[alsa-devel] [PATCH - alsa-lib 1/1] Change card/pid get functions to return -ENOSYS if the kernel is too old

[Takashi Iwai]

On Mon, 06 Jun 2016 15:57:33 +0200,
Adam Goode wrote:
> 
> On Fri, Apr 8, 2016 at 3:53 PM, Adam Goode <agoode at google.com> wrote:
> 
> > On Fri, Apr 8, 2016 at 1:56 PM, Martin Koegler <martin.koegler at chello.at>
> > wrote:
> >
> >> On Fri, Apr 08, 2016 at 11:21:57AM -0400, Adam Goode wrote:
> >> > So there are 2 alternatives I can think of:
> >> >
> >> > option 1: introduce snd_seq_client_info_get_pid_2() and leave the
> >> existing
> >> > function alone. This new function can return a new constant,
> >> > SND_SEQ_CLIENT_INFO_PID_UNKNOWN for missing kernel support. Same for
> >> card.
> >> > option 2: introduce snd_seq_client_info_pid_known() that returns a bool
> >> if
> >> > there is kernel support. Same for card.
> >>
> >> I wouldn't implement option 1. Using that methode for API changes will
> >> result in lots of rarely used functions.
> >>
> >> Chrome will probable need that information to enable a fallback-code - if
> >> a application has no fallback code,
> >> it has no need for different returncodes. In a few years, most
> >> distributions will likely ship kernel including that functions,
> >> so there is no need to for fallback-code and the kernel support check any
> >> more.
> >>
> >> Why not a snd_seq_client_info_get_capatibilites() returing bit flags:
> >> .._CAP_CARD_NUMBER | .._CAP_PID ?
> >>
> >>
> > This approach is fine by me as well, though it does introduce more
> > constants and bitwise logic.
> >
> >
> Since returning to this problem after a while, I've realized that we don't
> need any changes at all to alsa-lib. This test is equivalent to what is
> needed:
> 
> snd_seq_get_client_info(seq, info); // get our client's info
> bool client_pid_supported = snd_seq_client_info_get_pid(info) != -1; // do
> we know our own PID?

Ah, yes, it works.  I thought of another, e.g. checking /dev/sequencer
version ioctl, but yours works more nicely.

> It might be helpful to put this into the documentation, but otherwise I
> think this is fine.

A patch is always welcome :)


thanks,

Takashi