linux-kernel - RE: framebuffer corruption due to overlapping stp instructions on arm64

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Date:   Wed, 8 Aug 2018 14:56:02 -0400 (EDT)
From:   Mikulas Patocka <mpatocka@...hat.com>
To:     David Laight <David.Laight@...LAB.COM>
cc:     "'Arnd Bergmann'" <arnd@...db.de>,
        Catalin Marinas <catalin.marinas@....com>,
        Thomas Petazzoni <thomas.petazzoni@...e-electrons.com>,
        Joao Pinto <Joao.Pinto@...opsys.com>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        linux-pci <linux-pci@...r.kernel.org>,
        Will Deacon <will.deacon@....com>,
        Russell King - ARM Linux <linux@...linux.org.uk>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        "neko@...uhatsu.net" <neko@...uhatsu.net>,
        Jingoo Han <jingoohan1@...il.com>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>
Subject: RE: framebuffer corruption due to overlapping stp instructions on
 arm64



On Wed, 8 Aug 2018, David Laight wrote:

> From: Arnd Bergmann
> > Sent: 08 August 2018 17:31
> ..
> > > They do modify the same byte, but with the same value. Suppose that you
> > > want to copy a piece of data that is between 8 and 16 bytes long. You can
> > > do this:
> > >
> > > add src_end, src, len
> > > add dst_end, dst, len
> > > ldr x0, [src]
> > > ldr x1, [src_end - 8]
> > > str x0, [dst]
> > > str x1, [dst_end - 8]
> 
> I've done that myself (on x86) copied the last 'word' first then
> everything else in increasing address order.
> 
> > > The ARM64 memcpy uses this trick heavily in order to reduce branching, and
> > > this is what makes the PCIe controller choke.
> 
> More likely the write combining buffer?

When I write to memory (using the NC mapping - that is also used in the 
PCI BAR), I get no corruption. So the corruption must be in the PCIe 
controller, not the core or memory subsystem.

I also tried to disable write streaming on NC mapping with a chicken bit, 
but it didn't help.

> > So when a single unaligned 'stp' gets translated into a PCIe with TLP
> > with length=5 (20 bytes) and LastBE = ~1stBE, write combining the
> > overlapping stores gives us a TLP with a longer length (5..8 for two
> > stores), and byte-enable bits that are not exactly a complement.
> 
> Write combining should generate a much longer TLP.
> Depending on the size of the write combining buffer.
> 
> But in the above case I'd have thought that the second write
> would fail to 'combine' - because it isn't contiguous with the
> stored data.
> 
> So something more complex will be going on.
> 
> 	David

Mikulas