Re: [PATCH V13 07/10] mmc: block: blk-mq: Add support for direct completion

[Adrian Hunter <adrian.hunter@intel.com>]

On 08/11/17 11:28, Linus Walleij wrote:
> On Fri, Nov 3, 2017 at 2:20 PM, Adrian Hunter <adrian.hunter@intel.com> wrote:
> 
>> For blk-mq, add support for completing requests directly in the ->done
>> callback. That means that error handling and urgent background operations
>> must be handled by recovery_work in that case.
>>
>> Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
> 
> I tried enabling this on my MMC host (mmci) but I got weird
> DMA error messages when I did.
> 
> I guess this has not been tested on a non-DMA-coherent
> system?

I don't see what DMA-coherence has to do with anything.

Possibilities:
	- DMA unmapping doesn't work in an atomic context
	- requests' DMA operations have to be synchronized with each other

> I think I might be seeing this because the .pre and .post
> callbacks need to be strictly sequenced, and this is
> maybe not taken into account here?

I looked at mmci but that did not seem to be the case.

> Isn't there as risk
> that the .post callback of the next request is called before
> the .post callback of the previous request has returned
> for example?

Of course, the requests are treated as independent.  If the separate DMA
operations require synchronization, that is for the host driver to fix.