[alsa-devel] [PATCH] ALSA: sb: remove needless evaluation in implementation for copy callback

[Takashi Iwai]

On Wed, 24 May 2017 17:01:07 +0200,
Takashi Sakamoto wrote:
> 
> Hi,
> 
> On May 23 2017 22:49, Takashi Iwai wrote:
> > On Tue, 23 May 2017 15:40:42 +0200,
> > Takashi Sakamoto wrote:
> >>
> >> In design of ALSA pcm core, 'struct snd_pcm_ops.copy' is expected to
> >> copy PCM frames, according to frame alignment on intermediate buffer for
> >> userspace and dedicated buffer for data transmission. In this callback,
> >> value of 'channel' argument depends on the frame alignment, which drivers
> >> registers to runtime of PCM substream. When target devices can handle
> >> non-interleaved buffer, this value has positive value, otherwise negative.
> >>
> >> ALSA driver for PCM component of EMU8000 chip is programmed with local
> >> macro to switch the frame alignment. The 'copy' operation in
> >> non-interleaved side has evaluation of the 'channel' argument (actually
> >> it's 'voice' argument). This is useless.
> >>
> >> This commit remove the evaluation.
> >>
> >> Signed-off-by: Takashi Sakamoto <o-takashi at sakamocchi.jp>
> >
> > Passing -1 to the channel was valid even for non-interleaved access.
> > It was meant to apply to all channels.
> >
> > The call with channel = -1 itself might have been dropped meanwhile,
> > so removing it may be OK now.  But the description is confusing as if
> > it were incorrectly implemented.
> 
> https://git.kernel.org/pub/scm/linux/kernel/git/tiwai/sound.git/tree/Documentation/sound/kernel-api/writing-an-alsa-driver.rst#n3598
> 
> > You need to check the channel argument, and if it's -1, copy the
> > whole channels. Otherwise, you have to copy only the specified
> > channel.
> 
> Hm. We need to correct this API documentation, because there's no
> codes to pass negative value to the argument for vector buffer
> operation. I'll post my proposal later.

Just leave them as is.  We're working on these codes in anyway, and
touching them just confuse the development.

If it were a fix, I'd take it quickly.  But it's nothing but a very
minor optimization, so it's really not worth to spend your time.


thanks,

Takashi