Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Date:   Fri, 3 Aug 2018 08:04:32 -0400 (EDT)
From:   Mikulas Patocka <mpatocka@redhat.com>
To:     David Laight <David.Laight@ACULAB.COM>
cc:     "'Ard Biesheuvel'" <ard.biesheuvel@linaro.org>,
        Ramana Radhakrishnan <ramana.gcc@googlemail.com>,
        Florian Weimer <fweimer@redhat.com>,
        Thomas Petazzoni <thomas.petazzoni@free-electrons.com>,
        GNU C Library <libc-alpha@sourceware.org>,
        Andrew Pinski <pinskia@gmail.com>,
        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
In-Reply-To: <f696ebe8605840e3bb04bb78b60a6cfa@AcuMS.aculab.com>
Message-ID: <alpine.LRH.2.02.1808030759480.12341@file01.intranet.prod.int.rdu2.redhat.com>
References: <alpine.LRH.2.02.1808021242320.31834@file01.intranet.prod.int.rdu2.redhat.com> <CA+=Sn1mWkjuwVnjw6OWWUM=UcP76bdFa680FebCseewHfx3NpA@mail.gmail.com> <9acdacdb-3bd5-b71a-3003-e48132ee1371@redhat.com> <CAJA7tRZbmnZq7RfvQeYEy_a1ZObWqpFpVdvgsXgsioQ3RyPMuA@mail.gmail.com>
 <CAKv+Gu97QvwoLLK_zueiA_gjg_4Q5cqU4YVUyHUVFFfffdyJaw@mail.gmail.com> <f696ebe8605840e3bb04bb78b60a6cfa@AcuMS.aculab.com>
User-Agent: Alpine 2.02 (LRH 1266 2009-07-14)
MIME-Version: 1.0
Content-Type: TEXT/PLAIN; charset=US-ASCII
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk


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.
> 
> 	David

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?

Mikulas