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 > > 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. > > 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 > > --
