Re: [PATCH V2] usb: gadget: f_uac1: add adaptive sync support for capture

From: Pavel Hofman
Date: Wed Oct 18 2023 - 06:02:08 EST



Dne 18. 10. 23 v 9:47 Charles Yi napsal(a):
UAC1 has it's own freerunning clock and can update Host about
real clock frequency through feedback endpoint so Host can align
number of samples sent to the UAC1 to prevent overruns/underruns.

Change UAC1 driver to make it configurable through additional
'c_sync' configfs file.

Default remains 'asynchronous' with possibility to switch it
to 'adaptive'.


Hi Charles,

Please can you clarify more the adaptive EP IN scenario? I am aware that the f_uac2.c also allows defining c_sync type (that's what your patch is based on).

IIUC the data production rate of adaptive source endpoint (i.e. EP IN) is controlled by feed forward messages from the host
Quoting http://sdpha2.ucsd.edu/Lab_Equip_Manuals/usb_20.pdf page 73:

"Adaptive source endpoints produce data at a rate that is controlled by the data sink. The sink provides feedback (refer to Section 5.12.4.2) to the source, which allows the source to know the desired data rate of the sink."

While the current f_uac2 implementation generates feedback for EP OUT async (unlike f_uac1), I cannot find any support for incoming feed-forward messages from the host for EP IN adaptive case. Neither in f_uac1, of course.

I am not sure if linux supports IN EP adaptive, but the MS UAC2 driver does not https://learn.microsoft.com/en-us/windows-hardware/drivers/audio/usb-2-0-audio-drivers#audio-streaming:

"For the Adaptive IN case the driver doesn't support a feed forward endpoint. If such an endpoint is present in the alternate setting, it will be ignored. The driver handles the Adaptive IN stream in the same way as an Asynchronous IN stream."

IIUC (and I may be wrong) all the c_sync param does in f_uac2 (and f_uac1 in your patch) is just changing the EP IN configuration flag, but the actual support for truly adaptive EP IN is not implemented. IMO there is no code which would accept the feed-forward message from the host and adjust the rate at which samples are consumed from the alsa buffer to EP IN packets (method u_audio_iso_complete https://elixir.bootlin.com/linux/latest/source/drivers/usb/gadget/function/u_audio.c#L193 )

That pertains a bit to the first sentence of your patch - IMO it describes EP OUT async, but not EP IN adaptive.

Thanks a lot for a bit of clarification.

Pavel.



Changes in V2:
- Updated the indentation of commit message.

Signed-off-by: Charles Yi <be286@xxxxxxx>
---
drivers/usb/gadget/function/f_uac1.c | 30 ++++++++++++++++++++++++++++
drivers/usb/gadget/function/u_uac1.h | 2 ++
2 files changed, 32 insertions(+)

