linux-kernel - Re: framebuffer corruption due to overlapping stp instructions on arm64

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Message-ID: <CAJA7tRZ69kbyA-Y0n4QBtz1JGFQdTXzNykewsySX0aj-CF5zqQ@mail.gmail.com>
Date:   Mon, 6 Aug 2018 09:04:33 +0100
From:   Ramana Radhakrishnan <ramana.gcc@...glemail.com>
To:     Pavel Machek <pavel@....cz>
Cc:     Andrew Pinski <pinskia@...il.com>,
        Mikulas Patocka <mpatocka@...hat.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>,
        Russell King <linux@...linux.org.uk>,
        Thomas Petazzoni <thomas.petazzoni@...e-electrons.com>,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
        LKML <linux-kernel@...r.kernel.org>,
        GNU C Library <libc-alpha@...rceware.org>
Subject: Re: framebuffer corruption due to overlapping stp instructions on arm64

On Sun, Aug 5, 2018 at 10:36 PM, Pavel Machek <pavel@....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