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RE: framebuffer corruption due to overlapping stp instructions on arm64
- From: Mikulas Patocka <mpatocka at redhat dot com>
- To: David Laight <David dot Laight at ACULAB 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 08:04:32 -0400 (EDT)
- Subject: RE: framebuffer corruption due to overlapping stp instructions on arm64
- References: <alpine.LRH.firstname.lastname@example.org> <CA+=Sn1mWkjuwVnjw6OWWUM=UcP76bdFa680FebCseewHfx3NpA@mail.gmail.com> <email@example.com> <CAJA7tRZbmnZq7RfvQeYEy_a1ZObWqpFpVdvgsXgsioQ3RyPMuA@mail.gmail.com> <CAKv+Gu97QvwoLLK_zueiA_gjg_4Q5cqU4YVUyHUVFFfffdyJaw@mail.gmail.com> <f696ebe8605840e3bb04bb78b60a6cfa@AcuMS.aculab.com>
On Fri, 3 Aug 2018, David Laight wrote:
> From: Ard Biesheuvel
> > Sent: 03 August 2018 10:30
> > The discussion about whether memcpy() should rely on unaligned
> > accesses, and whether you should use it on device memory is orthogonal
> > to that, and not the heart of the matter IMO
> Even on x86 using memcpy() on PCIe memory (maybe mmap()ed into userspace)
> isn't a good idea.
> In the kernel memcpy_to/fromio() ought to be a better choice but that
> is just an alternate name for memcpy().
> The problem on x86 is that memcpy() is likely to be implemented as
> 'rep movsb' on modern cpu - relying on the cpu hardware to perform
> cache-line sized transfers (etc).
> Unfortunately on uncached locations it has to revert to byte copies.
> So PCIe transfers (especially reads) are very slow.
> The transfers need to use the largest size register available.
On x86, the framebuffer is mapped as write-combining memory type, so "rep
movsb" could merge the byte writes to larger chunks. I don't have a cpu
with the ERMS feature - could anyone try it if rep movsb works worse or
better than explicit writes to the framebuffer?