diff --git a/drivers/usb/gadget/function/f_uac1.c b/drivers/usb/gadget/function/f_uac1.c
index 6f0e1d803dc2..7a6fcb40bb46 100644
--- a/drivers/usb/gadget/function/f_uac1.c
+++ b/drivers/usb/gadget/function/f_uac1.c
@@ -33,6 +33,8 @@
#define FUOUT_EN(_opts) ((_opts)->c_mute_present \
|| (_opts)->c_volume_present)
+#define EPOUT_FBACK_IN_EN(_opts) ((_opts)->c_sync == USB_ENDPOINT_SYNC_ASYNC)
+
struct f_uac1 {
struct g_audio g_audio;
u8 ac_intf, as_in_intf, as_out_intf;
@@ -227,6 +229,16 @@ static struct uac_iso_endpoint_descriptor as_iso_out_desc = {
.wLockDelay = cpu_to_le16(1),
};
+static struct usb_endpoint_descriptor as_fback_ep_desc = {
+ .bLength = USB_DT_ENDPOINT_SIZE,
+ .bDescriptorType = USB_DT_ENDPOINT,
+
+ .bEndpointAddress = USB_DIR_IN,
+ .bmAttributes = USB_ENDPOINT_XFER_ISOC | USB_ENDPOINT_USAGE_FEEDBACK,
+ .wMaxPacketSize = cpu_to_le16(3),
+ .bInterval = 1,
+};
+
static struct uac_format_type_i_discrete_descriptor as_in_type_i_desc = {
.bLength = 0, /* filled on rate setup */
.bDescriptorType = USB_DT_CS_INTERFACE,
@@ -280,6 +292,7 @@ static struct usb_descriptor_header *f_audio_desc[] = {
(struct usb_descriptor_header *)&as_out_ep_desc,
(struct usb_descriptor_header *)&as_iso_out_desc,
+ (struct usb_descriptor_header *)&as_fback_ep_desc,
(struct usb_descriptor_header *)&as_in_interface_alt_0_desc,
(struct usb_descriptor_header *)&as_in_interface_alt_1_desc,
@@ -1107,6 +1120,9 @@ static void setup_descriptor(struct f_uac1_opts *opts)
f_audio_desc[i++] = USBDHDR(&as_out_type_i_desc);
f_audio_desc[i++] = USBDHDR(&as_out_ep_desc);
f_audio_desc[i++] = USBDHDR(&as_iso_out_desc);
+ if (EPOUT_FBACK_IN_EN(opts)) {
+ f_audio_desc[i++] = USBDHDR(&as_fback_ep_desc);
+ }
}
if (EPIN_EN(opts)) {
f_audio_desc[i++] = USBDHDR(&as_in_interface_alt_0_desc);
@@ -1317,6 +1333,12 @@ static int f_audio_bind(struct usb_configuration *c, struct usb_function *f)
ac_header_desc->baInterfaceNr[ba_iface_id++] = status;
uac1->as_out_intf = status;
uac1->as_out_alt = 0;
+
+ if (EPOUT_FBACK_IN_EN(audio_opts)) {
+ as_out_ep_desc.bmAttributes =
+ USB_ENDPOINT_XFER_ISOC | USB_ENDPOINT_SYNC_ASYNC;
+ as_out_interface_alt_1_desc.bNumEndpoints++;
+ }
}
if (EPIN_EN(audio_opts)) {
@@ -1354,6 +1376,12 @@ static int f_audio_bind(struct usb_configuration *c, struct usb_function *f)
goto err_free_fu;
audio->out_ep = ep;
audio->out_ep->desc = &as_out_ep_desc;
+ if (EPOUT_FBACK_IN_EN(audio_opts)) {
+ audio->in_ep_fback = usb_ep_autoconfig(gadget, &as_fback_ep_desc);
+ if (!audio->in_ep_fback) {
+ goto err_free_fu;
+ }
+ }
}
if (EPIN_EN(audio_opts)) {
@@ -1685,6 +1713,8 @@ static struct usb_function_instance *f_audio_alloc_inst(void)
opts->req_number = UAC1_DEF_REQ_NUM;
+ opts->c_sync = UAC1_DEF_CSYNC;
+
snprintf(opts->function_name, sizeof(opts->function_name), "AC Interface");
return &opts->func_inst;
diff --git a/drivers/usb/gadget/function/u_uac1.h b/drivers/usb/gadget/function/u_uac1.h
index f7a616760e31..c6e2271e8cdd 100644
--- a/drivers/usb/gadget/function/u_uac1.h
+++ b/drivers/usb/gadget/function/u_uac1.h
@@ -27,6 +27,7 @@
#define UAC1_DEF_MAX_DB 0 /* 0 dB */
#define UAC1_DEF_RES_DB (1*256) /* 1 dB */
+#define UAC1_DEF_CSYNC USB_ENDPOINT_SYNC_ASYNC
struct f_uac1_opts {
struct usb_function_instance func_inst;
@@ -56,6 +57,7 @@ struct f_uac1_opts {
struct mutex lock;
int refcnt;
+ int c_sync;
};
#endif /* __U_UAC1_H */