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Date:   Fri, 3 Aug 2018 20:58:18 -0400 (EDT)
From:   Mikulas Patocka <mpatocka@redhat.com>
To:     "Richard Earnshaw (lists)" <Richard.Earnshaw@arm.com>
cc:     Ard Biesheuvel <ard.biesheuvel@linaro.org>,
        Ramana Radhakrishnan <ramana.gcc@googlemail.com>,
        Florian Weimer <fweimer@redhat.com>,
        Thomas Petazzoni <thomas.petazzoni@free-electrons.com>,
        GNU C Library <libc-alpha@sourceware.org>,
        Andrew Pinski <pinskia@gmail.com>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Will Deacon <will.deacon@arm.com>,
        Russell King <linux@armlinux.org.uk>,
        LKML <linux-kernel@vger.kernel.org>,
        linux-arm-kernel <linux-arm-kernel@lists.infradead.org>
Subject: Re: framebuffer corruption due to overlapping stp instructions on
 arm64
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On Fri, 3 Aug 2018, Richard Earnshaw (lists) wrote:

> Whoa, hold on.
> 
> Memcpy should never be used on device memory.  Period.  Memcpy doesn't
> know anything about what size of access is needed for accessing a device.
> 
> But why is the buffer in device memory rather than some other form of
> uncached memory?
> 
> If you change memcpy to deal with an aspect of the system hardware,
> you'll end up hosing performance EVERYWHERE.  DON'T DO IT!

memcpy in glibc uses ifunc selection and it already has optimized variants 
for Falkor and Thunder-X. You can add just another variant for Armada-8040 
that works around this bug and you won't be harming anyone but users of 
Armada-8040.

Furthermore, you can detect in the kernel that the PCI bus has some device 
with prefetchable BAR and activate the workaround only if there is 
videocard plugged in the PCIe slot.

> If you must, create a new API with tighter semantics, but don't change
> memcpy to accommodate this.
> 
> Anyway, back to the original report.  What memory mapping is being used?
>  In detail?

It is PCI prefetchable BAR. It is mapped using pgprot_writecombine, which 
results in MT_NORMAL_NC page attributes. (the MT_DEVICE_nGnRE can't be 
used because it results in crashes due to unaligned accesses to videoram).

> R.

Mikulas