Re: [PATCH] powerpc/dma: Fix invalid DMA mmap behavior

[Michael Ellerman <mpe@ellerman.id.au>]

Arnd Bergmann <arnd@arndb.de> writes:
> On Thu, Jul 18, 2019 at 11:52 AM Christoph Hellwig <hch@lst.de> wrote:
>> On Thu, Jul 18, 2019 at 10:49:34AM +0200, Christoph Hellwig wrote:
>> > On Thu, Jul 18, 2019 at 01:45:16PM +1000, Oliver O'Halloran wrote:
>> > > > Other than m68k, mips, and arm64, everybody else that doesn't have
>> > > > ARCH_NO_COHERENT_DMA_MMAP set uses this default implementation, so
>> > > > I assume this behavior is acceptable on those architectures.
>> > >
>> > > It might be acceptable, but there's no reason to use pgport_noncached
>> > > if the platform supports cache-coherent DMA.
>> > >
>> > > Christoph (+cc) made the change so maybe he saw something we're missing.
>> >
>> > I always found the forcing of noncached access even for coherent
>> > devices a little odd, but this was inherited from the previous
>> > implementation, which surprised me a bit as the different attributes
>> > are usually problematic even on x86.  Let me dig into the history a
>> > bit more, but I suspect the righ fix is to default to cached mappings
>> > for coherent devices.
>>
>> Ok, some history:
>>
>> The generic dma mmap implementation, which we are effectively still
>> using today was added by:
>>
>> commit 64ccc9c033c6089b2d426dad3c56477ab066c999
>> Author: Marek Szyprowski <m.szyprowski@samsung.com>
>> Date:   Thu Jun 14 13:03:04 2012 +0200
>>
>>     common: dma-mapping: add support for generic dma_mmap_* calls
>>
>> and unconditionally uses pgprot_noncached in dma_common_mmap, which is
>> then used as the fallback by dma_mmap_attrs if no ->mmap method is
>> present.  At that point we already had the powerpc implementation
>> that only uses pgprot_noncached for non-coherent mappings, and
>> the arm one, which uses pgprot_writecombine if DMA_ATTR_WRITE_COMBINE
>> is set and otherwise pgprot_dmacoherent, which seems to be uncached.
>> Arm did support coherent platforms at that time, but they might have
>> been an afterthought and not handled properly.
>
> Cache-coherent devices are still very rare on 32-bit ARM.
>
> Among the callers of dma_mmap_coherent(), almost all are in platform
> specific device drivers that only ever run on noncoherent ARM SoCs,
> which explains why nobody would have noticed problems.
>
> There is also a difference in behavior between ARM and PowerPC
> when dealing with mismatched cacheability attributes: If the same
> page is mapped as both cached and uncached to, this may
> cause silent undefined behavior on ARM, while PowerPC should
> enter a checkstop as soon as it notices.

On newer Power CPUs it's actually more like the ARM behaviour.

I don't know for sure that it will *never* checkstop but there are at
least cases where it won't. There's some (not much) detail in the
Power8/9 user manuals.

cheers