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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: Wed, 8 Aug 2018 10:21:46 -0400 (EDT)
- Subject: RE: framebuffer corruption due to overlapping stp instructions on arm64
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On Tue, 7 Aug 2018, David Laight wrote:
> From: Mikulas Patocka
> > Sent: 07 August 2018 15:07
> > Unaccelerated scrolling is still painfully slow
> > even on modern computers because of slow framebuffer read.
> I solved that many years ago on a strongarm system by mapping
> the screen memory at two separate virtual addresses.
> One uncached used for writes, the second cached using the
> 'minicache' for reads.
> (and immediately fell foul of a memcpy() function that compared
> the two virtual addresses and decided to copy backwards)
> I suspect some modern cpus don't like you doing that and the
> graphics 'drivers' won't use different mappings.
Intel says that you can't mix PAT memory attributes - but the non-temporal
store instructions use write-combining semantics on a memory that is
normally cacheable - and it is allowed to mix non-temporal stores with
other cacheable memory accesses - so I believe that the CPU will snoop the
cache for wc accesses and handle the conflict.
> Even in glibc you want a more general copy_to/from_io_memory()
> rather than just 'copy_from_framebuffer()'.
> Best to define both - even if they end up identical.
> Other drivers allow PCIe space be mmap()ed into user space.
> While your tests show vmovntdqa being slightly slower than an
> avx read for uncached mappings it is still much better than
> all the other options.
Tihs was a measuring glitch - movntdqa is as fast as movdqa on non-cached
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