[Intel-gfx] [PATCH v2] drm/i915: Drop support for I915_EXEC_CONSTANTS_* execbuf parameters.

[Chris Wilson]

On Tue, Feb 14, 2017 at 08:17:51PM -0800, Kenneth Graunke wrote:
> This patch makes the I915_PARAM_HAS_EXEC_CONSTANTS getparam return 0
> (indicating the optional feature is not supported), and makes execbuf
> always return -EINVAL if the flags are used.
> 
> Apparently, no userspace ever shipped which used this optional feature:
> I checked the git history of Mesa, xf86-video-intel, libva, and Beignet,
> and there were zero commits showing a use of these flags.  Kernel commit
> 72bfa19c8deb4 apparently introduced the feature prematurely.

It was actually cairo where we had patches to use it first. And we then
realised the use was broken for gen6.
 
> 'relative_constants_mode' has always been tracked per-device, but this
> has actually been wrong ever since hardware contexts were introduced, as
> the INSTPM register is saved (and automatically restored) as part of the
> render ring context. The software per-device value could therefore get
> out of sync with the hardware per-context value.  This meant that using
> them is actually unsafe: a client which tried to use them could damage
> the state of other clients, causing the GPU to interpret their BO
> offsets as absolute pointers, leading to bogus memory reads.
> 
> These flags were also never ported to execlist mode, making them no-ops
> on Gen9+ (which requires execlists), and Gen8 in the default mode.
> 
> On Gen8+, userspace can write these registers directly, achieving the
> same effect.  On Gen6-7.5, it likely makes sense to extend the command
> parser to support them.  I don't think anyone wants this on Gen4-5.
> 
> Based on a patch by Dave Gordon.
> 
> Cc: Dave Gordon <david.s.gordon at intel.com>
> Cc: Chris Wilson <chris at chris-wilson.co.uk>
> Cc: stable at vger.kernel.org
> Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=92448
> Signed-off-by: Kenneth Graunke <kenneth at whitecape.org>
> ---
>  drivers/gpu/drm/i915/i915_drv.c            |  2 +-
>  drivers/gpu/drm/i915/i915_drv.h            |  2 --
>  drivers/gpu/drm/i915/i915_gem.c            |  2 --
>  drivers/gpu/drm/i915/i915_gem_execbuffer.c | 50 ++----------------------------
>  4 files changed, 3 insertions(+), 53 deletions(-)
> 
> diff --git a/drivers/gpu/drm/i915/i915_drv.c b/drivers/gpu/drm/i915/i915_drv.c
> index 309c29c84c54..1a53a4bb09c8 100644
> --- a/drivers/gpu/drm/i915/i915_drv.c
> +++ b/drivers/gpu/drm/i915/i915_drv.c
> @@ -276,7 +276,7 @@ static int i915_getparam(struct drm_device *dev, void *data,
>  		value = !!dev_priv->engine[VCS2];
>  		break;
>  	case I915_PARAM_HAS_EXEC_CONSTANTS:
> -		value = INTEL_GEN(dev_priv) >= 4;
> +		value = 0;

The style for obsolete param is to return -ENODEV. If that makes sense
we can move this to the start of this block with the other dead params.

Reviewed-by: Chris Wilson <chris at chris-wilson.co.uk>
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre