[PATCH v2] Pioneer devices: engage implicit feedback sync for playback

[Geraldo]

Dear Linux users of Pioneer gear, please disregard v1 of this patch and
test this instead if at all possible.

My initial assessment that we needed to let the capture EP be opened twice
was clearly proven wrong by further testing. This is good because if we
really needed that it would require a lot of reengineering inside ALSA.

One thing that still boggles my mind though is why we need a special
Pioneer case inside snd_usb_parse_implicit_fb_quirk that returns 1 like a
capture quirk was found.

This is a highly experimental patch on top of 5.12-rc6 that's supposed to
engage implicit feedback sync on the playback for Pioneer devices. Without
implicit feedback sync the inputs started glitching due to clock drift in
about an hour in my Pioneer DJ DDJ-SR2.

Once again I'd like to thank Takashi Iwai. He's the true author of the bulk
of this code, I just adapted it and coerced it into working.

Signed-off-by: Geraldo Nascimento <geraldogabriel at gmail.com>

--- implicit.c.git      2021-04-04 20:51:57.226754632 -0300
+++ implicit.c  2021-04-05 10:02:41.059816581 -0300
@@ -177,30 +177,31 @@ static int add_roland_implicit_fb(struct
                                       ifnum, alts);
 }

-/* Playback and capture EPs on Pioneer devices share the same iface/altset,
- * but they don't seem working with the implicit fb mode well, hence we
- * just return as if the sync were already set up.
+/* Pioneer devices: playback and capture streams sharing the same
iface/altset
  */
-static int skip_pioneer_sync_ep(struct snd_usb_audio *chip,
-                               struct audioformat *fmt,
-                               struct usb_host_interface *alts)
-{
-       struct usb_endpoint_descriptor *epd;
-
-       if (alts->desc.bNumEndpoints != 2)
-               return 0;
-
-       epd = get_endpoint(alts, 1);
-       if (!usb_endpoint_is_isoc_in(epd) ||
-           (epd->bmAttributes & USB_ENDPOINT_SYNCTYPE) !=
USB_ENDPOINT_SYNC_ASYNC ||
-           ((epd->bmAttributes & USB_ENDPOINT_USAGE_MASK) !=
-            USB_ENDPOINT_USAGE_DATA &&
-            (epd->bmAttributes & USB_ENDPOINT_USAGE_MASK) !=
-            USB_ENDPOINT_USAGE_IMPLICIT_FB))
-               return 0;
-       return 1; /* don't handle with the implicit fb, just skip sync EP */
+static int add_pioneer_implicit_fb(struct snd_usb_audio *chip,
+                                  struct audioformat *fmt,
+                                  struct usb_host_interface *alts)
+{
+       struct usb_endpoint_descriptor *epd;
+
+       if (alts->desc.bNumEndpoints != 2)
+               return 0;
+
+       epd = get_endpoint(alts, 1);
+
+       if (!usb_endpoint_is_isoc_in(epd) ||
+           (epd->bmAttributes & USB_ENDPOINT_SYNCTYPE) !=
USB_ENDPOINT_SYNC_ASYNC ||
+           ((epd->bmAttributes & USB_ENDPOINT_USAGE_MASK) !=
+            USB_ENDPOINT_USAGE_DATA &&
+            (epd->bmAttributes & USB_ENDPOINT_USAGE_MASK) !=
+            USB_ENDPOINT_USAGE_IMPLICIT_FB))
+               return 0;
+       return add_implicit_fb_sync_ep(chip, fmt, epd->bEndpointAddress, 1,
+                                      alts->desc.bInterfaceNumber, alts);
 }

+
 static int __add_generic_implicit_fb(struct snd_usb_audio *chip,
                                     struct audioformat *fmt,
                                     int iface, int altset)
@@ -306,7 +307,7 @@ static int audioformat_implicit_fb_quirk
            alts->desc.bInterfaceClass == USB_CLASS_VENDOR_SPEC &&
            (USB_ID_VENDOR(chip->usb_id) == 0x2b73 || /* Pioneer */
             USB_ID_VENDOR(chip->usb_id) == 0x08e4    /* Pioneer */)) {
-               if (skip_pioneer_sync_ep(chip, fmt, alts))
+               if (add_pioneer_implicit_fb(chip, fmt, alts))
                        return 1;
        }

@@ -339,8 +340,20 @@ int snd_usb_parse_implicit_fb_quirk(stru
                                    struct audioformat *fmt,
                                    struct usb_host_interface *alts)
 {
-       if (fmt->endpoint & USB_DIR_IN)
-               return audioformat_capture_quirk(chip, fmt, alts);
+        bool isPioneer;
+
+        if (alts->desc.bInterfaceClass == USB_CLASS_VENDOR_SPEC &&
+            (USB_ID_VENDOR(chip->usb_id) == 0x2b73 || /* Pioneer */
+             USB_ID_VENDOR(chip->usb_id) == 0x08e4    /* Pioneer */))
+            isPioneer = true;
+
+       if (fmt->endpoint & USB_DIR_IN) {
+                if (isPioneer == true)
+                    return 1;
+                else
+                   return audioformat_capture_quirk(chip, fmt, alts);
+        }
+
        else
                return audioformat_implicit_fb_quirk(chip, fmt, alts);
 }