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References: <20200430221016.3866-1-Jason@zx2c4.com> <20200501104215.s2eftchxm66lmbvj@linutronix.de>
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From:   "Jason A. Donenfeld" <Jason@zx2c4.com>
Date:   Fri, 1 May 2020 15:54:19 -0600
Message-ID: <CAHmME9othY=_3szXKh-+4uAgcLpuvgXm4HX2-SU+Hg7KztXTFw@mail.gmail.com>
Subject: Re: [PATCH] drm/i915: check to see if SIMD registers are available
 before using SIMD
To:     Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc:     LKML <linux-kernel@vger.kernel.org>,
        intel-gfx@lists.freedesktop.org,
        dri-devel <dri-devel@lists.freedesktop.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Chris Wilson <chris@chris-wilson.co.uk>,
        stable <stable@vger.kernel.org>
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On Fri, May 1, 2020 at 4:42 AM Sebastian Andrzej Siewior
<bigeasy@linutronix.de> wrote:
>    Reviewed-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>

Thanks.

>
> 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.

Yea I was worried about that too. I'm not an i915 developer, but so
far as I can tell:

- The path from intel_engine_cmd_parser is  <= 256 KiB for "known
users", so that's rather large.
- The path from perf_memcpy is either 4k, 64k, or 4M, depending on the
type of object, so that seems gigantic, but I think that might be
selftest code.
- The path from compress_page appears to be PAGE_SIZE, so 4k, which
meshes with the limits we set agreed on few weeks ago for the crypto
stuff.
- The path from guc_read_update_log_buffer appears to be 8k, 32k, 2M,
or 8M, depending on the type of object, so that seems absurdly huge
and doesn't appear to be selftest code either like the other case.

I have no doubt the i915 developers will jump in here waving their
arms, but either way, it sure seems to me like you might have a point.