diff options
Diffstat (limited to 'Documentation/driver-api')
| -rw-r--r-- | Documentation/driver-api/80211/mac80211-advanced.rst | 8 | ||||
| -rw-r--r-- | Documentation/driver-api/dmaengine/index.rst | 4 | ||||
| -rw-r--r-- | Documentation/driver-api/dmaengine/provider.rst | 12 | ||||
| -rw-r--r-- | Documentation/driver-api/driver-model/driver.rst | 2 | ||||
| -rw-r--r-- | Documentation/driver-api/edid.rst | 58 | ||||
| -rw-r--r-- | Documentation/driver-api/index.rst | 4 | ||||
| -rw-r--r-- | Documentation/driver-api/io-mapping.rst | 97 | ||||
| -rw-r--r-- | Documentation/driver-api/io_ordering.rst | 51 | ||||
| -rw-r--r-- | Documentation/driver-api/ioctl.rst | 253 |
9 files changed, 421 insertions, 68 deletions
diff --git a/Documentation/driver-api/80211/mac80211-advanced.rst b/Documentation/driver-api/80211/mac80211-advanced.rst index 9f1c5bb7ac35..24cb64b3b715 100644 --- a/Documentation/driver-api/80211/mac80211-advanced.rst +++ b/Documentation/driver-api/80211/mac80211-advanced.rst @@ -272,8 +272,8 @@ STA information lifetime rules .. kernel-doc:: net/mac80211/sta_info.c :doc: STA information lifetime rules -Aggregation -=========== +Aggregation Functions +===================== .. kernel-doc:: net/mac80211/sta_info.h :functions: sta_ampdu_mlme @@ -284,8 +284,8 @@ Aggregation .. kernel-doc:: net/mac80211/sta_info.h :functions: tid_ampdu_rx -Synchronisation -=============== +Synchronisation Functions +========================= TBD diff --git a/Documentation/driver-api/dmaengine/index.rst b/Documentation/driver-api/dmaengine/index.rst index b9df904d0a79..bdc45d8b4cfb 100644 --- a/Documentation/driver-api/dmaengine/index.rst +++ b/Documentation/driver-api/dmaengine/index.rst @@ -5,8 +5,8 @@ DMAEngine documentation DMAEngine documentation provides documents for various aspects of DMAEngine framework. -DMAEngine documentation ------------------------ +DMAEngine development documentation +----------------------------------- This book helps with DMAengine internal APIs and guide for DMAEngine device driver writers. diff --git a/Documentation/driver-api/dmaengine/provider.rst b/Documentation/driver-api/dmaengine/provider.rst index 790a15089f1f..56e5833e8a07 100644 --- a/Documentation/driver-api/dmaengine/provider.rst +++ b/Documentation/driver-api/dmaengine/provider.rst @@ -266,11 +266,15 @@ to use. attached (via the dmaengine_desc_attach_metadata() helper to the descriptor. From the DMA driver the following is expected for this mode: + - DMA_MEM_TO_DEV / DEV_MEM_TO_MEM + The data from the provided metadata buffer should be prepared for the DMA controller to be sent alongside of the payload data. Either by copying to a hardware descriptor, or highly coupled packet. + - DMA_DEV_TO_MEM + On transfer completion the DMA driver must copy the metadata to the client provided metadata buffer before notifying the client about the completion. After the transfer completion, DMA drivers must not touch the metadata @@ -284,10 +288,14 @@ to use. and dmaengine_desc_set_metadata_len() is provided as helper functions. From the DMA driver the following is expected for this mode: - - get_metadata_ptr + + - get_metadata_ptr() + Should return a pointer for the metadata buffer, the maximum size of the metadata buffer and the currently used / valid (if any) bytes in the buffer. - - set_metadata_len + + - set_metadata_len() + It is called by the clients after it have placed the metadata to the buffer to let the DMA driver know the number of valid bytes provided. diff --git a/Documentation/driver-api/driver-model/driver.rst b/Documentation/driver-api/driver-model/driver.rst index baa6a85c8287..63887b813005 100644 --- a/Documentation/driver-api/driver-model/driver.rst +++ b/Documentation/driver-api/driver-model/driver.rst @@ -210,7 +210,7 @@ probed. While the typical use case for sync_state() is to have the kernel cleanly take over management of devices from the bootloader, the usage of sync_state() is not restricted to that. Use it whenever it makes sense to take an action after -all the consumers of a device have probed. +all the consumers of a device have probed:: int (*remove) (struct device *dev); diff --git a/Documentation/driver-api/edid.rst b/Documentation/driver-api/edid.rst deleted file mode 100644 index b1b5acd501ed..000000000000 --- a/Documentation/driver-api/edid.rst +++ /dev/null @@ -1,58 +0,0 @@ -.. SPDX-License-Identifier: GPL-2.0 - -==== -EDID -==== - -In the good old days when graphics parameters were configured explicitly -in a file called xorg.conf, even broken hardware could be managed. - -Today, with the advent of Kernel Mode Setting, a graphics board is -either correctly working because all components follow the standards - -or the computer is unusable, because the screen remains dark after -booting or it displays the wrong area. Cases when this happens are: -- The graphics board does not recognize the monitor. -- The graphics board is unable to detect any EDID data. -- The graphics board incorrectly forwards EDID data to the driver. -- The monitor sends no or bogus EDID data. -- A KVM sends its own EDID data instead of querying the connected monitor. -Adding the kernel parameter "nomodeset" helps in most cases, but causes -restrictions later on. - -As a remedy for such situations, the kernel configuration item -CONFIG_DRM_LOAD_EDID_FIRMWARE was introduced. It allows to provide an -individually prepared or corrected EDID data set in the /lib/firmware -directory from where it is loaded via the firmware interface. The code -(see drivers/gpu/drm/drm_edid_load.c) contains built-in data sets for -commonly used screen resolutions (800x600, 1024x768, 1280x1024, 1600x1200, -1680x1050, 1920x1080) as binary blobs, but the kernel source tree does -not contain code to create these data. In order to elucidate the origin -of the built-in binary EDID blobs and to facilitate the creation of -individual data for a specific misbehaving monitor, commented sources -and a Makefile environment are given here. - -To create binary EDID and C source code files from the existing data -material, simply type "make". - -If you want to create your own EDID file, copy the file 1024x768.S, -replace the settings with your own data and add a new target to the -Makefile. Please note that the EDID data structure expects the timing -values in a different way as compared to the standard X11 format. - -X11: - HTimings: - hdisp hsyncstart hsyncend htotal - VTimings: - vdisp vsyncstart vsyncend vtotal - -EDID:: - - #define XPIX hdisp - #define XBLANK htotal-hdisp - #define XOFFSET hsyncstart-hdisp - #define XPULSE hsyncend-hsyncstart - - #define YPIX vdisp - #define YBLANK vtotal-vdisp - #define YOFFSET vsyncstart-vdisp - #define YPULSE vsyncend-vsyncstart diff --git a/Documentation/driver-api/index.rst b/Documentation/driver-api/index.rst index 0ebe205efd0c..d4e78cb3ef4d 100644 --- a/Documentation/driver-api/index.rst +++ b/Documentation/driver-api/index.rst @@ -17,6 +17,7 @@ available subsections can be seen below. driver-model/index basics infrastructure + ioctl early-userspace/index pm/index clk @@ -74,11 +75,12 @@ available subsections can be seen below. connector console dcdbas - edid eisa ipmb isa isapnp + io-mapping + io_ordering generic-counter lightnvm-pblk memory-devices/index diff --git a/Documentation/driver-api/io-mapping.rst b/Documentation/driver-api/io-mapping.rst new file mode 100644 index 000000000000..a966239f04e4 --- /dev/null +++ b/Documentation/driver-api/io-mapping.rst @@ -0,0 +1,97 @@ +======================== +The io_mapping functions +======================== + +API +=== + +The io_mapping functions in linux/io-mapping.h provide an abstraction for +efficiently mapping small regions of an I/O device to the CPU. The initial +usage is to support the large graphics aperture on 32-bit processors where +ioremap_wc cannot be used to statically map the entire aperture to the CPU +as it would consume too much of the kernel address space. + +A mapping object is created during driver initialization using:: + + struct io_mapping *io_mapping_create_wc(unsigned long base, + unsigned long size) + +'base' is the bus address of the region to be made +mappable, while 'size' indicates how large a mapping region to +enable. Both are in bytes. + +This _wc variant provides a mapping which may only be used +with the io_mapping_map_atomic_wc or io_mapping_map_wc. + +With this mapping object, individual pages can be mapped either atomically +or not, depending on the necessary scheduling environment. Of course, atomic +maps are more efficient:: + + void *io_mapping_map_atomic_wc(struct io_mapping *mapping, + unsigned long offset) + +'offset' is the offset within the defined mapping region. +Accessing addresses beyond the region specified in the +creation function yields undefined results. Using an offset +which is not page aligned yields an undefined result. The +return value points to a single page in CPU address space. + +This _wc variant returns a write-combining map to the +page and may only be used with mappings created by +io_mapping_create_wc + +Note that the task may not sleep while holding this page +mapped. + +:: + + void io_mapping_unmap_atomic(void *vaddr) + +'vaddr' must be the value returned by the last +io_mapping_map_atomic_wc call. This unmaps the specified +page and allows the task to sleep once again. + +If you need to sleep while holding the lock, you can use the non-atomic +variant, although they may be significantly slower. + +:: + + void *io_mapping_map_wc(struct io_mapping *mapping, + unsigned long offset) + +This works like io_mapping_map_atomic_wc except it allows +the task to sleep while holding the page mapped. + + +:: + + void