docs: move botching-up-ioctls.rst to the process guide

This is overall information for kernel developers, and not part of the
user-space API.

Cc: Daniel Vetter <daniel@ffwll.ch>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

2 files changed, 0 insertions, 226 deletions

diff --git a/Documentation/ioctl/botching-up-ioctls.rst b/Documentation/ioctl/botching-up-ioctls.rst
deleted file mode 100644
index ac697fef3545..000000000000
--- a/Documentation/ioctl/botching-up-ioctls.rst
+++ /dev/null
@@ -1,225 +0,0 @@
-=================================
-(How to avoid) Botching up ioctls
-=================================
-
-From: http://blog.ffwll.ch/2013/11/botching-up-ioctls.html
-
-By: Daniel Vetter, Copyright © 2013 Intel Corporation
-
-One clear insight kernel graphics hackers gained in the past few years is that
-trying to come up with a unified interface to manage the execution units and
-memory on completely different GPUs is a futile effort. So nowadays every
-driver has its own set of ioctls to allocate memory and submit work to the GPU.
-Which is nice, since there's no more insanity in the form of fake-generic, but
-actually only used once interfaces. But the clear downside is that there's much
-more potential to screw things up.
-
-To avoid repeating all the same mistakes again I've written up some of the
-lessons learned while botching the job for the drm/i915 driver. Most of these
-only cover technicalities and not the big-picture issues like what the command
-submission ioctl exactly should look like. Learning these lessons is probably
-something every GPU driver has to do on its own.
-
-
-Prerequisites
--------------
-
-First the prerequisites. Without these you have already failed, because you
-will need to add a 32-bit compat layer:
-
- * Only use fixed sized integers. To avoid conflicts with typedefs in userspace
-   the kernel has special types like __u32, __s64. Use them.
-
- * Align everything to the natural size and use explicit padding. 32-bit
-   platforms don't necessarily align 64-bit values to 64-bit boundaries, but
-   64-bit platforms do. So we always need padding to the natural size to get
-   this right.
-
- * Pad the entire struct to a multiple of 64-bits if the structure contains
-   64-bit types - the structure size will otherwise differ on 32-bit versus
-   64-bit. Having a different structure size hurts when passing arrays of
-   structures to the kernel, or if the kernel checks the structure size, which
-   e.g. the drm core does.
-
- * Pointers are __u64, cast from/to a uintprt_t on the userspace side and
-   from/to a void __user * in the kernel. Try really hard not to delay this
-   conversion or worse, fiddle the raw __u64 through your code since that
-   diminishes the checking tools like sparse can provide. The macro
-   u64_to_user_ptr can be used in the kernel to avoid warnings about integers
-   and pointres of different sizes.
-
-
-Basics
-------
-
-With the joys of writing a compat layer avoided we can take a look at the basic
-fumbles. Neglecting these will make backward and forward compatibility a real
-pain. And since getting things wrong on the first attempt is guaranteed you
-will have a second iteration or at least an extension for any given interface.
-
- * Have a clear way for userspace to figure out whether your new ioctl or ioctl
-   extension is supported on a given kernel. If you can't rely on old kernels
-   rejecting the new flags/modes or ioctls (since doing that was botched in the
-   past) then you need a driver feature flag or revision number somewhere.
-
- * Have a plan for extending ioctls with new flags or new fields at the end of
-   the structure. The drm core checks the passed-in size for each ioctl call
-   and zero-extends any mismatches between kernel and userspace. That helps,
-   but isn't a complete solution since newer userspace on older kernels won't
-   notice that the newly added fields at the end get ignored. So this still
-   needs a new driver feature flags.
-
- * Check all unused fields and flags and all the padding for whether it's 0,
-   and reject the ioctl if that's not the case. Otherwise your nice plan for
-   future extensions is going right down the gutters since someone will submit
-   an ioctl struct with random stack garbage in the yet unused parts. Which
-   then bakes in the ABI that those fields can never be used for anything else
-   but garbage. This is also the reason why you must explicitly pad all
-   structures, even if you never use them in an array - the padding the compiler
-   might insert could contain garbage.
-
- * Have simple testcases for all of the above.
-
-
-Fun with Error Paths
---------------------
-
-Nowadays we don't have any excuse left any more for drm drivers being neat
-little root exploits. This means we both need full input validation and solid
-error handling paths - GPUs will die eventually in the oddmost corner cases
-anyway:
-
- * The ioctl must check for array overflows. Also it needs to check for
-   over/underflows and clamping issues of integer values in general. The usual
-   example is sprite positioning values fed directly into the hardware with the
-   hardware just having 12 bits or so. Works nicely until some odd display
-   server doesn't bother with clamping itself and the cursor wraps around the
-   screen.
-
- * Have simple testcases for every input validation failure case in your ioctl.
-   Check that the error code matches your expectations. And finally make sure
-   that you only test for one single error path in each subtest by submitting
-   otherwise perfectly valid data. Without this an earlier check might reject
-   the ioctl already and shadow the codepath you actually want to test, hiding
-   bugs and regressions.
-
- * Make all your ioctls restartable. First X really loves signals and second
-   this will allow you to test 90% of all error handling paths by just
-   interrupting your main test suite constantly with signals. Thanks to X's
-   love for signal you'll get an excellent base coverage of all your error
-   paths pretty much for free for graphics drivers. Also, be consistent with
-   how you handle ioctl restarting - e.g. drm has a tiny drmIoctl helper in its
-   userspace library. The i915 driver botched this with the set_tiling ioctl,
