[alsa-devel] of_dma_request_slave_channel() failed ?

[Kuninori Morimoto]

Hi Geert

Thank you for your feedback

> > -----------------------------------------------------
> > commit ac6bbf0cdf4206c517ac9789814c23e372ebce4d
> > Author: Rob Herring <robh at kernel.org>
> > Date:   Mon Jul 9 09:41:52 2018 -0600
> >
> >     iommu: Remove IOMMU_OF_DECLARE
> >
> >     Now that we use the driver core to stop deferred probe for missing
> >     drivers, IOMMU_OF_DECLARE can be removed.
> >
> >     This is slightly less optimal than having a list of built-in drivers in
> >     that we'll now defer probe twice before giving up. This shouldn't have a
> >     significant impact on boot times as past discussions about deferred
> >     probe have given no evidence of deferred probe having a substantial
> >     impact.
> >         ...
> > -----------------------------------------------------
(snip)
> I assume you wrote commit 6c92d5a2744e2761 ("ASoC: rsnd: don't fallback
> to PIO mode when -EPROBE_DEFER") to work around this?

Yes, it is work around for it.

> While this got rid of the error messages, and postpones sound initialization
> until the audio DMAC is available, it does mean that the driver will _never_
> fall back to PIO.
> 
> I.e. if CONFIG_RCAR_DMAC=n, audio will fail to probe instead of falling
> back to PIO.

If I focus only for sound here, the purpose of PIO mode is
checking basic HW connection, clock settings, etc.
Without DMAC support, it can't use many sound features.
And PIO mode is supporting "SSI" only.

If DT has SRC/CTU/DVC settings for sound,
it needs DMA support anyway.

&rcar_sound {
	...
	ports {
		rsnd_port0: port at 0 {
			rsnd_endpoint0: endpoint {
				...
=>				playback = <&ssi0 &src0 &dvc0>;
			};
		};
	};
};

Before 6c92d5a2744e2761 patch, driver will forcibly ignore
non-SSI modules, and try to use PIO mode.
I'm not sure it is "kindly support" or "overkill support".

After this patch, it needs DMA, otherwise, probe will be failed.
DT shouldn't have non-SSI modules if you want to use PIO mode.

	+ /* use PIO mode */ 
	- playback = <&ssi0 &src0 &dvc0>;
	+ playback = <&ssi0>;