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

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Date:   Fri, 1 May 2020 12:42:15 +0200
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     "Jason A. Donenfeld" <Jason@...c4.com>
Cc:     linux-kernel@...r.kernel.org, intel-gfx@...ts.freedesktop.org,
        dri-devel@...ts.freedesktop.org, tglx@...utronix.de,
        chris@...is-wilson.co.uk, stable@...r.kernel.org
Subject: Re: [PATCH] drm/i915: check to see if SIMD registers are available
 before using SIMD

On 2020-04-30 16:10:16 [-0600], Jason A. Donenfeld wrote:
> 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.

That would indicate that these functions are used from IRQ/softirq which
break otherwise if the kernel is also using the registers. The crypto
code uses it for that purpose.

So
   Reviewed-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>

May I ask how large the memcpy can be? I'm asking in case it is large
and an explicit rescheduling point might be needed.

> Cc: stable@...r.kernel.org
> Signed-off-by: Jason A. Donenfeld <Jason@...c4.com>

Sebastian