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

From: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
To: Kenneth Graunke <kenneth@whitecape.org>, intel-gfx@lists.freedesktop.org
Cc: Dave Gordon <david.s.gordon@intel.com>, stable@vger.kernel.org
Subject: Re: [PATCH v2] drm/i915: Drop support for I915_EXEC_CONSTANTS_* execbuf parameters.
Date: Wed, 15 Feb 2017 09:38:43 +0200	[thread overview]
Message-ID: <1487144323.8490.3.camel@linux.intel.com> (raw)
In-Reply-To: <20170215041751.11441-1-kenneth@whitecape.org>

On ti, 2017-02-14 at 20:17 -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.
> 
> '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@intel.com>
> Cc: Chris Wilson <chris@chris-wilson.co.uk>
> Cc: stable@vger.kernel.org
> Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=92448
> Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>

Explanation makes sense, only Google hits seem to be on Mesa detecting
the feature and later, removing the detection when unused.

Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>

Regards, Joonas
-- 
Joonas Lahtinen
Open Source Technology Center
Intel Corporation
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