr_t and the interfaces described below. + +A dma_addr_t can hold any valid DMA address for the platform. It can be +given to a device to use as a DMA source or target. A CPU cannot reference +a dma_addr_t directly because there may be translation between its physical +address space and the DMA address space. + +Part Ia - Using large DMA-coherent buffers +------------------------------------------ + +:: + + void * + dma_alloc_coherent(struct device *dev, size_t size, + dma_addr_t *dma_handle, gfp_t flag) + +Coherent memory is memory for which a write by either the device or +the processor can immediately be read by the processor or device +without having to worry about caching effects. (You may however need +to make sure to flush the processor's write buffers before telling +devices to read that memory.) + +This routine allocates a region of <size> bytes of coherent memory. + +It returns a pointer to the allocated region (in the processor's virtual +address space) or NULL if the allocation failed. + +It also returns a <dma_handle> which may be cast to an unsigned integer the +same width as the bus and given to the device as the DMA address base of +the region. + +Note: coherent memory can be expensive on some platforms, and the +minimum allocation length may be as big as a page, so you should +consolidate your requests for coherent memory as much as possible. +The simplest way to do that is to use the dma_pool calls (see below). + +The flag parameter allows the caller to specify the ``GFP_`` flags (see +kmalloc()) for the allocation (the implementation may ignore flags that affect +the location of the returned memory, like GFP_DMA). + +:: + + void + dma_free_coherent(struct device *dev, size_t size, void *cpu_addr, + dma_addr_t dma_handle) + +Free a previously allocated region of coherent memory. dev, size and dma_handle +must all be the same as those passed into dma_alloc_coherent(). cpu_addr must +be the virtual address returned by dma_alloc_coherent(). + +Note that unlike the sibling allocation call, this routine may only be called +with IRQs enabled. + + +Part Ib - Using small DMA-coherent buffers +------------------------------------------ + +To get this part of the DMA API, you must #include <linux/dmapool.h> + +Many drivers need lots of small DMA-coherent memory regions for DMA +descriptors or I/O buffers. Rather than allocating in units of a page +or more using dma_alloc_coherent(), you can use DMA pools. These work +much like a struct kmem_cache, except that they use the DMA-coherent allocator, +not __get_free_pages(). Also, they understand common hardware constraints +for alignment, like queue heads needing to be aligned on N-byte boundaries. + +.. kernel-doc:: mm/dmapool.c + :export: + +.. kernel-doc:: include/linux/dmapool.h + + +Part Ic - DMA addressing limitations +------------------------------------ + +DMA mask is a bit mask of the addressable region for the device. In other words, +if applying the DMA mask (a bitwise AND operation) to the DMA address of a +memory region does not clear any bits in the address, then the device can +perform DMA to that memory region. + +All the below functions which set a DMA mask may fail if the requested mask +cannot be used with the device, or if the device is not capable of doing DMA. + +:: + + int + dma_set_mask_and_coherent(struct device *dev, u64 mask) + +Updates both streaming and coherent DMA masks. + +Returns: 0 if successful and a negative error if not. + +:: + + int + dma_set_mask(struct device *dev, u64 mask) + +Updates only the streaming DMA mask. + +Returns: 0 if successful and a negative error if not. + +:: + + int + dma_set_coherent_mask(struct device *dev, u64 mask) + +Updates only the coherent DMA mask. + +Returns: 0 if successful and a negative error if not. + +:: + + u64 + dma_get_required_mask(struct device *dev) + +This API returns the mask that the platform requires to +operate efficiently. Usually this means the returned mask +is the minimum required to cover all of memory. Examining the +required mask gives drivers with variable descriptor sizes the +opportunity to use smaller descriptors as necessary. + +Requesting the required mask does not alter the current mask. If you +wish to take advantage of it, you should issue a dma_set_mask() +call to set the mask to the value returned. + +:: + + size_t + dma_max_mapping_size(struct device *dev); + +Returns the maximum size of a mapping for the device. The size parameter +of the mapping functions like dma_map_single(), dma_map_page() and +others should not be larger