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

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Date:   Wed, 20 May 2020 19:22:19 +0800
From:   Shengjiu Wang <shengjiu.wang@...il.com>
To:     Lucas Stach <l.stach@...gutronix.de>
Cc:     Shengjiu Wang <shengjiu.wang@....com>,
        Timur Tabi <timur@...nel.org>,
        Nicolin Chen <nicoleotsuka@...il.com>,
        Xiubo Li <Xiubo.Lee@...il.com>,
        Fabio Estevam <festevam@...il.com>,
        Liam Girdwood <lgirdwood@...il.com>,
        Mark Brown <broonie@...nel.org>, perex@...ex.cz,
        Takashi Iwai <tiwai@...e.com>, shawnguo@...nel.org,
        s.hauer@...gutronix.de, kernel@...gutronix.de, linux-imx@....com,
        sumit.semwal@...aro.org, Linux-ALSA <alsa-devel@...a-project.org>,
        linuxppc-dev@...ts.ozlabs.org,
        linux-arm-kernel@...ts.infradead.org,
        linux-kernel <linux-kernel@...r.kernel.org>,
        linux-media@...r.kernel.org, dri-devel@...ts.freedesktop.org,
        linaro-mm-sig@...ts.linaro.org
Subject: Re: [PATCH] ASoC: fsl: imx-pcm-dma: Don't request dma channel in probe

Hi

On Wed, May 20, 2020 at 5:42 PM Lucas Stach <l.stach@...gutronix.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@...gutronix.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