On Tue, Aug 30, 2016 at 04:09:33PM +0900, Takashi Sakamoto wrote:
> On Aug 30 2016 16:05, Clemens Ladisch wrote:
> >Takashi Sakamoto wrote:
> >>[...] APIs return operated length, while TLV feature
> >>can't. This is inconvenient to applications.
> >>
> >>This commit enables control core to return operated length of TLV feature.
> >>This changes the prototype of 'snd_kcontrol_tlv_rw_t' to get a pointer to
> >>size variable so that each implementation of the prototype can modify the
> >>variable with operated length.
> >
> >I'll use this function as an example:
> >
> >>--- a/sound/usb/mixer.c
> >>+++ b/sound/usb/mixer.c
> >>@@ -535,17 +535,20 @@ int snd_usb_set_cur_mix_value(struct usb_mixer_elem_info *cval, int channel,
> >> * TLV callback for mixer volume controls
> >> */
> >> int snd_usb_mixer_vol_tlv(struct snd_kcontrol *kcontrol, int op_flag,
> >>- unsigned int size, unsigned int __user *_tlv)
> >>+ unsigned int *size, unsigned int __user *_tlv)
> >> {
> >> struct usb_mixer_elem_info *cval = kcontrol->private_data;
> >> DECLARE_TLV_DB_MINMAX(scale, 0, 0);
> >>
> >>- if (size < sizeof(scale))
> >>+ if (*size < sizeof(scale))
> >> return -ENOMEM;
> >> scale[2] = cval->dBmin;
> >> scale[3] = cval->dBmax;
> >> if (copy_to_user(_tlv, scale, sizeof(scale)))
> >> return -EFAULT;
> >>+
> >>+ *size = sizeof(scale);
> >>+
> >> return 0;
> >> }
> >
> >The size is already returned in scale[1] (it's initialized by
> >DECLARE_TLV_DB_MINMAX()). That's exactly what the "L" in "TLV" means.
> >
> >All other TLV callbacks also take care to set this field correctly.
> >
> >If there were any TLV callback that did not set _tlv[1] to the actual
> >size, it would be buggy, and just needed to be fixed to do so.
>
> As I described, TLV feature of ALSA control interface is not only
> used to transfer threshold level information, but also arbitrary
> data for I/O by developers in ALSA SoC part. The '_tlv[1]' protocol
> is not necessarily kept by them.
can you explain what you mean by 'to transfer threshold level information'
And on this discussion, IIUC, we should fill tlv[1] with size being returned
right? For the asoc part, I think we should fix snd_soc_bytes_tlv_callback()
to update this.
Thanks
--
~Vinod
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