than the returned value. + +:: + + size_t + dma_opt_mapping_size(struct device *dev); + +Returns the maximum optimal size of a mapping for the device. + +Mapping larger buffers may take much longer in certain scenarios. In +addition, for high-rate short-lived streaming mappings, the upfront time +spent on the mapping may account for an appreciable part of the total +request lifetime. As such, if splitting larger requests incurs no +significant performance penalty, then device drivers are advised to +limit total DMA streaming mappings length to the returned value. + +:: + + bool + dma_need_sync(struct device *dev, dma_addr_t dma_addr); + +Returns %true if dma_sync_single_for_{device,cpu} calls are required to +transfer memory ownership. Returns %false if those calls can be skipped. + +:: + + unsigned long + dma_get_merge_boundary(struct device *dev); + +Returns the DMA merge boundary. If the device cannot merge any DMA address +segments, the function returns 0. + +Part Id - Streaming DMA mappings +-------------------------------- + +Streaming DMA allows to map an existing buffer for DMA transfers and then +unmap it when finished. Map functions are not guaranteed to succeed, so the +return value must be checked. + +.. note:: + + In particular, mapping may fail for memory not addressable by the + device, e.g. if it is not within the DMA mask of the device and/or a + connecting bus bridge. Streaming DMA functions try to overcome such + addressing constraints, either by using an IOMMU (a device which maps + I/O DMA addresses to physical memory addresses), or by copying the + data to/from a bounce buffer if the kernel is configured with a + :doc:`SWIOTLB <swiotlb>`. However, these methods are not always + available, and even if they are, they may still fail for a number of + reasons. + + In short, a device driver may need to be wary of where buffers are + located in physical memory, especially if the DMA mask is less than 32 + bits. + +:: + + dma_addr_t + dma_map_single(struct device *dev, void *cpu_addr, size_t size, + enum dma_data_direction direction) + +Maps a piece of processor virtual memory so it can be accessed by the +device and returns the DMA address of the memory. + +The DMA API uses a strongly typed enumerator for its direction: + +======================= ============================================= +DMA_NONE no direction (used for debugging) +DMA_TO_DEVICE data is going from the memory to the device +DMA_FROM_DEVICE data is coming from the device to the memory +DMA_BIDIRECTIONAL direction isn't known +======================= ============================================= + +.. note:: + + Contiguous kernel virtual space may not be contiguous as + physical memory. Since this API does not provide any scatter/gather + capability, it will fail if the user tries to map a non-physically + contiguous piece of memory. For this reason, memory to be mapped by + this API should be obtained from sources which guarantee it to be + physically contiguous (like kmalloc). + +.. warning:: + + Memory coherency operates at a granularity called the cache + line width. In order for memory mapped by this API to operate + correctly, the mapped region must begin exactly on a cache line + boundary and end exactly on one (to prevent two separately mapped + regions from sharing a single cache line). Since the cache line size + may not be known at compile time, the API will not enforce this + requirement. Therefore, it is recommended that driver writers who + don't take special care to determine the cache line size at run time + only map virtual regions that begin and end on page boundaries (which + are guaranteed also to be cache line boundaries). + + DMA_TO_DEVICE synchronisation must be done after the last modification + of the memory region by the software and before it is handed off to + the device. Once this primitive is used, memory covered by this + primitive should be treated as read-only by the device. If the device + may write to it at any point, it should be DMA_BIDIRECTIONAL (see + below). + + DMA_FROM_DEVICE synchronisation must be done before the driver + accesses data that may be changed by the device. This memory should + be treated as read-only by the driver. If the driver needs to write + to it at any point, it should be DMA_BIDIRECTIONAL (see below). + + DMA_BIDIRECTIONAL requires special handling: it means that the driver + isn't sure if the memory was modified before being handed off to the + device and also isn't sure if the device will also modify it. Thus, + you must always