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Subject: Re: framebuffer corruption due to overlapping stp instructions on
 arm64
To:     Mikulas Patocka <mpatocka@redhat.com>,
        Will Deacon <will.deacon@arm.com>,
        Jingoo Han <jingoohan1@gmail.com>,
        Joao Pinto <Joao.Pinto@synopsys.com>
Cc:     Ard Biesheuvel <ard.biesheuvel@linaro.org>,
        Matt Sealey <neko@bakuhatsu.net>,
        Thomas Petazzoni <thomas.petazzoni@free-electrons.com>,
        libc-alpha@sourceware.org,
        Catalin Marinas <catalin.marinas@arm.com>,
        Russell King <linux@armlinux.org.uk>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        linux-arm-kernel <linux-arm-kernel@lists.infradead.org>,
        linux-pci@vger.kernel.org
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From:   Sinan Kaya <okaya@kernel.org>
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Date:   Fri, 3 Aug 2018 13:32:10 -0400
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On 8/3/2018 1:09 PM, Mikulas Patocka wrote:
>>> Most accelerated graphics drivers rely heavily on the ability to map
>>> the VRAM normal-non-cacheable (ioremap_wc, basically), and treat it as
>>> ordinary memory.
>> Yeah, I'd expect framebuffers to be mapped as normal NC. That should be
>> fine for prefetchable BARs, no?
>>
>> Will
> So - why does it corrupt data then? I've created this program that
> reproduces the data corruption quicky. If I run it on /dev/fb0, I get an
> instant failure. Sometimes a few bytes are not written, sometimes a few
> bytes are written with a value that should be 16 bytes apart.
> 
> I tried to run it on system RAM mapped with the NC attribute and I didn't
> get any corruption - that suggests the the bug may be in the PCIE
> subsystem.

Note that normal-NC gives you write combining whereas device
nGnRE doesn't have any write-combining support.
normal-NC is typically mapped to prefetchable BAR space where
write-combining is welcome.

It could be an issue on the SOC itself too. I suggest you contact your
board vendor.