io_mapping_unmap(void *vaddr) + +This works like io_mapping_unmap_atomic, except it is used +for pages mapped with io_mapping_map_wc. + +At driver close time, the io_mapping object must be freed:: + + void io_mapping_free(struct io_mapping *mapping) + +Current Implementation +====================== + +The initial implementation of these functions uses existing mapping +mechanisms and so provides only an abstraction layer and no new +functionality. + +On 64-bit processors, io_mapping_create_wc calls ioremap_wc for the whole +range, creating a permanent kernel-visible mapping to the resource. The +map_atomic and map functions add the requested offset to the base of the +virtual address returned by ioremap_wc. + +On 32-bit processors with HIGHMEM defined, io_mapping_map_atomic_wc uses +kmap_atomic_pfn to map the specified page in an atomic fashion; +kmap_atomic_pfn isn't really supposed to be used with device pages, but it +provides an efficient mapping for this usage. + +On 32-bit processors without HIGHMEM defined, io_mapping_map_atomic_wc and +io_mapping_map_wc both use ioremap_wc, a terribly inefficient function which +performs an IPI to inform all processors about the new mapping. This results +in a significant performance penalty. diff --git a/Documentation/driver-api/io_ordering.rst b/Documentation/driver-api/io_ordering.rst new file mode 100644 index 000000000000..2ab303ce9a0d --- /dev/null +++ b/Documentation/driver-api/io_ordering.rst @@ -0,0 +1,51 @@ +============================================== +Ordering I/O writes to memory-mapped addresses +============================================== + +On some platforms, so-called memory-mapped I/O is weakly ordered. On such +platforms, driver writers are responsible for ensuring that I/O writes to +memory-mapped addresses on their device arrive in the order intended. This is +typically done by reading a 'safe' device or bridge register, causing the I/O +chipset to flush pending writes to the device before any reads are posted. A +driver would usually use this technique immediately prior to the exit of a +critical section of code protected by spinlocks. This would ensure that +subsequent writes to I/O space arrived only after all prior writes (much like a +memory barrier op, mb(), only with respect to I/O). + +A more concrete example from a hypothetical device driver:: + + ... + CPU A: spin_lock_irqsave(&dev_lock, flags) + CPU A: val = readl(my_status); + CPU A: ... + CPU A: writel(newval, ring_ptr); + CPU A: spin_unlock_irqrestore(&dev_lock, flags) + ... + CPU B: spin_lock_irqsave(&dev_lock, flags) + CPU B: val = readl(my_status); + CPU B: ... + CPU B: writel(newval2, ring_ptr); + CPU B: spin_unlock_irqrestore(&dev_lock, flags) + ... + +In the case above, the device may receive newval2 before it receives newval, +which could cause problems. Fixing it is easy enough though:: + + ... + CPU A: spin_lock_irqsave(&dev_lock, flags) + CPU A: val = readl(my_status); + CPU A: ... + CPU A: writel(newval, ring_ptr); + CPU A: (void)readl(safe_register); /* maybe a config register? */ + CPU A: spin_unlock_irqrestore(&dev_lock, flags) + ... + CPU B: spin_lock_irqsave(&dev_lock, flags) + CPU B: val = readl(my_status); + CPU B: ... + CPU B: writel(newval2, ring_ptr); + CPU B: (void)readl(safe_register); /* maybe a config register? */ + CPU B: spin_unlock_irqrestore(&dev_lock, flags) + +Here, the reads from safe_register will cause the I/O chipset to flush any +pending writes before actually posting the read to the chipset, preventing +possible data corruption. diff --git a/Documentation/driver-api/ioctl.rst b/Documentation/driver-api/ioctl.rst new file mode 100644 index 000000000000..c455db0e1627 --- /dev/null +++ b/Documentation/driver-api/ioctl.rst @@ -0,0 +1,253 @@ +====================== +ioctl based interfaces +====================== + +ioctl() is the most common way for applications to interface +with device drivers. It is flexible and easily extended by adding new +commands and can be passed through character devices, block devices as +well as sockets and other special file descriptors. + +However, it is also very easy to get ioctl command definitions wrong, +and hard to fix them later without breaking existing applications, +so this documentation tries to help developers get it right. + +Command number definitions +========================== + +The command number, or request number, is the second argument passed to +the ioctl system call. While this can be any 32-bit number that uniquely +identifies an action for a particular driver, there are a number of +conventions around defining them. + +``include/uapi/asm-generic/ioctl.h`` provides four macros for defining +ioctl commands that follow modern conventions: ``_IO``, ``_IOR``, +``_IOW``, and ``_IOWR``. These should be used for all new commands, +with the