[PATCH 3/3] usb: gadget: u_audio: add real feedback implementation

[Ruslan Bilovol <ruslan.bilovol@gmail.com>]

This adds interface between userspace and feedback
endpoint to report real feedback frequency to the Host.

Current implementation adds new userspace interface
ALSA mixer control "PCM Feedback Frequency Hz" (similar
to aloop driver's "PCM Rate Shift 100000" mixer control)

We allow +/-20% deviation of nominal sampling frequency,
that usually is more than enough in real-world usecases

Usage of this new control is easy to implement in
existing userspace tools like alsaloop from alsa-utils.

Signed-off-by: Ruslan Bilovol <ruslan.bilovol@gmail.com>
---
 drivers/usb/gadget/function/f_uac2.c  |  4 ++
 drivers/usb/gadget/function/u_audio.c | 93 +++++++++++++++++++++++++++++++++++
 drivers/usb/gadget/function/u_audio.h |  7 +++
 3 files changed, 104 insertions(+)

diff --git a/drivers/usb/gadget/function/f_uac2.c b/drivers/usb/gadget/function/f_uac2.c
index 3187ad3..ebdb2a1 100644
--- a/drivers/usb/gadget/function/f_uac2.c
+++ b/drivers/usb/gadget/function/f_uac2.c
@@ -487,6 +487,10 @@ static void set_ep_max_packet_size(const struct f_uac2_opts *uac2_opts,
 
 	max_packet_size = num_channels(chmask) * ssize *
 		DIV_ROUND_UP(srate, factor / (1 << (ep_desc->bInterval - 1)));
+
+	if (!is_playback && (uac2_opts->c_sync == USB_ENDPOINT_SYNC_ASYNC))
+		max_packet_size = max_packet_size * FBACK_FREQ_MAX / 100;
+
 	ep_desc->wMaxPacketSize = cpu_to_le16(min_t(u16, max_packet_size,
 				le16_to_cpu(ep_desc->wMaxPacketSize)));
 }
diff --git a/drivers/usb/gadget/function/u_audio.c b/drivers/usb/gadget/function/u_audio.c
index 33c9288..ae9e1f3 100644
--- a/drivers/usb/gadget/function/u_audio.c
+++ b/drivers/usb/gadget/function/u_audio.c
@@ -16,6 +16,7 @@
 #include <sound/core.h>
 #include <sound/pcm.h>
 #include <sound/pcm_params.h>
+#include <sound/control.h>
 
 #include "u_audio.h"
 
@@ -596,12 +597,87 @@ void u_audio_stop_playback(struct g_audio *audio_dev)
 }
 EXPORT_SYMBOL_GPL(u_audio_stop_playback);
 
+static int u_audio_rate_shift_info(struct snd_kcontrol *kcontrol,
+				   struct snd_ctl_elem_info *uinfo)
+{
+	struct uac_rtd_params *prm = snd_kcontrol_chip(kcontrol);
+	struct snd_uac_chip *uac = prm->uac;
+	struct g_audio *audio_dev = uac->audio_dev;
+	struct uac_params *params = &audio_dev->params;
+	unsigned int ffback_min, ffback_max;
+
+	ffback_min = params->c_srate * FBACK_FREQ_MIN / 100;
+	ffback_max = params->c_srate * FBACK_FREQ_MIN / 100;
+
+	uinfo->type = SNDRV_CTL_ELEM_TYPE_INTEGER;
+	uinfo->count = 1;
+	uinfo->value.integer.min = ffback_min;
+	uinfo->value.integer.max = ffback_max;
+	uinfo->value.integer.step = 1;
+	return 0;
+}
+
+static int u_audio_rate_shift_get(struct snd_kcontrol *kcontrol,
+				   struct snd_ctl_elem_value *ucontrol)
+{
+	struct uac_rtd_params *prm = snd_kcontrol_chip(kcontrol);
+	unsigned long flags;
+
+	spin_lock_irqsave(&prm->lock, flags);
+	ucontrol->value.integer.value[0] = prm->ffback;
+	spin_unlock_irqrestore(&prm->lock, flags);
+
+	return 0;
+}
+
+static int u_audio_rate_shift_put(struct snd_kcontrol *kcontrol,
+				  struct snd_ctl_elem_value *ucontrol)
+{
+	struct uac_rtd_params *prm = snd_kcontrol_chip(kcontrol);
+	struct snd_uac_chip *uac = prm->uac;
+	struct g_audio *audio_dev = uac->audio_dev;
+	struct uac_params *params = &audio_dev->params;
+	unsigned int val;
+	unsigned int ffback_min, ffback_max;
+	unsigned long flags;
+	int change = 0;
+
+	ffback_min = params->c_srate * FBACK_FREQ_MIN / 100;
+	ffback_max = params->c_srate * FBACK_FREQ_MAX / 100;
+
+	val = ucontrol->value.integer.value[0];
+	if (val < ffback_min)
+		val = ffback_min;
+	if (val > ffback_max)
+		val = ffback_max;
+
+	spin_lock_irqsave(&prm->lock, flags);
+	if (prm->ffback != val) {
+		prm->ffback = val;
+		change = 1;
+	}
+	spin_unlock_irqrestore(&prm->lock, flags);
+
+	return change;
+}
+
+static const struct snd_kcontrol_new u_audio_controls[]  = {
+{
+	.iface =        SNDRV_CTL_ELEM_IFACE_PCM,
+	.name =         "PCM Feedback Frequency Hz",
+	.info =         u_audio_rate_shift_info,
+	.get =          u_audio_rate_shift_get,
+	.put =          u_audio_rate_shift_put,
+},
+};
+
 int g_audio_setup(struct g_audio *g_audio, const char *pcm_name,
 					const char *card_name)
 {
 	struct snd_uac_chip *uac;
 	struct snd_card *card;
 	struct snd_pcm *pcm;
+	struct snd_kcontrol *kctl;
 	struct uac_params *params;
 	int p_chmask, c_chmask;
 	int err;
@@ -687,6 +763,23 @@ int g_audio_setup(struct g_audio *g_audio, const char *pcm_name,
 	snd_pcm_set_ops(pcm, SNDRV_PCM_STREAM_PLAYBACK, &uac_pcm_ops);
 	snd_pcm_set_ops(pcm, SNDRV_PCM_STREAM_CAPTURE, &uac_pcm_ops);
 
+	if (c_chmask && g_audio->in_ep_fback) {
+		strlcpy(card->mixername, card_name, sizeof(card->driver));
+
+		kctl = snd_ctl_new1(&u_audio_controls[0], &uac->c_prm);
+		if (!kctl) {
+			err = -ENOMEM;
+			goto snd_fail;
+		}
+
+		kctl->id.device = pcm->device;
+		kctl->id.subdevice = 0;
+
+		err = snd_ctl_add(card, kctl);
+		if (err < 0)
+			goto snd_fail;
+	}
+
 	strlcpy(card->driver, card_name, sizeof(card->driver));
 	strlcpy(card->shortname, card_name, sizeof(card->shortname));
 	sprintf(card->longname, "%s %i", card_name, card->dev->id);
diff --git a/drivers/usb/gadget/function/u_audio.h b/drivers/usb/gadget/function/u_audio.h
index 53e6baf..fd70808 100644
--- a/drivers/usb/gadget/function/u_audio.h
+++ b/drivers/usb/gadget/function/u_audio.h
@@ -11,6 +11,13 @@
 
 #include <linux/usb/composite.h>
 
+/*
+ * Min/max percentage of nominal sampling frequency deviation
+ * reported through feedback endpoint to the host
+ */
+#define FBACK_FREQ_MIN	80
+#define FBACK_FREQ_MAX	120
+
 struct uac_params {
 	/* playback */
 	int p_chmask;	/* channel mask */
-- 
1.9.1