[PATCH] usb: gadget: f_uac2: prevent invalid bInterval auto-selection

[PATCH] usb: gadget: f_uac2: prevent invalid bInterval auto-selection

[Kyle Farrell]

When p_hs_bint or c_hs_bint is 0, the driver selects the largest
bInterval from 4 down to 1 for which the required bandwidth fits the
maximum packet size. The original implementation intends to keep
bInterval at 1 when required bandwidth exceeds the single-packet maximum,
but the search loop instead decrements bInterval to 0 in that case.

A bInterval of 0 is invalid for an isochronous endpoint. A Linux host
reports it and substitutes a default:

  usb 3-8: ... endpoint 0x1 has an invalid bInterval 0, changing to 7

Stop the search at bInterval=1. Maintain existing clamping of
wMaxPacketSize and warning about possible data loss when the required
bandwidth cannot be accommodated.

Fixes: 355a05dc8367 ("usb: gadget: f_uac2: Optionally determine bInterval for HS and SS")
Signed-off-by: Kyle Farrell <kaf@nwlink.com>
---
 drivers/usb/gadget/function/f_uac2.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/drivers/usb/gadget/function/f_uac2.c b/drivers/usb/gadget/function/f_uac2.c
index d8cf710085a0..9bd59ea5f7fb 100644
--- a/drivers/usb/gadget/function/f_uac2.c
+++ b/drivers/usb/gadget/function/f_uac2.c
@@ -728,11 +728,11 @@ static int set_ep_max_packet_size_bint(struct device *dev, const struct f_uac2_o
 			bint = opts_bint;
 			max_size_bw = get_max_bw_for_bint(uac2_opts, bint, 8000, is_playback);
 		} else {
-			/* checking bInterval from 4 to 1 whether the required bandwidth fits */
-			for (bint = 4; bint > 0; --bint) {
+			/* select the largest bInterval that fits, or 1 if none fit */
+			for (bint = 4; ; --bint) {
 				max_size_bw = get_max_bw_for_bint(
 					uac2_opts, bint, 8000, is_playback);
-				if (max_size_bw <= max_size_ep)
+				if (max_size_bw <= max_size_ep || bint == 1)
 					break;
 			}
 		}

base-commit: bdf12cde30830d46a8ef1ba93986356f1164b4f1
-- 
2.34.1

Re: [PATCH] usb: gadget: f_uac2: prevent invalid bInterval auto-selection

[sashiko-bot]

> When p_hs_bint or c_hs_bint is 0, the driver selects the largest
> bInterval from 4 down to 1 for which the required bandwidth fits the
> maximum packet size. The original implementation intends to keep
> bInterval at 1 when required bandwidth exceeds the single-packet maximum,
> but the search loop instead decrements bInterval to 0 in that case.
> 
> A bInterval of 0 is invalid for an isochronous endpoint. A Linux host
> reports it and substitutes a default:
> 
>   usb 3-8: ... endpoint 0x1 has an invalid bInterval 0, changing to 7
> 
> Stop the search at bInterval=1. Maintain existing clamping of
> wMaxPacketSize and warning about possible data loss when the required
> bandwidth cannot be accommodated.
> 
> Fixes: 355a05dc8367 ("usb: gadget: f_uac2: Optionally determine bInterval for HS and SS")
> Signed-off-by: Kyle Farrell <kaf@nwlink.com>

Sashiko has reviewed this patch and found no issues.

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20261028222800.782988-1-kaf@nwlink.com?part=1