correct parameters: + +_IO/_IOR/_IOW/_IOWR + The macro name specifies how the argument will be used. It may be a + pointer to data to be passed into the kernel (_IOW), out of the kernel + (_IOR), or both (_IOWR). _IO can indicate either commands with no + argument or those passing an integer value instead of a pointer. + It is recommended to only use _IO for commands without arguments, + and use pointers for passing data. + +type + An 8-bit number, often a character literal, specific to a subsystem + or driver, and listed in :doc:`../userspace-api/ioctl/ioctl-number` + +nr + An 8-bit number identifying the specific command, unique for a give + value of 'type' + +data_type + The name of the data type pointed to by the argument, the command number + encodes the ``sizeof(data_type)`` value in a 13-bit or 14-bit integer, + leading to a limit of 8191 bytes for the maximum size of the argument. + Note: do not pass sizeof(data_type) type into _IOR/_IOW/IOWR, as that + will lead to encoding sizeof(sizeof(data_type)), i.e. sizeof(size_t). + _IO does not have a data_type parameter. + + +Interface versions +================== + +Some subsystems use version numbers in data structures to overload +commands with different interpretations of the argument. + +This is generally a bad idea, since changes to existing commands tend +to break existing applications. + +A better approach is to add a new ioctl command with a new number. The +old command still needs to be implemented in the kernel for compatibility, +but this can be a wrapper around the new implementation. + +Return code +=========== + +ioctl commands can return negative error codes as documented in errno(3); +these get turned into errno values in user space. On success, the return +code should be zero. It is also possible but not recommended to return +a positive 'long' value. + +When the ioctl callback is called with an unknown command number, the +handler returns either -ENOTTY or -ENOIOCTLCMD, which also results in +-ENOTTY being returned from the system call. Some subsystems return +-ENOSYS or -EINVAL here for historic reasons, but this is wrong. + +Prior to Linux 5.5, compat_ioctl handlers were required to return +-ENOIOCTLCMD in order to use the fallback conversion into native +commands. As all subsystems are now responsible for handling compat +mode themselves, this is no longer needed, but it may be important to +consider when backporting bug fixes to older kernels. + +Timestamps +========== + +Traditionally, timestamps and timeout values are passed as ``struct +timespec`` or ``struct timeval``, but these are problematic because of +incompatible definitions of these structures in user space after the +move to 64-bit time_t. + +The ``struct __kernel_timespec`` type can be used instead to be embedded +in other data structures when separate second/nanosecond values are +desired, or passed to user space directly. This is still not ideal though, +as the structure matches neither the kernel's timespec64 nor the user +space timespec exactly. The get_timespec64() and put_timespec64() helper +functions can be used to ensure that the layout remains compatible with +user space and the padding is treated correctly. + +As it is cheap to convert seconds to nanoseconds, but the opposite +requires an expensive 64-bit division, a simple __u64 nanosecond value +can be simpler and more efficient. + +Timeout values and timestamps should ideally use CLOCK_MONOTONIC time, +as returned by ktime_get_ns() or ktime_get_ts64(). Unlike +CLOCK_REALTIME, this makes the timestamps immune from jumping backwards +or forwards due to leap second adjustments and clock_settime() calls. + +ktime_get_real_ns() can be used for CLOCK_REALTIME timestamps that +need to be persistent across a reboot or between multiple machines. + +32-bit compat mode +================== + +In order to support 32-bit user space running on a 64-bit machine, each +subsystem or driver that implements an ioctl callback handler must also +implement the corresponding compat_ioctl handler. + +As long as all the rules for data structures are followed, this is as +easy as setting the .compat_ioctl pointer to a helper function such as +compat_ptr_ioctl() or blkdev_compat_ptr_ioctl(). + +compat_ptr() +------------ + +On the s390 architecture, 31-bit user space has ambiguous representations +for data pointers, with the upper bit being ignored. When running such +a process in compat mode, the compat_ptr() helper must be used to +clear the upper bit of a compat_uptr_t and turn it into a valid 64-bit +pointer. On other architectures, this macro only performs a cast to a +``void __user *`` pointer. + +In an compat_ioctl() callback, the last argument is an unsigned long, +which can be interpreted as either a pointer or a scalar depending on +the command. If it is a scalar, then