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

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Message-ID: <CAHmME9pDtoPOwMGZuFAyYyWpOs8cnVO8t3FeOTR+YTeKL6PETg@mail.gmail.com>
Date:   Fri, 1 May 2020 15:55:14 -0600
From:   "Jason A. Donenfeld" <Jason@...c4.com>
To:     Christoph Hellwig <hch@...radead.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        intel-gfx@...ts.freedesktop.org,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        Sebastian Siewior <bigeasy@...utronix.de>,
        Thomas Gleixner <tglx@...utronix.de>,
        Chris Wilson <chris@...is-wilson.co.uk>,
        stable <stable@...r.kernel.org>
Subject: Re: [PATCH] drm/i915: check to see if SIMD registers are available
 before using SIMD

On Fri, May 1, 2020 at 12:07 PM Christoph Hellwig <hch@...radead.org> wrote:
>
> On Thu, Apr 30, 2020 at 04:10:16PM -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.
>
> Err, why does i915 implements its own uncached memcpy instead of relying
> on core functionality to start with?

I was wondering the same. It sure does seem like this ought to be more
generalized functionality, with a name that represents the type of
transfer it's optimized for (wc or similar).