This is the mail archive of the
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: Ard Biesheuvel <ard dot biesheuvel at linaro dot org>
- Cc: Florian Weimer <fweimer at redhat dot com>, Andrew Pinski <pinskia at gmail dot com>, Richard Earnshaw <Richard dot Earnshaw at arm dot com>, Ramana Radhakrishnan <ramana dot gcc at googlemail dot com>, Thomas Petazzoni <thomas dot petazzoni at free-electrons dot com>, GNU C Library <libc-alpha at sourceware dot org>, 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: Mon, 6 Aug 2018 06:42:01 -0400 (EDT)
- Subject: Re: framebuffer corruption due to overlapping stp instructions on arm64
- References: <alpine.LRH.email@example.com> <CA+=Sn1mWkjuwVnjw6OWWUM=UcP76bdFa680FebCseewHfx3NpA@mail.gmail.com> <firstname.lastname@example.org> <CAJA7tRZbmnZq7RfvQeYEy_a1ZObWqpFpVdvgsXgsioQ3RyPMuA@mail.gmail.com> <CAKv+Gu97QvwoLLK_zueiA_gjg_4Q5cqU4YVUyHUVFFfffdyJaw@mail.gmail.com> <email@example.com> <alpine.LRH.firstname.lastname@example.org> <CA+=Sn1=6F-fGLmXE0VqQHTqTCUKUT=Fz29mMT7349SPFisbZ9A@mail.gmail.com> <alpine.LRH.email@example.com> <firstname.lastname@example.org> <alpine.LRH.email@example.com> <CAKv+Gu_y6La77sWOSROrwfRy65jrFVYVvRZt+ewNheYhdhHCkQ@mail.gmail.com> <alpine.LRH.firstname.lastname@example.org> <CAKv+Gu87YqLM=FFZ6V8rHhuhskDdsBUuXFpqNKVbXWqdnuNEyA@mail.gmail.com>
On Mon, 6 Aug 2018, Ard Biesheuvel wrote:
> On 6 August 2018 at 12:31, Mikulas Patocka <email@example.com> wrote:
> > On Mon, 6 Aug 2018, Ard Biesheuvel wrote:
> >> On 6 August 2018 at 10:02, Mikulas Patocka <firstname.lastname@example.org> wrote:
> >> >
> >> >
> >> > On Sun, 5 Aug 2018, Florian Weimer wrote:
> >> >
> >> >> On 08/04/2018 01:04 PM, Mikulas Patocka wrote:
> >> >> > There's plenty of memcpy's in the graphics stack. No one will be rewriting
> >> >> > all the graphics drivers because of tiny market share that ARM has in
> >> >> > desktop computers. So if you refuse to fix things and blame everyone else,
> >> >> > you can as well announce that you don't want to have PCIe graphics on ARM
> >> >> > at all.
> >> >>
> >> >> The POWER toolchain maintainers said pretty much the same thing not too
> >> >> long ago. I wonder how many architectures need to fail until the
> >> >> graphics stack is finally fixed.
> >> >>
> >> >> Thanks,
> >> >> Florian
> >> >
> >> > If you say that your architecture doesn't support unaligned accesses at
> >> > all, there's no problem - the compiler won't generate them and the libc
> >> > won't contain them.
> >> >
> >> > But if you say that your architecture supports unaligned accesses except
> >> > for the framebuffer, then you have a problem - the compiler can't know
> >> > which pointers point to the framebuffer and libc can't know either - you
> >> > caused this problem by your architectural decision.
> >> >
> >> > You can use 'volatile' to suppress memory optimizations, but it's
> >> > impossible to go through the whole Linux graphics stack and add volatile
> >> > to every pointer that may point to videoram. Even if you succeesed, new
> >> > videoram accesses without volatile will appear after a year of
> >> > development.
> >> >
> >> > See for example the macros READ_ONCE and WRITE_ONCE in Linux kernel - they
> >> > should be used when there's concurrent access to the particular variable,
> >> > but mainstream architectures don't require them, so many kernel developers
> >> > are omitting them in their code.
> >> >
> >> > If you are building a supercomputer with a particular GPU, you can force
> >> > the GPU vendor to provide POWER-compliant drivers. If you are building a
> >> > workstation where the user can plug any GPU, forcing developers will go
> >> > nowhere. You have to emulate the unaligned accesses and make sure that the
> >> > next versions of your architecture support them in hardware.
> >> >
> >> I have the feeling this discussion is going off the rails again.
> >> The original report is about corruption when doing overlapping writes.
> >> Matt Sealey said you cannot have PCI outbound windows with memory
> >> semantics on ARM, and so you should be using device mappings (which do
> >> not tolerate unaligned accesses)
> >> In this context, 'device mapping' does not mean 'any non-DRAM region',
> >> but it refers to a particular type of MMU mapping attribute defined by
> >> the ARM architecture.
> >> I think we can all agree that memcpy() should be usable on any region
> >> of memory that has true memory semantics, even if it is backed by VRAM
> >> on a graphics card.
> >> The question is if PCIe can provide such regions on ARM.
> > I think there are three possible solutions:
> > 1. provide an alternative memcpy implementation that doesn't do unaligned
> > accesses and recompile the graphics software with -mstrict-align
> > 2. map the PCI BAR as device memory and emulate the unaligned instructions
> > 3. find some hardware workaround that could insert delays between the PCIe
> > accesses (but the hardware engineers need to cooperate on this instead of
> > asserting that they refuse tu support it)
> Are we talking about a quirk for the Armada 8040 or about PCIe on ARM
> in general?
I don't know - there are not any other easily available PCIe ARM boards
except for Armada 8040.
> If the latter, I still haven't seen an explanation why the particulars
> of AMBA justify overlapped writes being dropped at will by the