Dne 30. 04. 21 v 19:11 Jerome Brunet napsal(a):

On Fri 30 Apr 2021 at 19:09, Jerome Brunet [off-list ref] wrote:

quoted

On Fri 30 Apr 2021 at 16:55, Pavel Hofman [off-list ref] wrote:

quoted

Dne 30. 04. 21 v 16:26 Jerome Brunet napsal(a):

quoted

From: Ruslan Bilovol <redacted>

As per USB and UAC2 specs, asynchronous audio sink endpoint
requires explicit synchronization mechanism (Isochronous
Feedback Endpoint)

Implement feedback companion endpoint for ISO OUT endpoint

This patch adds all required infrastructure and USB requests
handling for feedback endpoint. Syncrhonization itself is
still dummy (feedback ep always reports 'nomimal frequency'
 e.g. no adjustement is needed). This satisfies hosts that
require feedback endpoint (like Win10) and poll it periodically

Actual synchronization mechanism should be implemented
separately

Signed-off-by: Ruslan Bilovol <redacted>
Signed-off-by: Jerome Brunet <jbrunet@baylibre.com>

Hi,

The HS calculation of Q16.16 feedback value
overflows at some 524kHz, disallowing use of larger samplerates (e.g.
768kHz or higher).

I tested the formula used in alsa USB driver
https://github.com/torvalds/linux/blob/d99676af540c2dc829999928fb81c58c80a1dce4/sound/usb/endpoint.c#L80
which uses only 10bit shift. The feedback control in UAC2 gadget now
works up to 4M samplerate with 1Hz precision (tested on RPi4 with
bInterval = 1, checked in stream0 proc file on linux host).

--- a/drivers/usb/gadget/function/u_audio.c
+++ b/drivers/usb/gadget/function/u_audio.c

@@ -118,7 +119,8 @@ static void u_audio_set_fback_frequency(enum

usb_device_speed speed,
                 * Prevent integer overflow by calculating in Q12.13
format and
                 * then shifting to Q16.16
                 */
-               ff = DIV_ROUND_UP((freq << 13), (8*1000)) << 3;
+               ff = ((freq << 10) + 62) / 125;

Pavel, The code posted is a little different from snip here.
While I understand the "<< 10" and "/ 125", the "+ 62" would welcome a
comment.

OOhhh I got it now ... I think using ROUND_UP() is lot more readable
(and maintainable) 

quoted

Also in the final patch, the calculation is a bit different and moved to
"long long" ... but I'm sure the same type of improvement could be done.

Jerome, I see (sorry for the misunderstanding) that you have modified
original Ruslan's patch and the freq variable being shifted by 13 is
long long. Your code is definitely better, thanks.

Pavel.