linux-kernel - Re: [PATCH 4/7] ALSA: pcm: use dma_can_mmap() to check if a device supports dma_mmap

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Date:   Mon, 05 Aug 2019 11:22:03 +0200
From:   Takashi Iwai <tiwai@...e.de>
To:     Christoph Hellwig <hch@....de>
Cc:     iommu@...ts.linux-foundation.org,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        Takashi Iwai <tiwai@...e.de>,
        Robin Murphy <robin.murphy@....com>,
        Michal Simek <monstr@...str.eu>,
        linux-arm-kernel@...ts.infradead.org,
        linux-m68k@...ts.linux-m68k.org, linux-parisc@...r.kernel.org,
        linux-sh@...r.kernel.org, linux-xtensa@...ux-xtensa.org,
        linuxppc-dev@...ts.ozlabs.org, x86@...nel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 4/7] ALSA: pcm: use dma_can_mmap() to check if a device supports dma_mmap_*

On Mon, 05 Aug 2019 11:11:56 +0200,
Christoph Hellwig wrote:
> 
> Replace the local hack with the dma_can_mmap helper to check if
> a given device supports mapping DMA allocations to userspace.
> 
> Signed-off-by: Christoph Hellwig <hch@....de>
> ---
>  sound/core/pcm_native.c | 5 ++---
>  1 file changed, 2 insertions(+), 3 deletions(-)
> 
> diff --git a/sound/core/pcm_native.c b/sound/core/pcm_native.c
> index 703857aab00f..81c82c3ee8a2 100644
> --- a/sound/core/pcm_native.c
> +++ b/sound/core/pcm_native.c
> @@ -220,12 +220,11 @@ static bool hw_support_mmap(struct snd_pcm_substream *substream)
>  {
>  	if (!(substream->runtime->hw.info & SNDRV_PCM_INFO_MMAP))
>  		return false;
> -	/* architecture supports dma_mmap_coherent()? */
> -#if defined(CONFIG_ARCH_NO_COHERENT_DMA_MMAP) || !defined(CONFIG_HAS_DMA)
> +	if (!dma_can_mmap(substream->dma_buffer.dev.dev))
> +		return false;
>  	if (!substream->ops->mmap &&
>  	    substream->dma_buffer.dev.type == SNDRV_DMA_TYPE_DEV)
>  		return false;
> -#endif

This won't work as expected, unfortunately.  It's a bit tricky check,
since the driver may have its own mmap implementation via
substream->ops->mmap, and the dma_buffer.dev.dev might point to
another object depending on the dma_buffer.dev.type.

So please replace with something like below:

--- a/sound/core/pcm_native.c
+++ b/sound/core/pcm_native.c
@@ -221,11 +221,10 @@ static bool hw_support_mmap(struct snd_pcm_substream *substream)
 	if (!(substream->runtime->hw.info & SNDRV_PCM_INFO_MMAP))
 		return false;
 	/* architecture supports dma_mmap_coherent()? */
-#if defined(CONFIG_ARCH_NO_COHERENT_DMA_MMAP) || !defined(CONFIG_HAS_DMA)
 	if (!substream->ops->mmap &&
-	    substream->dma_buffer.dev.type == SNDRV_DMA_TYPE_DEV)
+	    substream->dma_buffer.dev.type == SNDRV_DMA_TYPE_DEV &&
+	    !dma_can_mmap(substream->dma_buffer.dev.dev))
 		return false;
-#endif
 	return true;
 }
 
 

Thanks!

Takashi