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Diffstat (limited to 'Documentation/core-api/dma-api.rst')
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diff --git a/Documentation/core-api/dma-api.rst b/Documentation/core-api/dma-api.rst index 2ad08517e626..3087bea715ed 100644 --- a/Documentation/core-api/dma-api.rst +++ b/Documentation/core-api/dma-api.rst @@ -8,15 +8,15 @@ 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-consistent memory +Part II describes extensions for supporting non-coherent memory machines. Unless you know that your driver absolutely has to support -non-consistent platforms (this is usually only legacy platforms) you +non-coherent platforms (this is usually only legacy platforms) you should only use the API described in part I. -Part I - dma_API +Part I - DMA API ---------------- -To get the dma_API, you must #include <linux/dma-mapping.h>. This +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 @@ -33,13 +33,13 @@ Part Ia - Using large DMA-coherent buffers dma_alloc_coherent(struct device *dev, size_t size, dma_addr_t *dma_handle, gfp_t flag) -Consistent memory is memory for which a write by either the device or +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 consistent 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. @@ -48,15 +48,14 @@ 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: consistent memory can be expensive on some platforms, and the +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 consistent memory as much as possible. +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 (dma_alloc_coherent() only) allows the caller to -specify the ``GFP_`` flags (see kmalloc()) for the allocation (the -implementation may choose to ignore flags that affect the location of -the returned memory, like GFP_DMA). +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). :: @@ -64,19 +63,18 @@ the returned memory, like GFP_DMA). dma_free_coherent(struct device *dev, size_t size, void *cpu_addr, dma_addr_t dma_handle) -Free a region of consistent memory you previously allocated. 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 -the dma_alloc_coherent(). +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 their sibling allocation calls, these routines -may only be called with IRQs enabled. +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> +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 @@ -85,78 +83,29 @@ 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: -:: - - struct dma_pool * - dma_pool_create(const char *name, struct device *dev, - size_t size, size_t align, size_t alloc); - -dma_pool_create() initializes a pool of DMA-coherent buffers -for use with a given device. It must be called in a context which -can sleep. - -The "name" is for diagnostics (like a struct kmem_cache name); dev and size -are like what you'd pass to dma_alloc_coherent(). The device's hardware -alignment requirement for this type of data is "align" (which is expressed -in bytes, and must be a power of two). If your device has no boundary -crossing restrictions, pass 0 for alloc; passing 4096 says memory allocated -from this pool must not cross 4KByte boundaries. - -:: - - void * - dma_pool_zalloc(struct dma_pool *pool, gfp_t mem_flags, - dma_addr_t *handle) - -Wraps dma_pool_alloc() and also zeroes the returned memory if the -allocation attempt succeeded. - - -:: - - void * - dma_pool_alloc(struct dma_pool *pool, gfp_t gfp_flags, - dma_addr_t *dma_handle); - -This allocates memory from the pool; the returned memory will meet the -size and alignment requirements specified at creation time. Pass -GFP_ATOMIC to prevent blocking, or if it's permitted (not -in_interrupt, not holding SMP locks), pass GFP_KERNEL to allow -blocking. Like dma_alloc_coherent(), this returns two values: an -address usable by the CPU, and the DMA address usable by the pool's -device. - -:: - - void - dma_pool_free(struct dma_pool *pool, void *vaddr, - dma_addr_t addr); - -This puts memory back into the pool. The pool is what was passed to -dma_pool_alloc(); the CPU (vaddr) and DMA addresses are what -were returned when that routine allocated the memory being freed. - -:: - - void - dma_pool_destroy(struct dma_pool *pool); - -dma_pool_destroy() frees the resources of the pool. It must be -called in a context which can sleep. Make sure you've freed all allocated -memory back to the pool before you destroy it. +.. 