sync bidirectional memory twice: once before the + memory is handed off to the device (to make sure all memory changes + are flushed from the processor) and once before the data may be + accessed after being used by the device (to make sure any processor + cache lines are updated with data that the device may have changed). + +:: + + void + dma_unmap_single(struct device *dev, dma_addr_t dma_addr, size_t size, + enum dma_data_direction direction) + +Unmaps the region previously mapped. All the parameters passed in +must be identical to those passed to (and returned by) dma_map_single(). + +:: + + dma_addr_t + dma_map_page(struct device *dev, struct page *page, + unsigned long offset, size_t size, + enum dma_data_direction direction) + + void + dma_unmap_page(struct device *dev, dma_addr_t dma_address, size_t size, + enum dma_data_direction direction) + +API for mapping and unmapping for pages. All the notes and warnings +for the other mapping APIs apply here. Also, although the <offset> +and <size> parameters are provided to do partial page mapping, it is +recommended that you never use these unless you really know what the +cache width is. + +:: + + dma_addr_t + dma_map_resource(struct device *dev, phys_addr_t phys_addr, size_t size, + enum dma_data_direction dir, unsigned long attrs) + + void + dma_unmap_resource(struct device *dev, dma_addr_t addr, size_t size, + enum dma_data_direction dir, unsigned long attrs) + +API for mapping and unmapping for MMIO resources. All the notes and +warnings for the other mapping APIs apply here. The API should only be +used to map device MMIO resources, mapping of RAM is not permitted. + +:: + + int + dma_mapping_error(struct device *dev, dma_addr_t dma_addr) + +In some circumstances dma_map_single(), dma_map_page() and dma_map_resource() +will fail to create a mapping. A driver can check for these errors by testing +the returned DMA address with dma_mapping_error(). A non-zero return value +means the mapping could not be created and the driver should take appropriate +action (e.g. reduce current DMA mapping usage or delay and try again later). + +:: + + int + dma_map_sg(struct device *dev, struct scatterlist *sg, + int nents, enum dma_data_direction direction) + +Maps a scatter/gather list for DMA. Returns the number of DMA address segments +mapped, which may be smaller than <nents> passed in if several consecutive +sglist entries are merged (e.g. with an IOMMU, or if some adjacent segments +just happen to be physically contiguous). + +Please note that the sg cannot be mapped again if it has been mapped once. +The mapping process is allowed to destroy information in the sg. + +As with the other mapping interfaces, dma_map_sg() can fail. When it +does, 0 is returned and a driver must take appropriate action. It is +critical that the driver do something, in the case of a block driver +aborting the request or even oopsing is better than doing nothing and +corrupting the filesystem. + +With scatterlists, you use the resulting mapping like this:: + + int i, count = dma_map_sg(dev, sglist, nents, direction); + struct scatterlist *sg; + + for_each_sg(sglist, sg, count, i) { + hw_address[i] = sg_dma_address(sg); + hw_len[i] = sg_dma_len(sg); + } + +where nents is the number of entries in the sglist. + +The implementation is free to merge several consecutive sglist entries +into one. The returned number is the actual number of sg entries it +mapped them to. On failure, 0 is returned. + +Then you should loop count times (note: this can be less than nents times) +and use sg_dma_address() and sg_dma_len() macros where you previously +accessed sg->address and sg->length as shown above. + +:: + + void + dma_unmap_sg(struct device *dev, struct scatterlist *sg, + int nents, enum dma_data_direction direction) + +Unmap the previously mapped scatter/gather list. All the parameters +must be the same as those and passed in to the scatter/gather mapping +API. + +Note: <nents> must be the number you passed in, *not* the number of +DMA address entries returned. + +:: + + void + dma_sync_single_for_cpu(struct device *dev, dma_addr_t dma_handle, + size_t size, + enum dma_data_direction direction) + + void + dma_sync_single_for_device(struct device *dev, dma_addr_t dma_handle, + size_t size, + enum dma_data_direction direction) + + void + dma_sync_sg_for_cpu(struct device *dev, struct scatterlist *sg, + int nents, + enum dma_data_direction direction) + + void + dma_sync_sg_for_device(struct device *dev, struct scatterlist *sg, + int nents, + enum dma_data_direction