[Intel-gfx] [PATCH 04/13] drm/i915: Fail engine initialization if LRCA is incorrectly aligned

Chris Wilson chris at chris-wilson.co.uk
Mon Jan 11 15:31:37 PST 2016

On Mon, Jan 11, 2016 at 04:02:09PM +0000, Dave Gordon wrote:
> IIRC the original version of this WARN (in intel_lr_context_descriptor()
> above) was added with the GuC submission code, because the context
> descriptor as used in execlist code is a 64-bit value, but the GuC
> requires that all the unique stuff fits in those 20 unmasked bits of
> a 32-bit value, with the low 12 bits being used for flags. So we
> wanted to check that we never got a context ID that couldn't be
> pruned down to just those 20 bits without losing information. It's
> never been seen to happen since GuC development finished, so we can
> reasonably lose the check now.

I am missing something here as the GuC doesn't use the high 32bits of
the context descriptor, i.e. it never touches the lrca portion?

Chris Wilson, Intel Open Source Technology Centre

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