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authorTakashi Iwai <tiwai@suse.de>2018-05-27 13:01:17 +0200
committerTakashi Iwai <tiwai@suse.de>2018-05-29 10:01:54 +0200
commitf274baa49be67dd8a9f318cd95da6ef9f565d06b (patch)
treea8d94fa80a0630b108c93a37ab5433081112e182 /Documentation/sound
parentf91f1806530d065b90718598fbe2fad426732418 (diff)
ALSA: usb-audio: Allow non-vmalloc buffer for PCM buffers
Currently, USB-audio driver allocates the PCM buffer via vmalloc(), as this serves merely as an intermediate buffer that is copied to each URB transfer buffer. This works well in general on x86, but on some archs this may result in cache coherency issues when mmap is used. OTOH, it works also on such arch unless mmap is used. This patch is a step for mitigating the inconvenience; a new module option "use_vmalloc" is provided so that user can choose to allocate the DMA coherent buffer instead of the existing vmalloc buffer. The drawback is that it'd be the standard dma_alloc_coherent() calls and the system would require contiguous pages on non-x86 archs. Note that it's a global option and not dynamically switchable since the buffer is pre-allocated at the probe time. In theory, it's possible to be switchable, but it'd be trickier and racier. As default use_vmalloc option is set to true, so that the old behavior is kept. For allowing the coherent mmap on ARM or MIPS, pass use_vmalloc=0 option explicitly. Reported-and-tested-by: Daniel Danzberger <daniel@dd-wrt.com> Signed-off-by: Takashi Iwai <tiwai@suse.de>
Diffstat (limited to 'Documentation/sound')
-rw-r--r--Documentation/sound/alsa-configuration.rst7
1 files changed, 7 insertions, 0 deletions
diff --git a/Documentation/sound/alsa-configuration.rst b/Documentation/sound/alsa-configuration.rst
index aed6b4fb8e46..b1052e18292d 100644
--- a/Documentation/sound/alsa-configuration.rst
+++ b/Documentation/sound/alsa-configuration.rst
@@ -2224,6 +2224,13 @@ quirk_alias
Quirk alias list, pass strings like ``0123abcd:5678beef``, which
applies the existing quirk for the device 5678:beef to a new
device 0123:abcd.
+use_vmalloc
+ Use vmalloc() for allocations of the PCM buffers (default: yes).
+ For architectures with non-coherent memory like ARM or MIPS, the
+ mmap access may give inconsistent results with vmalloc'ed
+ buffers. If mmap is used on such architectures, turn off this
+ option, so that the DMA-coherent buffers are allocated and used
+ instead.
This module supports multiple devices, autoprobe and hotplugging.