direction) + +Synchronise a single contiguous or scatter/gather mapping for the CPU +and device. With the sync_sg API, all the parameters must be the same +as those passed into the sg mapping API. With the sync_single API, +you can use dma_handle and size parameters that aren't identical to +those passed into the single mapping API to do a partial sync. + + +.. note:: + + You must do this: + + - Before reading values that have been written by DMA from the device + (use the DMA_FROM_DEVICE direction) + - After writing values that will be written to the device using DMA + (use the DMA_TO_DEVICE) direction + - before *and* after handing memory to the device if the memory is + DMA_BIDIRECTIONAL + +See also dma_map_single(). + +:: + + dma_addr_t + dma_map_single_attrs(struct device *dev, void *cpu_addr, size_t size, + enum dma_data_direction dir, + unsigned long attrs) + + void + dma_unmap_single_attrs(struct device *dev, dma_addr_t dma_addr, + size_t size, enum dma_data_direction dir, + unsigned long attrs) + + int + dma_map_sg_attrs(struct device *dev, struct scatterlist *sgl, + int nents, enum dma_data_direction dir, + unsigned long attrs) + + void + dma_unmap_sg_attrs(struct device *dev, struct scatterlist *sgl, + int nents, enum dma_data_direction dir, + unsigned long attrs) + +The four functions above are just like the counterpart functions +without the _attrs suffixes, except that they pass an optional +dma_attrs. + +The interpretation of DMA attributes is architecture-specific, and +each attribute should be documented in +Documentation/core-api/dma-attributes.rst. + +If dma_attrs are 0, the semantics of each of these functions +is identical to those of the corresponding function +without the _attrs suffix. As a result dma_map_single_attrs() +can generally replace dma_map_single(), etc. + +As an example of the use of the ``*_attrs`` functions, here's how +you could pass an attribute DMA_ATTR_FOO when mapping memory +for DMA:: + + #include <linux/dma-mapping.h> + /* DMA_ATTR_FOO should be defined in linux/dma-mapping.h and + * documented in Documentation/core-api/dma-attributes.rst */ + ... + + unsigned long attr; + attr |= DMA_ATTR_FOO; + .... + n = dma_map_sg_attrs(dev, sg, nents, DMA_TO_DEVICE, attr); + .... + +Architectures that care about DMA_ATTR_FOO would check for its +presence in their implementations of the mapping and unmapping +routines, e.g.::: + + void whizco_dma_map_sg_attrs(struct device *dev, dma_addr_t dma_addr, + size_t size, enum dma_data_direction dir, + unsigned long attrs) + { + .... + if (attrs & DMA_ATTR_FOO) + /* twizzle the frobnozzle */ + .... + } + +Part Ie - IOVA-based DMA mappings +--------------------------------- + +These APIs allow a very efficient mapping when using an IOMMU. They are an +optional path that requires extra code and are only recommended for drivers +where DMA mapping performance, or the space usage for storing the DMA addresses +matter. All the considerations from the previous section apply here as well. + +:: + + bool dma_iova_try_alloc(struct device *dev, struct dma_iova_state *state, + phys_addr_t phys, size_t size); + +Is used to try to allocate IOVA space for mapping operation. If it returns +false this API can't be used for the given device and the normal streaming +DMA mapping API should be used. The ``struct dma_iova_state`` is allocated +by the driver and must be kept around until unmap time. + +:: + + static inline bool dma_use_iova(struct dma_iova_state *state) + +Can be used by the driver to check if the IOVA-based API is used after a +call to dma_iova_try_alloc. This can be useful in the unmap path. + +:: + + int dma_iova_link(struct device *dev, struct dma_iova_state *state, + phys_addr_t phys, size_t offset, size_t size, + enum dma_data_direction dir, unsigned long attrs); + +Is used to link ranges to the IOVA previously allocated. The start of all +but the first call to dma_iova_link for a given state must be aligned +to the DMA merge boundary returned by ``dma_get_merge_boundary())``, and +the size of all but the last range must be aligned to the DMA merge boundary +as well. + +:: + + int dma_iova_sync(struct device *dev, struct dma_iova_state *state, + size_t offset, size_t size); + +Must be called to sync the IOMMU page tables for IOVA-range mapped by one or +more calls to ``dma_iova_link()``. + +For drivers that use a one-shot mapping, all ranges can be unmapped and the +IOVA freed by calling: + +:: + + void dma_iova_destroy(struct device *dev, struct dma_iova_state *state, + size_t mapped_len, enum