Re: framebuffer corruption due to overlapping stp instructions on arm64

From: Sinan Kaya <okaya@kernel.org>
To: Mikulas Patocka <mpatocka@redhat.com>,
	Will Deacon <will.deacon@arm.com>,
	Jingoo Han <jingoohan1@gmail.com>,
	Joao Pinto <Joao.Pinto@synopsys.com>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>,
	libc-alpha@sourceware.org,
	Ard Biesheuvel <ard.biesheuvel@linaro.org>,
	Catalin Marinas <catalin.marinas@arm.com>,
	Russell King <linux@armlinux.org.uk>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	Matt Sealey <neko@bakuhatsu.net>,
	linux-pci@vger.kernel.org,
	linux-arm-kernel <linux-arm-kernel@lists.infradead.org>
Subject: Re: framebuffer corruption due to overlapping stp instructions on arm64
Date: Fri, 3 Aug 2018 13:32:10 -0400	[thread overview]
Message-ID: <f0ab7c55-c104-241e-99d1-8247b2c567d1@kernel.org> (raw)
In-Reply-To: <alpine.LRH.2.02.1808031235410.31584@file01.intranet.prod.int.rdu2.redhat.com>

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.

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