[Intel-gfx] [PATCH 1/2] drm/i915: Allow DRM_ROOT_ONLY|DRM_MASTER to submit privileged batchbuffers

[Daniel Vetter]

On Wed, Oct 17, 2012 at 8:47 PM, Eric Anholt <eric at anholt.net> wrote:
> Chris Wilson <chris at chris-wilson.co.uk> writes:
>
>> With the introduction of per-process GTT space, the hardware designers
>> thought it wise to also limit the ability to write to MMIO space to only
>> a "secure" batch buffer. The ability to rewrite registers is the only
>> way to program the hardware to perform certain operations like scanline
>> waits (required for tear-free windowed updates). So we either have a
>> choice of adding an interface to perform those synchronized updates
>> inside the kernel, or we permit certain processes the ability to write
>> to the "safe" registers from within its command stream. This patch
>> exposes the ability to submit a SECURE batch buffer to
>> DRM_ROOT_ONLY|DRM_MASTER processes.
>
> This seems like a major blow to ever getting non-root X.

Currently we only need this to get scanline waits going on snb+ and on
ivb+ that will neatly keep the gpu out of rc6 while waiting for that
scanline. So there rather clearly a massive legacy feature, ill
supported by the hw. So that's the only thing that the X driver will
lose, to which I highly suggest to simply install a pageflipping
compositor of sorts and call it a day.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch