Re: pcm|dmaengine|imx-sdma race condition on i.MX6

[Sascha Hauer <s.hauer@pengutronix.de>]

On Wed, Aug 19, 2020 at 01:08:29PM +0200, Lars-Peter Clausen wrote:
> > For the first option, which is potentially more performant, we have to leave the atomic PCM context
> > and we are not sure if we are allowed to.
> > For the second option, we would have to divide the dma_device terminate_all into an atomic sync and
> > an async one, which would align with the dmaengine API, giving it the option to ensure termination
> > in an atomic context.
> > Based on my understanding, most of them are synchronous anyways, for the currently async ones we
> > would have to implement busy waits.
> > However, with this approach, we reach the WARN_ON [6] inside of an atomic context,
> > indicating we might not do the right thing.
> 
> I don't know how feasible this is to implement in the SDMA dmaengine driver.
> But I think what is should do is to have some flag to indicate if a
> terminate is in progress. If a new transfer is issued while terminate is in
> progress the transfer should go on a list. Once terminate finishes it should
> check the list and start the transfer if there are any on the list.

The list is already there in form of the vchan helpers the driver uses.

I think the big mistake the driver makes is to configure fields in
struct sdma_channel and also the hardware directly in
sdma_prep_memcpy(), sdma_prep_slave_sg() and sdma_prep_dma_cyclic(). All
information should be stored in the struct sdma_desc allocated in the
prep functions and only be used when it's time to fire that specific
descriptor.

More specifically sdma_config_write() may not be called from
sdma_prep_slave_sg() or sdma_prep_dma_cyclic(), but instead must be
called from sdma_start_desc().  sdma_config_ownership() also must be
called later in sdma_start_desc(). 'direction' must be a member of
struct sdma_desc, not of struct sdma_channel.

Overall this sounds like a fair amount of work to do, but should be
feasible and IMO is a step in the right direction.

Sascha

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