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

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Date:   Sun, 5 Aug 2018 23:36:15 +0200
From:   Pavel Machek <pavel@....cz>
To:     Andrew Pinski <pinskia@...il.com>
Cc:     mpatocka@...hat.com, Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>, linux@...linux.org.uk,
        thomas.petazzoni@...e-electrons.com,
        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

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?

I'm pretty sure that gcc is allowed to do memcpy-like tricks even when
memcpy is not mentioned explicitely.

									Pavel
-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html

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