Re: [PATCH] ASoC: fsl: imx-pcm-dma: Don't request dma channel in probe

[Shengjiu Wang <shengjiu.wang@gmail.com>]

Hi

On Wed, May 20, 2020 at 5:42 PM Lucas Stach <l.stach@pengutronix.de> wrote:
>
> Am Mittwoch, den 20.05.2020, 16:20 +0800 schrieb Shengjiu Wang:
> > Hi
> >
> > On Tue, May 19, 2020 at 6:04 PM Lucas Stach <l.stach@pengutronix.de> wrote:
> > > Am Dienstag, den 19.05.2020, 17:41 +0800 schrieb Shengjiu Wang:
> > > > There are two requirements that we need to move the request
> > > > of dma channel from probe to open.
> > >
> > > How do you handle -EPROBE_DEFER return code from the channel request if
> > > you don't do it in probe?
> >
> > I use the dma_request_slave_channel or dma_request_channel instead
> > of dmaengine_pcm_request_chan_of. so there should be not -EPROBE_DEFER
> > return code.
>
> This is a pretty weak argument. The dmaengine device might probe after
> you try to get the channel. Using a function to request the channel
> that doesn't allow you to handle probe deferral is IMHO a bug and
> should be fixed, instead of building even more assumptions on top of
> it.
>

I see some driver also request dma channel in open() or hw_params().
how can they avoid the defer probe issue?
for example：
sound/arm/pxa2xx-pcm-lib.c
sound/soc/sprd/sprd-pcm-dma.c

> > > > - When dma device binds with power-domains, the power will
> > > > be enabled when we request dma channel. If the request of dma
> > > > channel happen on probe, then the power-domains will be always
> > > > enabled after kernel boot up,  which is not good for power
> > > > saving,  so we need to move the request of dma channel to .open();
> > >
> > > This is certainly something which could be fixed in the dmaengine
> > > driver.
> >
> > Dma driver always call the pm_runtime_get_sync in
> > device_alloc_chan_resources, the device_alloc_chan_resources is
> > called when channel is requested. so power is enabled on channel
> > request.
>
> So why can't you fix the dmaengine driver to do that RPM call at a
> later time when the channel is actually going to be used? This will
> allow further power savings with other slave devices than the audio
> PCM.
>
> Regards,
> Lucas
>

It seems the best place for calling pm_runtime_get_sync is the
device_alloc_chan_resources, and calling pm_runtime_put_sync
in the .device_free_chan_resources

For the slave_sg mode, the .device_prep_slave_sg and
.device_issue_pending  will be called many times after
.device_alloc_chan_resources. so it is not good to call
pm_runtime_get_sync in .device_prep_slave_sg or
.device_issue_pending

best regards
wang shengjiu