linux-kernel - Re: [PATCH v3 5/7] ARM: NOMMU: Introduce dma operations for noMMU

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Date:   Wed, 19 Apr 2017 12:10:58 +0200
From:   Arnd Bergmann <arnd@...db.de>
To:     Vladimir Murzin <vladimir.murzin@....com>
Cc:     Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Alexandre Torgue <alexandre.torgue@...com>,
        Russell King - ARM Linux <linux@...linux.org.uk>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        kbuild-all@...org,
        Benjamin Gaignard <benjamin.gaignard@...aro.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Robin Murphy <robin.murphy@....com>, sza@....hu
Subject: Re: [PATCH v3 5/7] ARM: NOMMU: Introduce dma operations for noMMU

On Fri, Mar 10, 2017 at 10:23 AM, Vladimir Murzin
<vladimir.murzin@....com> wrote:
> R/M classes of cpus can have memory covered by MPU which in turn might
> configure RAM as Normal i.e. bufferable and cacheable. It breaks
> dma_alloc_coherent() and friends, since data can stuck in caches now
> or be buffered.
>
> This patch factors out DMA support for NOMMU configuration into
> separate entity which provides dedicated dma_ops. We have to handle
> there several cases:
> - configurations with MMU/MPU setup
> - configurations without MMU/MPU setup
> - special case for M-class, since caches and MPU there are optional
>
> In general we rely on default DMA area for coherent allocations or/and
> per-device memory reserves suitable for coherent DMA, so if such
> regions are set coherent allocations go from there.
>
> In case MPU/MPU was not setup we fallback to normal page allocator for
> DMA memory allocation.
>
> In case we run M-class cpus, for configuration without cache support
> (like Cortex-M3/M4) dma operations are forced to be coherent and wired
> with dma-noop (such decision is made based on cacheid global
> variable); however, if caches are detected there and no DMA coherent
> region is given (either default or per-device), dma is disallowed even
> MPU is not set - it is because M-class implement system memory map
> which defines part of address space as Normal memory.
>
> Reported-by: Alexandre Torgue <alexandre.torgue@...com>
> Reported-by: Andras Szemzo <sza@....hu>
> Tested-by: Benjamin Gaignard <benjamin.gaignard@...aro.org>
> Tested-by: Andras Szemzo <sza@....hu>
> Tested-by: Alexandre TORGUE <alexandre.torgue@...com>
> Reviewed-by: Robin Murphy <robin.murphy@....com>
> Signed-off-by: Vladimir Murzin <vladimir.murzin@....com>

I thought I found a couple of problems with this, but after reading more
carefully I was wrong about all of them and I think the code is doing
exactly the right thing. I also like the way this is split out into a separate
set of dma_map_ops to simplify the normal MMU case.

Acked-by: Arnd Bergmann <arnd@...db.de>