Dne 01. 03. 21 v 14:05 Ruslan Bilovol napsal(a):
> +static void u_audio_set_fback_frequency(enum usb_device_speed speed,
> + unsigned int freq, void *buf)
> +{
> + u32 ff = 0;
> +
> + if (speed == USB_SPEED_FULL) {
> + /*
> + * Full-speed feedback endpoints report frequency
> + * in samples/microframe
> + * Format is encoded in Q10.10 left-justified in the 24 bits,
> + * so that it has a Q10.14 format.
> + */
> + ff = DIV_ROUND_UP((freq << 14), 1000);
> + } else {
> + /*
> + * High-speed feedback endpoints report frequency
> + * in samples/microframe.
> + * Format is encoded in Q12.13 fitted into four bytes so that
> + * the binary point is located between the second and the third
> + * byte format (that is Q16.16)
> + *
> + * Prevent integer overflow by calculating in Q12.13 format and
> + * then shifting to Q16.16
> + */
> + ff = DIV_ROUND_UP((freq << 13), (8*1000)) << 3;
> + }
Hi Ruslan,
Thanks a lot for your patch. 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 = DIV_ROUND_UP((freq << 13), (8*1000)) << 3;
+ ff = ((freq << 10) + 62) / 125;
}
*(__le32 *)buf = cpu_to_le32(ff);
}
Best regards,
Pavel.
