This is the mail archive of the
libc-alpha@sourceware.org
mailing list for the glibc project.
Re: framebuffer corruption due to overlapping stp instructions on arm64
- From: Mikulas Patocka <mpatocka at redhat dot com>
- To: "Richard Earnshaw (lists)" <Richard dot Earnshaw at arm dot com>
- Cc: Ard Biesheuvel <ard dot biesheuvel at linaro dot org>, Ramana Radhakrishnan <ramana dot gcc at googlemail dot com>, Florian Weimer <fweimer at redhat dot com>, Thomas Petazzoni <thomas dot petazzoni at free-electrons dot com>, GNU C Library <libc-alpha at sourceware dot org>, Andrew Pinski <pinskia at gmail dot com>, Catalin Marinas <catalin dot marinas at arm dot com>, Will Deacon <will dot deacon at arm dot com>, Russell King <linux at armlinux dot org dot uk>, LKML <linux-kernel at vger dot kernel dot org>, linux-arm-kernel <linux-arm-kernel at lists dot infradead dot org>
- Date: Fri, 3 Aug 2018 20:58:18 -0400 (EDT)
- Subject: Re: framebuffer corruption due to overlapping stp instructions on arm64
- References: <alpine.LRH.2.02.1808021242320.31834@file01.intranet.prod.int.rdu2.redhat.com> <CA+=Sn1mWkjuwVnjw6OWWUM=UcP76bdFa680FebCseewHfx3NpA@mail.gmail.com> <9acdacdb-3bd5-b71a-3003-e48132ee1371@redhat.com> <CAJA7tRZbmnZq7RfvQeYEy_a1ZObWqpFpVdvgsXgsioQ3RyPMuA@mail.gmail.com> <CAKv+Gu97QvwoLLK_zueiA_gjg_4Q5cqU4YVUyHUVFFfffdyJaw@mail.gmail.com> <11f9185a-7f71-83df-3a57-0a0ae9c1f934@arm.com>
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