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From:   Andrew Pinski <pinskia@gmail.com>
Date:   Fri, 3 Aug 2018 00:11:16 -0700
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Subject: Re: framebuffer corruption due to overlapping stp instructions on arm64
To:     mpatocka@redhat.com
Cc:     Catalin Marinas <catalin.marinas@arm.com>,
        Will Deacon <will.deacon@arm.com>, linux@armlinux.org.uk,
        thomas.petazzoni@free-electrons.com,
        linux-arm-kernel@lists.infradead.org,
        LKML <linux-kernel@vger.kernel.org>,
        GNU C Library <libc-alpha@sourceware.org>
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On Thu, Aug 2, 2018 at 12:31 PM Mikulas Patocka <mpatocka@redhat.com> wrote:
>
> Hi
>
> I tried to use a PCIe graphics card on the MacchiatoBIN board and I hit a
> strange problem.
>
> When I use the links browser in graphics mode on the framebuffer, I get
> occasional pixel corruption. Links does memcpy, memset and 4-byte writes
> on the framebuffer - nothing else.
>
> I found out that the pixel corruption is caused by overlapping unaligned
> stp instructions inside memcpy. In order to avoid branching, the arm64
> memcpy implementation may write the same destination twice with different
> alignment. If I put "dmb sy" between the overlapping stp instructions, the
> pixel corruption goes away.
>
> This seems like a hardware bug. Is it a known errata? Do you have any
> workarounds for it?

Yes fix Links not to use memcpy on the framebuffer.
It is undefined behavior to use device memory with memcpy.

Thanks,
Andrew Pinski

>
> I tried AMD card (HD 6350) and NVidia (NVS 285) and both exhibit the same
> corruption. OpenGL doesn't work (it results in artifacts on the AMD card
> and lock-up on the NVidia card), but it's quite expected if even simple
> writing to the framebuffer doesn't work.
>
> Mikulas