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

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

On Fri, Aug 21, 2020 at 09:52:00AM +0000, Robin Gong wrote:
> On 2020/08/20 14:52 Sascha Hauer <s.hauer@pengutronix.de> wrote:
> > 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.
> Seems Lars major concern is on the race condition between next descriptor
> and sdma_channel_terminate_work which free the last terminated descriptor,
> not the ability of vchan to support multi descriptors. But anyway, I think we
> should take care vchan_get_all_descriptors to free descriptors during terminate
> phase in case it's done in worker like sdma_channel_terminate_work, since that
> may free the next descriptor wrongly. That's what my patch attached in
> 0001-dmaengine-imx-sdma-add-terminated-list-for-freed-des.patch
> https://www.spinics.net/lists/arm-kernel/msg829972.html

Indeed this should solve the problem of freeing descriptors allocated
between terminate_all and a following prep_slave*.

> 
> > 
> > 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.
> Sorry Sascha, seems that's another topic and your intention is to make sure only
> software involved in sdma_prep_* and all HW moved into one function inside
> sdma_start_desc. I agree that will make code more clean but my concern is
> sdma_start_desc is protect by spin_lock which should be short as possible while
> some HW touch as context_load may cost some time. Anyway, that's another topic,
> maybe we can refine it in the future.

Yes, you are right. This is another topic.

Sascha

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