[alsa-devel] [PATCH 1/4] ALSA: control: return payload length for TLV operation

[Charles Keepax]

On Wed, Aug 31, 2016 at 01:30:02PM +0900, Takashi Sakamoto wrote:
> On Aug 31 2016 13:20, Vinod Koul wrote:
> >> The layout of TLV packet is:
> >> struct snd_ctl_tlv {
> >>     unsigned int numid;   # numerical ID of a control element
> >>     unsigned int length;  # length of payload
> >>     unsigned int tlv[0];  # payload
> >> };
> >> http://git.kernel.org/cgit/linux/kernel/git/tiwai/sound.git/tree/include/uapi/sound/asound.h?h=sound-4.8-rc4#n945
> >>
> >> In our implementaion, TLV packet payload (struct snd_ctl_tlv.tlv) is
> >> used to transfer data. For pure threshold level information, we expects
> >> applications and drivers to fill the payload with this protocol:
> >> struct snd_ctl_tlv.tlv[0]:  one of SNDRV_CTL_TLVT_XXX
> >> struct snd_ctl_tlv.tlv[1]:  length of data
> >> struct snd_ctl_tlv.tlv[2..]: data
> >>
> >> (You can see SNDRV_CTL_TLVT_XXX in this header.
> >> http://git.kernel.org/cgit/linux/kernel/git/tiwai/sound.git/tree/include/uapi/sound/tlv.h?h=sound-4.8-rc4
> >> )
> >>
> >> On the other hand, ALSA SoC part performs:
> >> struct snd_ctl_tlv.tlv[0..]: arbitrary data
> >>
> >> If your 'tlv[1]' means the 'struct snd_ctl_tlv.tlv[1]', no sense.
> >>
> >> The issue I address is current implementation cannot correctly handle
> >> this case:
> >>  - applications request a buffer with a certain size
> >>  - drivers processes the request with smaller size
> >>  - application cannot get the size

Is this an expected use-case? The TLV controls were implemented
to allow ALSA controls of greater than 512 bytes, I am not sure
the intention was to provide completely generalised binary pipe. In
general the expection for reading a control is that you can
always read the whole control (AFAIK), so it feels like something
returning less than the requested amount of data is buggy.

Thanks,
Charles