Re: [PATCH] ALSA: usb-audio: Fix max bytes-per-interval calculation

[Michal Pecio <michal.pecio@gmail.com>]

On Mon, 24 Nov 2025 16:05:18 -0500, Dylan Robinson wrote:
> 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.

Hi,

Since v6.18 we have usb_endpoint_max_periodic_payload(), which looks
like the exact function you need. It is already used by uvcvideo and
xhci_hcd, the latter being particularly important because it ensures
that your endpoints will get the bandwidth allocation you expect.

The copy-pasta in urb.c should probably be cleaned up at this point,
but that would be a separate and unrelated patch, of course.

Regards,
Michal

> Signed-off-by: Dylan Robinson <dylan_robinson@motu.com>
> ---
>  sound/usb/stream.c | 36 ++++++++++++++++++++++++++++++++----
>  1 file changed, 32 insertions(+), 4 deletions(-)
> 
> diff --git a/sound/usb/stream.c b/sound/usb/stream.c
> index 5c235a5ba7e1..074a61215de6 100644
> --- 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);
>  
> -- 
> 2.51.1
>