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From:   Ramana Radhakrishnan <ramana.gcc@googlemail.com>
Date:   Mon, 6 Aug 2018 09:04:33 +0100
Message-ID: <CAJA7tRZ69kbyA-Y0n4QBtz1JGFQdTXzNykewsySX0aj-CF5zqQ@mail.gmail.com>
Subject: Re: framebuffer corruption due to overlapping stp instructions on arm64
To:     Pavel Machek <pavel@ucw.cz>
Cc:     Andrew Pinski <pinskia@gmail.com>,
        Mikulas Patocka <mpatocka@redhat.com>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Will Deacon <will.deacon@arm.com>,
        Russell King <linux@armlinux.org.uk>,
        Thomas Petazzoni <thomas.petazzoni@free-electrons.com>,
        linux-arm-kernel <linux-arm-kernel@lists.infradead.org>,
        LKML <linux-kernel@vger.kernel.org>,
        GNU C Library <libc-alpha@sourceware.org>
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On Sun, Aug 5, 2018 at 10:36 PM, Pavel Machek <pavel@ucw.cz> 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.
>
> No, I don't think so. Why do you think so?

It is undefined behaviour in the architecture.


Ramana