of_dma_request_slave_channel() failed ?

[Kuninori Morimoto <kuninori.morimoto.gx@xxxxxxxxxxx>]

Hi Renesas ML
Cc Rob

I noticed that Sound driver is using PIO mode
on latest kernel.

	...
	rcar_sound ec500000.sound: src[0] : probe error -11
	rcar_sound ec500000.sound: ssi[0] fallback to PIO mode
	rcar_sound ec500000.sound: src[1] : probe error -11
	rcar_sound ec500000.sound: ssi[1] fallback to PIO mode
	rcar_sound ec500000.sound: ssi[2] : probe error -11
	rcar_sound ec500000.sound: ssi[2] fallback to PIO mode
	rcar_sound ec500000.sound: ssi[3] : probe error -11
	rcar_sound ec500000.sound: ssi[3] fallback to PIO mode
	rcar_sound ec500000.sound: probed
	...

Sound can use DMA mode and PIO mode.
It automatically fallbacks to PIO mode if it can't use DMA.

I bisected this issue, and found the 1st bad commit

-----------------------------------------------------
commit ac6bbf0cdf4206c517ac9789814c23e372ebce4d
Author: Rob Herring <robh@xxxxxxxxxx>
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.
	...    
-----------------------------------------------------

In more detail, it seems it can't find DMA controller,
and of_dma_request_slave_channel() returns
-EPROBE_DEFER from this commit.

struct dma_chan *of_dma_request_slave_channel(struct device_node *np,
					      const char *name)
{
	...
		ofdma = of_dma_find_controller(&dma_spec);

		if (ofdma) {
			chan = ofdma->of_dma_xlate(&dma_spec, ofdma);
		} else {
=>			ret_no_channel = -EPROBE_DEFER;
			chan = NULL;
		}
	...
=>	return ERR_PTR(ret_no_channel);
}

Is this known issue ?
Or sound shouldn't fallbacks to PIO mode ?

Best regards
---
Kuninori Morimoto