linux-kernel - Re: [PATCH] dmaengine: pl330: get rid of pm_runtime_irq

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Date:	Fri, 10 Apr 2015 10:04:08 +0200
From:	Lars-Peter Clausen <lars@...afoo.de>
To:	Robert Baldyga <r.baldyga@...sung.com>, vinod.koul@...el.com
CC:	dan.j.williams@...el.com, dmaengine@...r.kernel.org,
	linux-kernel@...r.kernel.org, m.szyprowski@...sung.com,
	k.kozlowski@...sung.com
Subject: Re: [PATCH] dmaengine: pl330: get rid of pm_runtime_irq_safe()

On 04/10/2015 08:57 AM, Robert Baldyga wrote:
> As using pm_runtime_irq_safe() causes power domain is always enabled,
> we want to get rid of it to reach better power efficiency. For this purpose
> we call pm_runtime_get()/pm_runtime_put() only in DMA channel allocate/free
> code. DMA channels are always requested and freed in non-atomic context,
> so we don't need pm_runtime_irq_safe().

I wonder how useful this is considering that pretty much always a channel is 
requested. I think we need an extension to the dmaengine API that allows a 
channel consumer to notify the driver that the channel that it requested is 
currently not in use. E.g. something like dmaengine_pm_{get,put}(struct 
dma_chan *). These functions would have the restriction that they can only 
be called from a non-atomic context, whereas issue_pending() and friends can 
still be called from a atomic context. So dmaengine_pm_get() would kind of 
be a notification that consumer intends to do something  in the near future 
whereas dmaengine_pm_put() would be a notification that it is not going to 
use the channel in the near future.

E.g. for audio DMA the audio driver could call dmaengine_pm_get() when the 
PCM device is opened and dmaengine_pm_put() when it is closed. Whereas 
issue_pending is called when the audio is started.

- Lars

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