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.
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