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Date:   Mon, 6 Aug 2018 06:42:01 -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:

> On 6 August 2018 at 12:31, Mikulas Patocka <mpatocka@redhat.com> wrote:
> >
> >
> > On Mon, 6 Aug 2018, Ard Biesheuvel wrote:
> >
> >> On 6 August 2018 at 10:02, Mikulas Patocka <mpatocka@redhat.com> 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
> interconnect.

Mikulas