Commit a748e1db authored by Dylan Robinson's avatar Dylan Robinson Committed by Takashi Iwai
Browse files

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: default avatarDylan Robinson <dylan_robinson@motu.com>
Link: https://patch.msgid.link/20251124210518.90054-1-dylan_robinson@motu.com


Signed-off-by: default avatarTakashi Iwai <tiwai@suse.de>
parent 9ef1203f

sound/usb/stream.c

+32 −4
Original line number Diff line number Diff line
@@ -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);