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From:   David Laight <David.Laight@ACULAB.COM>
To:     'Christoph Hellwig' <hch@infradead.org>,
        Chris Wilson <chris@chris-wilson.co.uk>
CC:     "Jason A. Donenfeld" <Jason@zx2c4.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "intel-gfx@lists.freedesktop.org" <intel-gfx@lists.freedesktop.org>,
        "dri-devel@lists.freedesktop.org" <dri-devel@lists.freedesktop.org>,
        "bigeasy@linutronix.de" <bigeasy@linutronix.de>,
        "tglx@linutronix.de" <tglx@linutronix.de>,
        "stable@vger.kernel.org" <stable@vger.kernel.org>
Subject: RE: [PATCH] drm/i915: check to see if SIMD registers are available
 before using SIMD
Thread-Topic: [PATCH] drm/i915: check to see if SIMD registers are available
 before using SIMD
Thread-Index: AQHWIi2IJD6S33S1cEGYb8jR4+2/zKiYGMQA
Date:   Mon, 4 May 2020 16:15:41 +0000
Message-ID: <cb9e8125a94e4efc8733967a5b47be88@AcuMS.aculab.com>
References: <20200430221016.3866-1-Jason@zx2c4.com>
 <20200501180731.GA2485@infradead.org>
 <158853721918.8377.18286963845226122104@build.alporthouse.com>
 <20200504160314.GA26373@infradead.org>
In-Reply-To: <20200504160314.GA26373@infradead.org>
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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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