linux-kernel - Re: [PATCH v3 1/6] ASoC: dmaengine: do not use a NULL prepare_slave

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Date:   Wed, 20 Apr 2022 12:06:59 +0200
From:   Sascha Hauer <sha@...gutronix.de>
To:     Codrin.Ciubotariu@...rochip.com
Cc:     alsa-devel@...a-project.org, linux-kernel@...r.kernel.org,
        devicetree@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        lars@...afoo.de, broonie@...nel.org, perex@...ex.cz,
        tiwai@...e.com, robh+dt@...nel.org, Nicolas.Ferre@...rochip.com
Subject: Re: [PATCH v3 1/6] ASoC: dmaengine: do not use a NULL
 prepare_slave_config() callback

On Wed, Apr 20, 2022 at 09:58:06AM +0000, Codrin.Ciubotariu@...rochip.com wrote:
> On 20.04.2022 12:15, Sascha Hauer wrote:
> > EXTERNAL EMAIL: Do not click links or open attachments unless you know the content is safe
> > 
> > Hi,
> 
> Hi Sascha,
> 
> > 
> > On Mon, Mar 07, 2022 at 02:21:57PM +0200, Codrin Ciubotariu wrote:
> >> Even if struct snd_dmaengine_pcm_config is used, prepare_slave_config()
> >> callback might not be set. Check if this callback is set before using it.
> >>
> >> Fixes: fa654e085300 ("ASoC: dmaengine-pcm: Provide default config")
> >> Signed-off-by: Codrin Ciubotariu <codrin.ciubotariu@...rochip.com>
> >> ---
> >>
> >> Changes in v2,v3:
> >>   - none
> >>
> >>   sound/soc/soc-generic-dmaengine-pcm.c | 6 +++---
> >>   1 file changed, 3 insertions(+), 3 deletions(-)
> >>
> >> diff --git a/sound/soc/soc-generic-dmaengine-pcm.c b/sound/soc/soc-generic-dmaengine-pcm.c
> >> index 285441d6aeed..2ab2ddc1294d 100644
> >> --- a/sound/soc/soc-generic-dmaengine-pcm.c
> >> +++ b/sound/soc/soc-generic-dmaengine-pcm.c
> >> @@ -86,10 +86,10 @@ static int dmaengine_pcm_hw_params(struct snd_soc_component *component,
> >>
> >>        memset(&slave_config, 0, sizeof(slave_config));
> >>
> >> -     if (!pcm->config)
> >> -             prepare_slave_config = snd_dmaengine_pcm_prepare_slave_config;
> >> -     else
> >> +     if (pcm->config && pcm->config->prepare_slave_config)
> >>                prepare_slave_config = pcm->config->prepare_slave_config;
> >> +     else
> >> +             prepare_slave_config = snd_dmaengine_pcm_prepare_slave_config;
> >>
> >>        if (prepare_slave_config) {
> >>                int ret = prepare_slave_config(substream, params, &slave_config);
> > 
> > I wonder if this patch is correct. There are drivers like
> > sound/soc/mxs/mxs-pcm.c which call snd_dmaengine_pcm_register() with a
> > config which has the prepare_slave_config callback unset. For these
> > drivers dmaengine_pcm_hw_params() previously was a no-op. Now with this
> > patch snd_dmaengine_pcm_prepare_slave_config() and
> > dmaengine_slave_config() are called. At least for the mxs-pcm driver
> > calling dmaengine_slave_config() will return -ENOSYS.
> > 
> > At least the "Check if this callback is set before using it" part is
> > wrong, the callback is checked before using it with
> > 
> >          if (prepare_slave_config) {
> >                  ...
> >          }
> > 
> > I don't have any mxs hardware at hand to test this. I just stumbled upon
> > the change of behaviour when rebasing
> > https://patchwork.kernel.org/project/alsa-devel/patch/20220301122111.1073174-1-s.hauer@pengutronix.de/
> > on current master.
> 
> You are right. I changed the behavior from:
> if (pmc->config && !pcm->config->prepare_slave_config)
> 	<do nothing>
> to:
> if (pmc->config && !pcm->config->prepare_slave_config)
> 	snd_dmaengine_pcm_prepare_slave_config()
> 
> It was not intended and I agree that the commit message is not accurate. 
> I guess some drivers might not need dmaengine_slave_config()...
> However, in my case, for the mchp-pdmc driver, I do have pcm->config 
> with pcm->config->prepare_slave_config NULL, but I still need 
> snd_dmaengine_pcm_prepare_slave_config() to be called. Should we add a 
> separate flag to call snd_dmaengine_pcm_prepare_slave_config() if 
> pcm->config->prepare_slave_config is NULL?

Other drivers set pcm->config->prepare_slave_config to
snd_dmaengine_pcm_prepare_slave_config() explicitly:

sound/soc/fsl/imx-pcm-dma.c:33: .prepare_slave_config = snd_dmaengine_pcm_prepare_slave_config,

I think that's the way to go.

Regards,
Sascha


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