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.
Regards
Takashi Sakamoto
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