dma_data_direction dir, + unsigned long attrs); + +Alternatively drivers can dynamically manage the IOVA space by unmapping +and mapping individual regions. In that case + +:: + + void dma_iova_unlink(struct device *dev, struct dma_iova_state *state, + size_t offset, size_t size, enum dma_data_direction dir, + unsigned long attrs); + +is used to unmap a range previously mapped, and + +:: + + void dma_iova_free(struct device *dev, struct dma_iova_state *state); + +is used to free the IOVA space. All regions must have been unmapped using +``dma_iova_unlink()`` before calling ``dma_iova_free()``. + +Part II - Non-coherent DMA allocations +-------------------------------------- + +These APIs allow to allocate pages that are guaranteed to be DMA addressable +by the passed in device, but which need explicit management of memory ownership +for the kernel vs the device. + +If you don't understand how cache line coherency works between a processor and +an I/O device, you should not be using this part of the API. + +:: + + struct page * + dma_alloc_pages(struct device *dev, size_t size, dma_addr_t *dma_handle, + enum dma_data_direction dir, gfp_t gfp) + +This routine allocates a region of <size> bytes of non-coherent memory. It +returns a pointer to first struct page for the region, or NULL if the +allocation failed. The resulting struct page can be used for everything a +struct page is suitable for. + +It also returns a <dma_handle> which may be cast to an unsigned integer the +same width as the bus and given to the device as the DMA address base of +the region. + +The dir parameter specified if data is read and/or written by the device, +see dma_map_single() for details. + +The gfp parameter allows the caller to specify the ``GFP_`` flags (see +kmalloc()) for the allocation, but rejects flags used to specify a memory +zone such as GFP_DMA or GFP_HIGHMEM. + +Before giving the memory to the device, dma_sync_single_for_device() needs +to be called, and before reading memory written by the device, +dma_sync_single_for_cpu(), just like for streaming DMA mappings that are +reused. + +:: + + void + dma_free_pages(struct device *dev, size_t size, struct page *page, + dma_addr_t dma_handle, enum dma_data_direction dir) + +Free a region of memory previously allocated using dma_alloc_pages(). +dev, size, dma_handle and dir must all be the same as those passed into +dma_alloc_pages(). page must be the pointer returned by dma_alloc_pages(). + +:: + + int + dma_mmap_pages(struct device *dev, struct vm_area_struct *vma, + size_t size, struct page *page) + +Map an allocation returned from dma_alloc_pages() into a user address space. +dev and size must be the same as those passed into dma_alloc_pages(). +page must be the pointer returned by dma_alloc_pages(). + +:: + + void * + dma_alloc_noncoherent(struct device *dev, size_t size, + dma_addr_t *dma_handle, enum dma_data_direction dir, + gfp_t gfp) + +This routine is a convenient wrapper around dma_alloc_pages that returns the +kernel virtual address for the allocated memory instead of the page structure. + +:: + + void + dma_free_noncoherent(struct device *dev, size_t size, void *cpu_addr, + dma_addr_t dma_handle, enum dma_data_direction dir) + +Free a region of memory previously allocated using dma_alloc_noncoherent(). +dev, size, dma_handle and dir must all be the same as those passed into +dma_alloc_noncoherent(). cpu_addr must be the virtual address returned by +dma_alloc_noncoherent(). + +:: + + struct sg_table * + dma_alloc_noncontiguous(struct device *dev, size_t size, + enum dma_data_direction dir, gfp_t gfp, + unsigned long attrs); + +This routine allocates <size> bytes of non-coherent and possibly non-contiguous +memory. It returns a pointer to struct sg_table that describes the allocated +and DMA mapped memory, or NULL if the allocation failed. The resulting memory +can be used for struct page mapped into a scatterlist are suitable for. + +The return sg_table is guaranteed to have 1 single DMA mapped segment as +indicated by sgt->nents, but it might have multiple CPU side segments as +indicated by sgt->orig_nents. + +The dir parameter specified if data is read and/or written by the device, +see dma_map_single() for details. + +The gfp parameter allows the caller to specify the ``GFP_`` flags (see +kmalloc()) for the allocation, but rejects flags used to specify a memory +zone such as GFP_DMA or GFP_HIGHMEM. + +The attrs argument must be either 0 or DMA_ATTR_ALLOC_SINGLE_PAGES. + +Before giving the memory to the device, dma_sync_sgtable_for_device() needs +to be called, and before reading memory written by the device, +dma_sync_sgtable_for_cpu(), just like for streaming DMA mappings that are +reused. + +:: + + void + dma_free_noncontiguous(struct device *dev, size_t size, + struct sg_table *sgt, + enum dma_data_direction dir) + +Free memory previously allocated using dma_alloc_noncontiguous(). dev, size, +and dir must all be the same as those passed into dma_alloc_noncontiguous(). +sgt must be the pointer returned by dma_alloc_noncontiguous(). + +:: + + void * + dma_vmap_noncontiguous(struct device *dev, size_t size, + struct sg_table *sgt) + +Return a contiguous kernel mapping for an allocation returned from +dma_alloc_noncontiguous(). dev and size must be the same as those passed into +dma_alloc_noncontiguous(). sgt must be the pointer returned by +dma_alloc_noncontiguous(). + +Once a non-contiguous allocation is mapped using this function, the +flush_kernel_vmap_range() and invalidate_kernel_vmap_range() APIs must be used +to manage the coherency between the kernel mapping, the device and user space +mappings (if any). + +:: + + void + dma_vunmap_noncontiguous(struct device *dev, void *vaddr) + +Unmap a kernel mapping returned by dma_vmap_noncontiguous(). dev must be the +same the one passed into dma_alloc_noncontiguous(). vaddr must be the pointer +returned by dma_vmap_noncontiguous(). + + +:: + + int + dma_mmap_noncontiguous(struct device *dev, struct vm_area_struct *vma, + size_t size, struct sg_table *sgt) + +Map an allocation returned from dma_alloc_noncontiguous() into a user address +space. dev and size must be the same as those passed into +dma_alloc_noncontiguous(). sgt must be the pointer returned by +dma_alloc_noncontiguous(). + +:: + + int + dma_get_cache_alignment(void) + +Returns the processor cache alignment. This is the absolute minimum +alignment *and* width that you must observe when either mapping +memory or doing partial flushes. + +.. note:: + + This API may return a number *larger* than the actual cache + line, but it will guarantee that one or more cache lines fit exactly + into the width returned by this call. It will also always be a power + of two for easy alignment. + + +Part III - Debug drivers use of the DMA API +------------------------------------------- + +The DMA API as described above has some constraints. DMA addresses must be +released with the corresponding function with the same size for example. With +the advent of hardware IOMMUs it becomes more and more important that drivers +do not violate those constraints. In the worst case such a violation can +result in data corruption up to destroyed filesystems. + +To debug drivers and find bugs in the usage of the DMA API checking code can +be compiled into the kernel which will tell the developer about those +violations. If your architecture supports it you can select the "Enable +debugging of DMA API usage" option in your kernel configuration. Enabling this +option has a performance impact. Do not enable it in production kernels. + +If you boot the resulting kernel will contain code which does some bookkeeping +about what DMA memory was allocated for which device. If this code detects an +error it prints a warning message with some details into your kernel log. An +example warning message may look like this:: + + WARNING: at /data2/repos/linux-2.6-iommu/lib/dma-debug.c:448 + check_unmap+0x203/0x490() + Hardware name: + forcedeth 0000:00:08.0: DMA-API: device driver frees DMA memory with wrong + function [device address=0x00000000640444be] [size=66 bytes] [mapped as + single] [unmapped as page] + Modules linked in: nfsd exportfs bridge stp llc r8169 + Pid: 0, comm: swapper Tainted: G W 2.6.28-dmatest-09289-g8bb99c0 #1 + Call Trace: + <IRQ> [<ffffffff80240b22>] warn_slowpath+0xf2/0x130 + [<ffffffff80647b70>] _spin_unlock+0x10/0x30 + [<ffffffff80537e75>] usb_hcd_link_urb_to_ep+0x75/0xc0 + [<ffffffff80647c22>] _spin_unlock_irqrestore+0x12/0x40 + [<ffffffff8055347f>] ohci_urb_enqueue+0x19f/0x7c0 + [<ffffffff80252f96>] queue_work+0x56/0x60 + [<ffffffff80237e10>] enqueue_task_fair+0x20/0x50 + [<ffffffff80539279>] usb_hcd_submit_urb+0x379/0xbc0 + [<ffffffff803b78c3>] cpumask_next_and+0x23/0x40 + [<ffffffff80235177>] find_busiest_group+0x207/0x8a0 + [<ffffffff8064784f>] _spin_lock_irqsave+0x1f/0x50 + [<ffffffff803c7ea3>] check_unmap+0x203/0x490 + [<ffffffff803c8259>] debug_dma_unmap_phys+0x49/0x50 + [<ffffffff80485f26>] nv_tx_done_optimized+0xc6/0x2c0 + [<ffffffff80486c13>] nv_nic_irq_optimized+0x73/0x2b0 + [<ffffffff8026df84>] handle_IRQ_event+0x34/0x70 + [<ffffffff8026ffe9>] handle_edge_irq+0xc9/0x150 + [<ffffffff8020e3ab>] do_IRQ+0xcb/0x1c0 + [<ffffffff8020c093>] ret_from_intr+0x0/0xa + <EOI> <4>---[ end trace f6435a98e2a38c0e ]--- + +The driver developer can find the driver and the device including a stacktrace +of the DMA API call which caused this warning. + +Per default only the first error will result in a warning message. All other +errors will only silently counted. This limitation exist to prevent the code +from flooding your kernel log. To support debugging a device driver this can +be disabled via debugfs. See the debugfs interface documentation below for +details. + +The debugfs directory for the DMA API debugging code is called dma-api/. In +this directory the following files can currently be found: + +=============================== =============================================== +dma-api/all_errors This file contains a numeric value. If this + value is not equal to zero the debugging code + will print a warning for every error it finds + into the kernel log. Be careful with this + option, as it can easily flood your logs. + +dma-api/disabled This read-only file contains the character 'Y' + if the debugging code is disabled. This can + happen when it runs out of memory or if it was + disabled at boot time + +dma-api/dump This read-only file contains current DMA + mappings. + +dma-api/error_count This file is read-only and shows the total + numbers of errors found. + +dma-api/num_errors The number in this file shows how many + warnings will be printed to the kernel log + before it stops. This number is initialized to + one at system boot and be set by writing into + this file + +dma-api/min_free_entries This read-only file can be read to get the + minimum number of free dma_debug_entries the + allocator has ever seen. If this value goes + down to zero the code will attempt to increase + nr_total_entries to compensate. + +dma-api/num_free_entries The current number of free dma_debug_entries + in the allocator. + +dma-api/nr_total_entries The total number of dma_debug_entries in the + allocator, both free and used. + +dma-api/driver_filter You can write a name of a driver into this file + to limit the debug output to requests from that + particular driver. Write an empty string to + that file to disable the filter and see + all errors again. +=============================== =============================================== + +If you have this code compiled into your kernel it will be enabled by default. +If you want to boot without the bookkeeping anyway you can provide +'dma_debug=off' as a boot parameter. This will disable DMA API debugging. +Notice that you can not enable it again at runtime. You have to reboot to do +so. + +If you want to see debug messages only for a special device driver you can +specify the dma_debug_driver=<drivername> parameter. This will enable the +driver filter at boot time. The debug code will only print errors for that +driver afterwards. This filter can be disabled or changed later using debugfs. + +When the code disables itself at runtime this is most likely because it ran +out of dma_debug_entries and was unable to allocate more on-demand. 65536 +entries are preallocated at boot - if this is too low for you boot with +'dma_debug_entries=<your_desired_number>' to overwrite the default. Note +that the code allocates entries in batches, so the exact number of +preallocated entries may be greater than the actual number requested. The +code will print to the kernel log each time it has dynamically allocated +as many entries as were initially preallocated. This is to indicate that a +larger preallocation size may be appropriate, or if it happens continually +that a driver may be leaking mappings. + +:: + + void + debug_dma_mapping_error(struct device *dev, dma_addr_t dma_addr); + +dma-debug interface debug_dma_mapping_error() to debug drivers that fail +to check DMA mapping errors on addresses returned by dma_map_single() and +dma_map_page() interfaces. This interface clears a flag set by +debug_dma_map_phys() to indicate that dma_mapping_error() has been called by +the driver. When driver does unmap, debug_dma_unmap() checks the flag and if +this flag is still set, prints warning message that includes call trace that +leads up to the unmap. This interface can be called from dma_mapping_error() +routines to enable DMA mapping error check debugging. + +Functions and structures +======================== + +.. kernel-doc:: include/linux/scatterlist.h +.. kernel-doc:: lib/scatterlist.c |