Re: [PATCH 1/4] ALSA: control: return payload length for TLV operation

[Charles Keepax <ckeepax@opensource.wolfsonmicro.com>]

On Mon, Sep 12, 2016 at 06:28:56PM +0200, Takashi Iwai wrote:
> On Mon, 12 Sep 2016 18:03:16 +0200,
> Charles Keepax wrote:
> > 
> > On Mon, Sep 12, 2016 at 05:28:58PM +0200, Takashi Iwai wrote:
> > > On Mon, 12 Sep 2016 17:25:31 +0200,
> > > Vinod Koul wrote:
> > > > 
> > > > On Mon, Sep 12, 2016 at 01:37:37PM +0100, Charles Keepax wrote:
> > > > > On Tue, Sep 06, 2016 at 12:30:35PM +0900, Takashi Sakamoto wrote:
> > > Also, still another point is to be decided: is passing an arbitrary
> > > size via info callback for an element without read/write access bits
> > > (but with TLV bit) a right behavior?
> > > 
> > 
> > So I guess the question would be if you couldn't read the
> > controls size from info how would you find out the control size?
> 
> Well, I'm not against the idea to expose the size in callback.  The
> behavior without read/write bits is just undefined, and we need a
> clear definition to avoid further confusion.  I guess the introduction
> of a new flag would be the start.

The trouble with that is you have to do a read/write to find out
the size you need to read or write though. So you end up doing
weird stuff like a dummy zero length read or something, which
feels a bit icky.

One other thing to consider is that if we add the new flag it
might be nice to allow normal read/write operations on the
controls as well (only accessing the first 512 bytes of the
control). Certainly for the ADSP control I would have
liked to have done this to preserve better backwards
compatibility with older user-spaces.

Thanks,
Charles