ALSA: usb-audio: Fix max bytes-per-interval calculation

The maxpacksize field in struct audioformat represents the maximum number
of bytes per isochronous interval. The current implementation only
special-cases high-speed endpoints and does not account for the different
computations required for SuperSpeed, SuperSpeedPlus, or eUSB2. As a
result, USB audio class devices operating at these speeds may fail to
stream correctly. The issue was observed on a MOTU 16A (2025) interface,
which requires more than 1024 bytes per interval at SuperSpeed.

This patch replaces the existing logic with a helper that computes the
correct maximum bytes-per-interval for all USB speeds, borrowing the logic
used in drivers/usb/core/urb.c.

Signed-off-by: Dylan Robinson <dylan_robinson@motu.com>
Link: https://patch.msgid.link/20251124210518.90054-1-dylan_robinson@motu.com
Signed-off-by: Takashi Iwai <tiwai@suse.de>

diff --git a/sound/usb/stream.c b/sound/usb/stream.c
index 5c235a5ba7e1b1ebd1b749c9cc4c64d9aa84c73b..074a61215de6b27588b95bf8fffbebcecd82e9b7 100644 (file)
--- a/sound/usb/stream.c
+++ b/sound/usb/stream.c
@@ -684,6 +684,37 @@ snd_usb_find_output_terminal_descriptor(struct usb_host_interface *ctrl_iface,
        return NULL;
 }
 
+static unsigned int
+snd_usb_max_bytes_per_interval(struct snd_usb_audio *chip,
+                                       struct usb_host_interface *alts)
+{
+       struct usb_host_endpoint *ep = &alts->endpoint[0];
+       unsigned int max_bytes = usb_endpoint_maxp(&ep->desc);
+
+       /* SuperSpeed isoc endpoints have up to 16 bursts of up to 3 packets each */
+       if (snd_usb_get_speed(chip->dev) >= USB_SPEED_SUPER) {
+               int burst = 1 + ep->ss_ep_comp.bMaxBurst;
+               int mult = USB_SS_MULT(ep->ss_ep_comp.bmAttributes);
+               max_bytes *= burst;
+               max_bytes *= mult;
+       }
+
+       if (snd_usb_get_speed(chip->dev) == USB_SPEED_SUPER_PLUS &&
+               USB_SS_SSP_ISOC_COMP(ep->ss_ep_comp.bmAttributes)) {
+               max_bytes = le32_to_cpu(ep->ssp_isoc_ep_comp.dwBytesPerInterval);
+       }
+
+       /* High speed, 1-3 packets/uframe, max 6 for eUSB2 double bw */
+       if (snd_usb_get_speed(chip->dev) == USB_SPEED_HIGH) {
+               if (usb_endpoint_is_hs_isoc_double(chip->dev, ep))
+                       max_bytes = le32_to_cpu(ep->eusb2_isoc_ep_comp.dwBytesPerInterval);
+               else
+                       max_bytes *= usb_endpoint_maxp_mult(&ep->desc);
+       }
+
+       return max_bytes;
+}
+
 static struct audioformat *
 audio_format_alloc_init(struct snd_usb_audio *chip,
                       struct usb_host_interface *alts,
@@ -703,11 +734,8 @@ audio_format_alloc_init(struct snd_usb_audio *chip,
        fp->ep_attr = get_endpoint(alts, 0)->bmAttributes;
        fp->datainterval = snd_usb_parse_datainterval(chip, alts);
        fp->protocol = protocol;
-       fp->maxpacksize = le16_to_cpu(get_endpoint(alts, 0)->wMaxPacketSize);
+       fp->maxpacksize = snd_usb_max_bytes_per_interval(chip, alts);
        fp->channels = num_channels;
-       if (snd_usb_get_speed(chip->dev) == USB_SPEED_HIGH)
-               fp->maxpacksize = (((fp->maxpacksize >> 11) & 3) + 1)
-                               * (fp->maxpacksize & 0x7ff);
        fp->clock = clock;
        INIT_LIST_HEAD(&fp->list);