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Date:   Tue, 7 Aug 2018 10:14:13 -0400 (EDT)
From:   Mikulas Patocka <mpatocka@redhat.com>
To:     Ard Biesheuvel <ard.biesheuvel@linaro.org>
cc:     Florian Weimer <fweimer@redhat.com>,
        Andrew Pinski <pinskia@gmail.com>,
        Richard Earnshaw <Richard.Earnshaw@arm.com>,
        Ramana Radhakrishnan <ramana.gcc@googlemail.com>,
        Thomas Petazzoni <thomas.petazzoni@free-electrons.com>,
        GNU C Library <libc-alpha@sourceware.org>,
        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 Mon, 6 Aug 2018, Ard Biesheuvel wrote:

> No that works fine for me. VDPAU acceleration works as well, but it
> depends on your chromium build whether it can actually use it, I
> think? In any case, mplayer can use vdpau to play 1080p h264 without
> breaking a sweat on this system.
> 
> Note that the VDPAU driver also relies on memory semantics, i.e., it
> may use DC ZVA (zero cacheline) instructions which are not permitted
> on device mappings. This is probably just glibc's memset() being
> invoked, but I remember hitting this on another PCIe-impaired arm64
> system with Synopsys PCIe IP

DC ZVA can be disabled with the SCTRL_EL1.DZE bit, so that neither kernel 
nor userspace will use it. If the mapping didn't support unaligned writes, 
it would be worse.

Mikulas