Re: [ALSA patch] [PATCH - alsa-lib 4/4] pcm_file: add infile read support for mmap mode

[Takashi Iwai <tiwai@suse.de>]

On Tue, 21 May 2019 10:37:43 +0200,
Miartus, Adam (Arion Recruitment; ADITG/ESM) wrote:
> 
> > On Mon, 20 May 2019 18:51:06 +0200,
> > Miartus, Adam (Arion Recruitment; ADITG/ESM) wrote:
> > >
> > > From: Adam Miartus <amiartus@de.adit-jv.com>
> > >
> > > mmap_begin callback is used to copy data from input file to mmaped
> > > buffer
> > >
> > > guard for corner use of api (multiple mmap_begin calls by user) is
> > > introduced to check if next continuous buffer was already overwritten
> > >
> > > buffer is overwritten with input file data only in case of stream
> > > capture
> > >
> > > Signed-off-by: Adam Miartus <amiartus@de.adit-jv.com>
> > > Reviewed-by: Timo Wischer <twischer@de.adit-jv.com>
> > 
> > Can't we copy the data in snd_pcm_file_mmap_commit() just like the
> > playback case?
> > 
> > 
> > thanks,
> > 
> > Takashi
> 
> My understanding is that in case of reading data in mmap mode user
> would call mmap_begin, read the buffer and then call mmap_commit.
> 
> In this case overwriting the buffer in mmap_commit with data from
> Input file would be too late. 

Hm, OK.  I basically hate to add a new ops, but this might be the only
way if we have to implement that.

OTOH, is this mmap support mandatory?  IOW, is wrapping with the plug
and mmap_emul performance-wise so bad?


thanks,

Takashi