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

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Date:   Mon, 4 May 2020 16:15:41 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Christoph Hellwig' <hch@...radead.org>,
        Chris Wilson <chris@...is-wilson.co.uk>
CC:     "Jason A. Donenfeld" <Jason@...c4.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "intel-gfx@...ts.freedesktop.org" <intel-gfx@...ts.freedesktop.org>,
        "dri-devel@...ts.freedesktop.org" <dri-devel@...ts.freedesktop.org>,
        "bigeasy@...utronix.de" <bigeasy@...utronix.de>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "stable@...r.kernel.org" <stable@...r.kernel.org>
Subject: RE: [PATCH] drm/i915: check to see if SIMD registers are available
 before using SIMD

From: Christoph Hellwig
> Sent: 04 May 2020 17:03
> 
> On Sun, May 03, 2020 at 09:20:19PM +0100, Chris Wilson wrote:
> > > Err, why does i915 implements its own uncached memcpy instead of relying
> > > on core functionality to start with?
> >
> > What is this core functionality that provides movntqda?
> 
> A sensible name might be memcpy_uncached or mempcy_nontemporal.
> But the important point is that this should be arch code with a common
> fallback rather than hacking it up in drivers.

More the point, you are trying to do a copy where:
1) The kernel isn't expected to read the data - so can bypass the cache.
and maybe:
2) The data needs flushing from the cache to actual memory.
and maybe:
3) The cache lines need invalidating.

The fallbacks depend on the required behaviour.

	David

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