ALSA: pcm: Fix mmap breakage without explicit buffer setup

commit dc0dc8a upstream.

The recent fix c4824ae ("ALSA: pcm: Fix mmap capability check")
restricts the mmap capability only to the drivers that properly set up
the buffers, but it caused a regression for a few drivers that manage
the buffer on its own way.

For those with UNKNOWN buffer type (i.e. the uninitialized / unused
substream->dma_buffer), just assume that the driver handles the mmap
properly and blindly trust the hardware info bit.

Fixes: c4824ae ("ALSA: pcm: Fix mmap capability check")
Reported-and-tested-by: Jeff Woods <jwoods@fnordco.com>
Cc: <stable@vger.kernel.org>
Link: https://lore.kernel.org/r/s5him0gpghv.wl-tiwai@suse.de
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

sound/core/pcm_native.c

@@ -251,7 +251,10 @@ static bool hw_support_mmap(struct snd_pcm_substream *substream)
 
 	switch (substream->dma_buffer.dev.type) {
 	case SNDRV_DMA_TYPE_UNKNOWN:
-		return false;
+		/* we can't know the device, so just assume that the driver does
+		 * everything right
+		 */
+		return true;
 	case SNDRV_DMA_TYPE_CONTINUOUS:
 	case SNDRV_DMA_TYPE_VMALLOC:
 		return true;