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pcm: fix wrong document references to PCM APIs which perform direct memory access with frame copying

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In a design of ALSA PCM interface, for PCM frame transmission to/from
kernel space, applications can select from two options; direct memory access
or ioctl(2). Available options are decided depending on device capacity and
machine architecture. Applications can get available options by the first
entry of 'struct snd_pcm_hw_params.masks'.

When the mask includes 'SNDRV_PCM_ACCESS_MMAP_xxx', applications can use
direct memory access. For this use case, userspace library has two types
of PCM API. One is to expose a pointer over the memory to start
reading/writing PCM frames. Another is to copy PCM frames between the
memory and a given buffer.

Current documentation includes wrong references to these APIs to describe
their advantages/disadvantages. This confuses application developers
because the references indicate PCM APIs to execute ioctl(2) operation to
read/write PCM frames.

This commit fixes the bug.

Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
This commit is contained in:
Takashi Sakamoto 2016-12-11 20:31:38 +09:00 • committed by Takashi Iwai
parent 60b68cb47e
commit d008e1ac20
1 changed files with 3 additions and 2 deletions
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5
src/pcm/pcm.c
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@ -260,8 +260,9 @@ If you like to use the compatibility functions in mmap mode, there are
read / write routines equaling to standard read / write transfers. Using
these functions discards the benefits of direct access to memory region.
See the #snd_pcm_mmap_readi(),
#snd_pcm_writei(), #snd_pcm_readn()
and #snd_pcm_writen() functions.
#snd_pcm_mmap_writei(), #snd_pcm_mmap_readn()
and #snd_pcm_mmap_writen() functions. These functions use
#snd_pcm_areas_copy() internally.
\section pcm_errors Error codes