compat_ptr() must not be used, to +ensure that the 64-bit kernel behaves the same way as a 32-bit kernel +for arguments with the upper bit set. + +The compat_ptr_ioctl() helper can be used in place of a custom +compat_ioctl file operation for drivers that only take arguments that +are pointers to compatible data structures. + +Structure layout +---------------- + +Compatible data structures have the same layout on all architectures, +avoiding all problematic members: + +* ``long`` and ``unsigned long`` are the size of a register, so + they can be either 32-bit or 64-bit wide and cannot be used in portable + data structures. Fixed-length replacements are ``__s32``, ``__u32``, + ``__s64`` and ``__u64``. + +* Pointers have the same problem, in addition to requiring the + use of compat_ptr(). The best workaround is to use ``__u64`` + in place of pointers, which requires a cast to ``uintptr_t`` in user + space, and the use of u64_to_user_ptr() in the kernel to convert + it back into a user pointer. + +* On the x86-32 (i386) architecture, the alignment of 64-bit variables + is only 32-bit, but they are naturally aligned on most other + architectures including x86-64. This means a structure like:: + + struct foo { + __u32 a; + __u64 b; + __u32 c; + }; + + has four bytes of padding between a and b on x86-64, plus another four + bytes of padding at the end, but no padding on i386, and it needs a + compat_ioctl conversion handler to translate between the two formats. + + To avoid this problem, all structures should have their members + naturally aligned, or explicit reserved fields added in place of the + implicit padding. The ``pahole`` tool can be used for checking the + alignment. + +* On ARM OABI user space, structures are padded to multiples of 32-bit, + making some structs incompatible with modern EABI kernels if they + do not end on a 32-bit boundary. + +* On the m68k architecture, struct members are not guaranteed to have an + alignment greater than 16-bit, which is a problem when relying on + implicit padding. + +* Bitfields and enums generally work as one would expect them to, + but some properties of them are implementation-defined, so it is better + to avoid them completely in ioctl interfaces. + +* ``char`` members can be either signed or unsigned, depending on + the architecture, so the __u8 and __s8 types should be used for 8-bit + integer values, though char arrays are clearer for fixed-length strings. + +Information leaks +================= + +Uninitialized data must not be copied back to user space, as this can +cause an information leak, which can be used to defeat kernel address +space layout randomization (KASLR), helping in an attack. + +For this reason (and for compat support) it is best to avoid any +implicit padding in data structures. Where there is implicit padding +in an existing structure, kernel drivers must be careful to fully +initialize an instance of the structure before copying it to user +space. This is usually done by calling memset() before assigning to +individual members. + +Subsystem abstractions +====================== + +While some device drivers implement their own ioctl function, most +subsystems implement the same command for multiple drivers. Ideally the +subsystem has an .ioctl() handler that copies the arguments from and +to user space, passing them into subsystem specific callback functions +through normal kernel pointers. + +This helps in various ways: + +* Applications written for one driver are more likely to work for + another one in the same subsystem if there are no subtle differences + in the user space ABI. + +* The complexity of user space access and data structure layout is done + in one place, reducing the potential for implementation bugs. + +* It is more likely to be reviewed by experienced developers + that can spot problems in the interface when the ioctl is shared + between multiple drivers than when it is only used in a single driver. + +Alternatives to ioctl +===================== + +There are many cases in which ioctl is not the best solution for a +problem. Alternatives include: + +* System calls are a better choice for a system-wide feature that + is not tied to a physical device or constrained by the file system + permissions of a character device node + +* netlink is the preferred way of configuring any network related + objects through sockets. + +* debugfs is used for ad-hoc interfaces for debugging functionality + that does not need to be exposed as a stable interface to applications. + +* sysfs is a good way to expose the state of an in-kernel object + that is not tied to a file descriptor. + +* configfs can be used for more complex configuration than sysfs + +* A custom file system can provide extra flexibility with a simple + user interface but adds a lot of complexity to the implementation. |