-   now we're stuck forever with some arcane semantics in both the kernel and
-   userspace.
-
- * If you can't make a given codepath restartable make a stuck task at least
-   killable. GPUs just die and your users won't like you more if you hang their
-   entire box (by means of an unkillable X process). If the state recovery is
-   still too tricky have a timeout or hangcheck safety net as a last-ditch
-   effort in case the hardware has gone bananas.
-
- * Have testcases for the really tricky corner cases in your error recovery code
-   - it's way too easy to create a deadlock between your hangcheck code and
-   waiters.
-
-
-Time, Waiting and Missing it
-----------------------------
-
-GPUs do most everything asynchronously, so we have a need to time operations and
-wait for outstanding ones. This is really tricky business; at the moment none of
-the ioctls supported by the drm/i915 get this fully right, which means there's
-still tons more lessons to learn here.
-
- * Use CLOCK_MONOTONIC as your reference time, always. It's what alsa, drm and
-   v4l use by default nowadays. But let userspace know which timestamps are
-   derived from different clock domains like your main system clock (provided
-   by the kernel) or some independent hardware counter somewhere else. Clocks
-   will mismatch if you look close enough, but if performance measuring tools
-   have this information they can at least compensate. If your userspace can
-   get at the raw values of some clocks (e.g. through in-command-stream
-   performance counter sampling instructions) consider exposing those also.
-
- * Use __s64 seconds plus __u64 nanoseconds to specify time. It's not the most
-   convenient time specification, but it's mostly the standard.
-
- * Check that input time values are normalized and reject them if not. Note
-   that the kernel native struct ktime has a signed integer for both seconds
-   and nanoseconds, so beware here.
-
- * For timeouts, use absolute times. If you're a good fellow and made your
-   ioctl restartable relative timeouts tend to be too coarse and can
-   indefinitely extend your wait time due to rounding on each restart.
-   Especially if your reference clock is something really slow like the display
-   frame counter. With a spec lawyer hat on this isn't a bug since timeouts can
-   always be extended - but users will surely hate you if their neat animations
-   starts to stutter due to this.
-
- * Consider ditching any synchronous wait ioctls with timeouts and just deliver
-   an asynchronous event on a pollable file descriptor. It fits much better
-   into event driven applications' main loop.
-
- * Have testcases for corner-cases, especially whether the return values for
-   already-completed events, successful waits and timed-out waits are all sane
-   and suiting to your needs.
-
-
-Leaking Resources, Not
-----------------------
-
-A full-blown drm driver essentially implements a little OS, but specialized to
-the given GPU platforms. This means a driver needs to expose tons of handles
-for different objects and other resources to userspace. Doing that right
-entails its own little set of pitfalls:
-
- * Always attach the lifetime of your dynamically created resources to the
-   lifetime of a file descriptor. Consider using a 1:1 mapping if your resource
-   needs to be shared across processes -  fd-passing over unix domain sockets
-   also simplifies lifetime management for userspace.
-
- * Always have O_CLOEXEC support.
-
- * Ensure that you have sufficient insulation between different clients. By
-   default pick a private per-fd namespace which forces any sharing to be done
-   explicitly. Only go with a more global per-device namespace if the objects
-   are truly device-unique. One counterexample in the drm modeset interfaces is
-   that the per-device modeset objects like connectors share a namespace with
-   framebuffer objects, which mostly are not shared at all. A separate
-   namespace, private by default, for framebuffers would have been more
-   suitable.
-
- * Think about uniqueness requirements for userspace handles. E.g. for most drm
-   drivers it's a userspace bug to submit the same object twice in the same
-   command submission ioctl. But then if objects are shareable userspace needs
-   to know whether it has seen an imported object from a different process
-   already or not. I haven't tried this myself yet due to lack of a new class
-   of objects, but consider using inode numbers on your shared file descriptors
-   as unique identifiers - it's how real files are told apart, too.
-   Unfortunately this requires a full-blown virtual filesystem in the kernel.
-
-
-Last, but not Least
--------------------
-
-Not every problem needs a new ioctl:
-
- * Think hard whether you really want a driver-private interface. Of course
-   it's much quicker to push a driver-private interface than engaging in
-   lengthy discussions for a more generic solution. And occasionally doing a
-   private interface to spearhead a new concept is what's required. But in the
-   end, once the generic interface comes around you'll end up maintainer two
-   interfaces. Indefinitely.
-
- * Consider other interfaces than ioctls. A sysfs attribute is much better for
-   per-device settings, or for child objects with fairly static lifetimes (like
-   output connectors in drm with all the detection override attributes). Or
-   maybe only your testsuite needs this interface, and then debugfs with its
-   disclaimer of not having a stable ABI would be better.
-
-Finally, the name of the game is to get it right on the first attempt, since if
-your driver proves popular and your hardware platforms long-lived then you'll
-be stuck with a given ioctl essentially forever. You can try to deprecate
-horrible ioctls on newer iterations of your hardware, but generally it takes
-years to accomplish this. And then again years until the last user able to
-complain about regressions disappears, too.
diff --git a/Documentation/ioctl/index.rst b/Documentation/ioctl/index.rst
index 0f0a857f6615..475675eae086 100644
--- a/Documentation/ioctl/index.rst
+++ b/Documentation/ioctl/index.rst
@@ -9,7 +9,6 @@ IOCTLs
 
    ioctl-number
 
-   botching-up-ioctls
    ioctl-decoding
 
    cdrom