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From:   David Laight <David.Laight@ACULAB.COM>
To:     'Mikulas Patocka' <mpatocka@redhat.com>
CC:     'Ard Biesheuvel' <ard.biesheuvel@linaro.org>,
        Ramana Radhakrishnan <ramana.gcc@googlemail.com>,
        Florian Weimer <fweimer@redhat.com>,
        "Thomas Petazzoni" <thomas.petazzoni@free-electrons.com>,
        GNU C Library <libc-alpha@sourceware.org>,
        Andrew Pinski <pinskia@gmail.com>,
        "Catalin Marinas" <catalin.marinas@arm.com>,
        Will Deacon <will.deacon@arm.com>,
        "Russell King" <linux@armlinux.org.uk>,
        LKML <linux-kernel@vger.kernel.org>,
        linux-arm-kernel <linux-arm-kernel@lists.infradead.org>
Subject: RE: framebuffer corruption due to overlapping stp instructions on
 arm64
Thread-Topic: framebuffer corruption due to overlapping stp instructions on
 arm64
Thread-Index: AQHUKwzKyzS7gP0u+Em6lFS72D3AkaSt4YCg///76QCAACCeQA==
Date:   Fri, 3 Aug 2018 13:04:22 +0000
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From: Mikulas Patocka
> Sent: 03 August 2018 13:05
...
> > Even on x86 using memcpy() on PCIe memory (maybe mmap()ed into userspace)
> > isn't a good idea.
> > In the kernel memcpy_to/fromio() ought to be a better choice but that
> > is just an alternate name for memcpy().
> >
> > The problem on x86 is that memcpy() is likely to be implemented as
> > 'rep movsb' on modern cpu - relying on the cpu hardware to perform
> > cache-line sized transfers (etc).
> > Unfortunately on uncached locations it has to revert to byte copies.
> > So PCIe transfers (especially reads) are very slow.
> >
> > The transfers need to use the largest size register available.
> >
> > 	David
> 
> On x86, the framebuffer is mapped as write-combining memory type, so "rep
> movsb" could merge the byte writes to larger chunks. I don't have a cpu
> with the ERMS feature - could anyone try it if rep movsb works worse or
> better than explicit writes to the framebuffer?

I don't think 'write combining' can help reads, and memcpy_to/fromio()
are likely to be used for normal memory mapped io areas.

	David

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