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From:   youling 257 <youling257@gmail.com>
Date:   Mon, 4 Nov 2019 16:27:20 +0800
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Subject: Re: ALSA: pcm: use dma_can_mmap() to check if a device supports dma_mmap_*
To:     Takashi Iwai <tiwai@suse.de>
Cc:     linux-xtensa@linux-xtensa.org, Michal Simek <monstr@monstr.eu>,
        Vladimir Murzin <vladimir.murzin@arm.com>,
        linux-parisc@vger.kernel.org, linux-sh@vger.kernel.org,
        linuxppc-dev@lists.ozlabs.org, Helge Deller <deller@gmx.de>,
        x86@kernel.org,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
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        Robin Murphy <robin.murphy@arm.com>,
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This driver https://android.googlesource.com/kernel/common/+/refs/heads/android-mainline/drivers/usb/gadget/function/f_audio_source.c

2019-11-04 16:02 GMT+08:00, Takashi Iwai <tiwai@suse.de>:

> Exactly which driver is hit?  The code path is via hw_support_mmap()
> and it's currently:
>
> static bool hw_support_mmap(struct snd_pcm_substream *substream)
> {
> 	if (!(substream->runtime->hw.info & SNDRV_PCM_INFO_MMAP))
> 		return false;
>
> 	if (substream->ops->mmap ||
> 	    substream->dma_buffer.dev.type != SNDRV_DMA_TYPE_DEV)
> 		return true;
>
> 	return dma_can_mmap(substream->dma_buffer.dev.dev);
> }
>
> so at least the driver has already set the SNDRV_DMA_TYPE_DEV
> explicitly (it's non-zero) and some device object, but the device
> object was invalid for dma_can_mmap() call.
>
> This smells more like a driver-side issue, not in the core side.
>
>
> thanks,
>
> Takashi
>