[Mesa-dev] [PATCH] i965: Fix comment about DRM_IOCTL_I915_GEM_WAIT.

[Kristian Høgsberg]

On Wed, Jul 15, 2015 at 10:22 AM, Kenneth Graunke <kenneth at whitecape.org> wrote:
> From: Chris Wilson <chris at chris-wilson.co.uk>
>
> The kernel actually waits forever when supplied a timeout value < 0,
> rather than returning immediately.  See i915_gem_wait_ioctl() in
> i915_gem.c's call to __i915_wait_request().
>
> (split by Ken from a large patch authored by Chris Wilson)
>
> Reviewed-by: Kenneth Graunke <kenneth at whitecape.org>
> ---
>  src/mesa/drivers/dri/i965/intel_syncobj.c | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
>
> diff --git a/src/mesa/drivers/dri/i965/intel_syncobj.c b/src/mesa/drivers/dri/i965/intel_syncobj.c
> index c44c4be..c2f4fa9 100644
> --- a/src/mesa/drivers/dri/i965/intel_syncobj.c
> +++ b/src/mesa/drivers/dri/i965/intel_syncobj.c
> @@ -105,9 +105,9 @@ brw_fence_client_wait(struct brw_context *brw, struct brw_fence *fence,
>     assert(fence->batch_bo);
>
>     /* DRM_IOCTL_I915_GEM_WAIT uses a signed 64 bit timeout and returns
> -    * immediately for timeouts <= 0.  The best we can do is to clamp the
> -    * timeout to INT64_MAX.  This limits the maximum timeout from 584 years to
> -    * 292 years - likely not a big deal.
> +    * immediately for timeout == 0, and indefinitely if timeout is negative.
> +    * The best we can do is to clamp the timeout to INT64_MAX.  This limits
> +    * the maximum timeout from 584 years to 292 years - likely not a big deal.

No, there are kernel versions in the wild that has this bug. The
comment after the patch says:

 /* DRM_IOCTL_I915_GEM_WAIT uses a signed 64 bit timeout and returns
 * immediately for timeout == 0, and indefinitely if timeout is negative.
 * The best we can do is to clamp the timeout to INT64_MAX.  This limits
 * the maximum timeout from 584 years to 292 years - likely not a big deal.

which doesn't make sense. If we feel like we need to point out that
we've fixed the bug, that's fine, but keep the part about how some
kernels are broken so it's clear why the workaround is needed.

Kristian

>      */
>     if (timeout > INT64_MAX)
>        timeout = INT64_MAX;
> --
> 2.4.5
>
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