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/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 |
8 files changed, 411 insertions, 66 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/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. |