DMA engine API issue (was: [PATCH/RFC 0/5] R-Car Gen2 DMAC hardware descriptor list support)

[Kuninori Morimoto]

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