On Fri, Aug 21, 2020 at 09:52:00AM +0000, Robin Gong wrote: > On 2020/08/20 14:52 Sascha Hauer <s.hauer@xxxxxxxxxxxxxx> 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 -- Pengutronix e.K. | | Steuerwalder Str. 21 | http://www.pengutronix.de/ | 31137 Hildesheim, Germany | Phone: +49-5121-206917-0 | Amtsgericht Hildesheim, HRA 2686 | Fax: +49-5121-206917-5555 |
