linux-kernel - Re: [PATCH] drm/i915: check to see if SIMD registers are available before using SIMD

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Date:   Sun, 03 May 2020 21:30:21 +0100
From:   Chris Wilson <chris@...is-wilson.co.uk>
To:     "Jason A. Donenfeld" <Jason@...c4.com>, bigeasy@...utronix.de,
        dri-devel@...ts.freedesktop.org, intel-gfx@...ts.freedesktop.org,
        linux-kernel@...r.kernel.org, tglx@...utronix.de
Cc:     "Jason A. Donenfeld" <Jason@...c4.com>, stable@...r.kernel.org
Subject: Re: [PATCH] drm/i915: check to see if SIMD registers are available before using SIMD

Quoting Jason A. Donenfeld (2020-04-30 23:10:16)
> Sometimes it's not okay to use SIMD registers, the conditions for which
> have changed subtly from kernel release to kernel release. Usually the
> pattern is to check for may_use_simd() and then fallback to using
> something slower in the unlikely case SIMD registers aren't available.
> So, this patch fixes up i915's accelerated memcpy routines to fallback
> to boring memcpy if may_use_simd() is false.
> 
> Cc: stable@...r.kernel.org

The same argument as on the previous submission is that we return to the
caller if we can't use movntqda as their fallback path should be faster
than uncached memcpy.
-Chris