[PATCH] ARM: mm: dma: Update coherent streaming apis with missing memory barrier

[Joel Fernandes]

Hey Will,

On Wed, Apr 23, 2014 at 12:17 PM, Will Deacon <will.deacon at arm.com> wrote:
> On Wed, Apr 23, 2014 at 05:02:16PM +0100, Catalin Marinas wrote:
>> On Wed, Apr 23, 2014 at 10:02:51AM +0100, Will Deacon wrote:
>> > On Tue, Apr 22, 2014 at 09:30:27PM +0100, Santosh Shilimkar wrote:
>> > > writel() or an explcit barrier in the driver will do the job. I was
>> > > just thinking that we are trying to work around the short comings
>> > > of streaming API by adding barriers in the driver. For example
>> > > on a non-coherent system, i don't need that barrier because
>> > > dma_ops does take care of that.
>> >
>> > I wonder whether we can remove those barriers altogether then (from the DMA
>> > cache operations). For the coherent case, the driver must provide the
>> > barrier (probably via writel) so the non-coherent case shouldn't be any
>> > different.
>>
>> For the DMA_TO_DEVICE case the effect should be the same as wmb()
>> implies dsb (and outer_sync() for write). But the reason we have
>> barriers in the DMA ops is slightly different - the completion of the
>> cache maintenance operation rather than ordering with any previous
>> writes to the DMA buffer.
>>
>> In the DMA_FROM_DEVICE scenario for example, the CPU gets an interrupt
>> for a finished DMA transfer and executes dma_unmap_single() prior to
>> accessing the page. However the CPU access after unmapping is done using
>> normal LDR/STR which do not imply any barrier. So we need to ensure the
>> completion of the cache invalidation in the dma operation.
>
> I don't think we necessarily need completion, we just need ordering. That
> is, the normal LDR/STR instructions must be observed after the cache
> maintenance. I'll have to revisit the ARM ARM to be sure of this, but a dmb
> should be sufficient for that guarantee.

Just wondering if you were convinced from the ARM ARM that a dsb is
not required after cache maintenance for the DMA_FROM_DEVICE case?

thanks,
-Joel