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) -Checks to see if the mask is possible and updates the device -streaming and coherent DMA mask parameters if it is. +Updates both streaming and coherent DMA masks. Returns: 0 if successful and a negative error if not. @@ -165,8 +114,7 @@ Returns: 0 if successful and a negative error if not. int dma_set_mask(struct device *dev, u64 mask) -Checks to see if the mask is possible and updates the device -parameters if it is. +Updates only the streaming DMA mask. Returns: 0 if successful and a negative error if not. @@ -175,8 +123,7 @@ Returns: 0 if successful and a negative error if not. int dma_set_coherent_mask(struct device *dev, u64 mask) -Checks to see if the mask is possible and updates the device -parameters if it is. +Updates only the coherent DMA mask. Returns: 0 if successful and a negative error if not. @@ -231,12 +178,32 @@ 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 the DMA address +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 @@ -246,9 +213,7 @@ Part Id - Streaming DMA mappings Maps a piece of processor virtual memory so it can be accessed by the device and returns the DMA address of the memory. -The direction for both APIs may be converted freely by casting. -However the dma_API uses a strongly typed enumerator for its -direction: +The DMA API uses a strongly typed enumerator for its direction: ======================= ============================================= DMA_NONE no direction (used for debugging) @@ -259,31 +224,13 @@ DMA_BIDIRECTIONAL direction isn't known .. note:: - Not all memory regions in a machine can be mapped by this API. - Further, contiguous kernel virtual space may not be contiguous as + 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). - Further, the DMA address of the memory must be within the - dma_mask of the device (the dma_mask is a bit mask of the - addressable region for the device, i.e., if the DMA address of - the memory ANDed with the dma_mask is still equal to the DMA - address, then the device can perform DMA to the memory). To - ensure that the memory allocated by kmalloc is within the dma_mask, - the driver may specify various platform-dependent flags to restrict - the DMA address range of the allocation (e.g., on x86, GFP_DMA - guarantees to be within the first 16MB of available DMA addresses, - as required by ISA devices). - - Note also that the above constraints on physical contiguity and - dma_mask may not apply if the platform has an IOMMU (a device which - maps an I/O DMA address to a physical memory address). However, to be - portable, device driver writers may *not* assume that such an IOMMU - exists. - .. warning:: Memory coherency operates at a granularity called the cache @@ -325,8 +272,7 @@ DMA_BIDIRECTIONAL direction isn't known enum dma_data_direction direction) Unmaps the region previously mapped. All the parameters passed in -must be identical to those passed in (and returned) by the mapping -API. +must be identical to those passed to (and returned by) dma_map_single(). :: @@ -376,10 +322,10 @@ action (e.g. reduce current DMA mapping usage or delay and try again later). dma_map_sg(struct device *dev, struct scatterlist *sg, int nents, enum dma_data_direction direction) -Returns: the number of DMA address segments mapped (this may be shorter -than <nents> passed in if some elements of the scatter/gather list are -physically or virtually adjacent and an IOMMU maps them with a single -entry). +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. @@ -403,9 +349,8 @@ With scatterlists, you use the resulting mapping like this:: where nents is the number of entries in the sglist. The implementation is free to merge several consecutive sglist entries -into one (e.g. with an IOMMU, or if several pages just happen to be -physically contiguous) and returns the actual number of sg entries it -mapped them to. On failure 0, is returned. +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 @@ -775,19 +720,19 @@ memory or doing partial flushes. of two for easy alignment. -Part III - Debug drivers use of the DMA-API +Part III - Debug drivers use of the DMA API ------------------------------------------- -The DMA-API as described above has some constraints. DMA addresses must be +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 +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 +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 @@ -826,7 +771,7 @@ example warning message may look like this:: <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. +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 @@ -834,7 +779,7 @@ 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 +The debugfs directory for the DMA API debugging code is called dma-api/. In this directory the following files can currently be found: =============================== =============================================== @@ -882,7 +827,7 @@ dma-api/driver_filter You can write a name of a driver into this file 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. +'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. @@ -915,3 +860,9 @@ 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 |