Hi Laurent
The rsnd_soc_dai_trigger() function takes a spinlock, making the context atomic, which regmap doesn't like as it locks a mutex.
It might be possible to fix this by setting the fast_io field in both the regmap_config and regmap_bus structures in sound/soc/sh/rcar/gen.c. regmap will then use a spinlock instead of a mutex. However, even if I believe that change makes sense and should be done, another atomic context issue will come from the rcar-dmac driver which allocates memory in the prep_dma_cyclic function, called by rsnd_dma_start() from rsnd_soc_dai_trigger() with the spinlock help.
What context is the rsnd_soc_dai_trigger() function called in by the alsa core ? If it's guaranteed to be a sleepable context, would it make sense to replace the rsnd_priv spinlock with a mutex ?
Answering myself here, that's not an option, as the trigger function is called in atomic context (replacing the spinlock with a mutex produced a clear BUG) due to snd_pcm_lib_write1() taking a spinlock.
Now we have a core issue. On one side there's rsnd_soc_dai_trigger() being called in atomic context, and on the other side the function ends up calling dmaengine_prep_dma_cyclic() which needs to allocate memory. To make this more func the DMA engine API is undocumented and completely silent on whether the prep functions are allowed to sleep. The question is, who's wrong ?
Now, if you're tempted to say that I'm wrong by allocating memory with GFP_KERNEL in the DMA engine driver, please read on first :-) I've tried replacing GFP_KERNEL with GFP_ATOMIC allocations, and then ran into a problem more complex to solve.
The rcar-dmac DMA engine driver uses runtime PM. When not used, the device is suspended. The driver calls pm_runtime_get_sync() to resume the device, and needs to do so when a descriptor is submitted. This operation, currently performed in the tx_submit handler, could be moved to the prep_dma_cyclic or issue_pending handler, but the three operations are called in a row from rsnd_dma_start(), itself ultimately called from snd_pcm_lib_write1() with the spinlock held. This means I have no place in my DMA engine driver where I can resume the device.
One could argue that the rcar-dmac driver could use a work queue to handle power management. That's correct, but the additional complexity, which would be required in *all* DMA engine drivers, seem too high to me. If we need to go that way, this is really a task that the DMA engine core should handle.
Let's start by answering the background question and updating the DMA engine API documentation once and for good : in which context are drivers allowed to call the prep, tx_submit and issue_pending functions ?
rsnd driver (and sound/soc/sh/fsi driver too) is using prep_dma_cyclic() now, but, it had been used prep_slave_single() before. Then, it used work queue in dai_trigger function. How about to use same method in prep_dma_cyclic() ? Do you think your issue will be solved if sound driver calls prep_dma_cyclic() from work queue ?
Best regards --- Kuninori Morimoto