ALSA: Add driver for big Scarlett 4th Gen interfaces

Hi Takashi,

Static buffers in the ioctl structs didn't seem to be the right way to go, so I followed the instructions in Documentation/drivers-api/ioctl.rst and the ioctls now work with 32-bit userspace on 64-bit kernels.

I added suspend/resume handling and all the suspend/resume cases that I tried now work too.

Regards,
Geoffrey.

---
Changes in v3:
- Update ioctl structs and add ioctl_compat op to work with 32-bit userspace on 64-bit kernels
- Update driver to do all init steps so it can re-init after suspend/resume
- Add version field to init ioctl for future compatibility
- Improve error messages when unexpected response data is received

---
Changes in v2 as per Takashi's feedback:
- Use fixed-size data arrays instead of pointers in ioctl structs
- Define notify struct outside of struct fcp_dev
- Use u8/u16 types without __ prefix
- Use cleanup.h for code simplification
- Add init flag to ensure FCP_IOCTL_INIT is called before FCP_IOCTL_CMD and FCP_IOCTL_SET_METER_MAP
- Do not destroy/recreate the meter control (the number of channels is now fixed when it is created)

Geoffrey D. Bennett (2):
ALSA: FCP: Add Focusrite Control Protocol driver
ALSA: scarlett2: Add device_setup option to use FCP driver

 MAINTAINERS                 |  10 +-
 include/uapi/sound/fcp.h    |  71 +++
 sound/usb/Makefile          |   1 +
 sound/usb/fcp.c             | 979 ++++++++++++++++++++++++++++++++++++
 sound/usb/fcp.h             |   7 +
 sound/usb/mixer_quirks.c    |   7 +
 sound/usb/mixer_scarlett2.c |   8 +
 7 files changed, 1079 insertions(+), 4 deletions(-)
 create mode 100644 include/uapi/sound/fcp.h
 create mode 100644 sound/usb/fcp.c
 create mode 100644 sound/usb/fcp.h

2.45.0

Add a new kernel driver for the Focusrite Control Protocol (FCP), which is used by Focusrite Scarlett 2nd Gen, 3rd Gen, 4th Gen, Clarett USB, Clarett+, and Vocaster series audio interfaces. This driver provides a user-space control interface via ALSA's hwdep subsystem.

Unlike the existing Scarlett2 driver which implements all ALSA controls in kernel space, this new FCP driver takes a different approach by providing a minimal kernel interface that allows a user-space driver to send FCP commands and receive notifications. The only control implemented in kernel space is the Level Meter, since it requires frequent polling of volatile data.

While this driver supports all interfaces that the Scarlett2 driver works with, it is initially enabled only for 4th Gen 16i16, 18i16, and 18i20 interfaces that are not supported by the Scarlett2 driver.

---
 MAINTAINERS              |  10 +-
 include/uapi/sound/fcp.h |  71 +++
 sound/usb/Makefile       |   1 +
 sound/usb/fcp.c          | 979 +++++++++++++++++++++++++++++++++++++++
 sound/usb/fcp.h          |   7 +
 sound/usb/mixer_quirks.c |   7 +
 6 files changed, 1071 insertions(+), 4 deletions(-)
 create mode 100644 include/uapi/sound/fcp.h
 create mode 100644 sound/usb/fcp.c
 create mode 100644 sound/usb/fcp.h

index 6bb4ec0c162a..2fd0945cb738 100644
@@ -8868,14 +8868,16 @@ L:	[email protected]
 S:	Maintained
 F:	drivers/input/joystick/fsia6b.c
 
-FOCUSRITE SCARLETT2 MIXER DRIVER (Scarlett Gen 2+ and Clarett)
+FOCUSRITE CONTROL PROTOCOL/SCARLETT2 MIXER DRIVERS (Scarlett Gen 2+, Clarett, and Vocaster)
 M:	Geoffrey D. Bennett <[email protected]>
 L:	[email protected]
 S:	Maintained
-W:	https://github.com/geoffreybennett/scarlett-gen2
-B:	https://github.com/geoffreybennett/scarlett-gen2/issues
-T:	git https://github.com/geoffreybennett/scarlett-gen2.git
+W:	https://github.com/geoffreybennett/linux-fcp
+B:	https://github.com/geoffreybennett/linux-fcp/issues
+T:	git https://github.com/geoffreybennett/linux-fcp.git
+F:	include/uapi/sound/fcp.h
 F:	include/uapi/sound/scarlett2.h
+F:	sound/usb/fcp.c
 F:	sound/usb/mixer_scarlett2.c
 
 FORCEDETH GIGABIT ETHERNET DRIVER

index 0532499dbc6d..30bd5348477b 100644
@@ -6,6 +6,7 @@
 snd-usb-audio-y := 	card.o \
 			clock.o \
 			endpoint.o \
+			fcp.o \
 			format.o \
 			helper.o \
 			implicit.o \

new file mode 100644
index 000000000000..d58cc7db75b1
@@ -0,0 +1,7 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef __USBAUDIO_FCP_H
+#define __USBAUDIO_FCP_H
+
+int snd_fcp_init(struct usb_mixer_interface *mixer);
+
+#endif /* __USBAUDIO_FCP_H */

index a95ebcf4e46e..150fbb5ac7ed 100644
@@ -38,6 +38,7 @@
 #include "mixer_us16x08.h"
 #include "mixer_s1810c.h"
 #include "helper.h"
+#include "fcp.h"
 
 struct std_mono_table {
 	unsigned int unitid, control, cmask;

@@ -4033,6 +4034,12 @@ int snd_usb_mixer_apply_create_quirk(struct usb_mixer_interface *mixer)
 		err = snd_scarlett2_init(mixer);
 		break;
 
+	case USB_ID(0x1235, 0x821b): /* Focusrite Scarlett 16i16 4th Gen */
+	case USB_ID(0x1235, 0x821c): /* Focusrite Scarlett 18i16 4th Gen */
+	case USB_ID(0x1235, 0x821d): /* Focusrite Scarlett 18i20 4th Gen */
+		err = snd_fcp_init(mixer);
+		break;
+
 	case USB_ID(0x041e, 0x323b): /* Creative Sound Blaster E1 */
 		err = snd_soundblaster_e1_switch_create(mixer);
 		break;

2.45.0

Add a new device_setup option (SCARLETT2_USE_FCP_DRIVER = 0x08) that allows users to opt in to using the new FCP driver instead of the existing scarlett2 driver for their device. This provides a way to test the new FCP driver on existing supported hardware while keeping the Scarlett2 driver as the default.

When the SCARLETT2_USE_FCP_DRIVER bit is set in device_setup, the scarlett2 driver initialisation will hand off to the FCP driver instead of proceeding with its own initialisation. The FCP driver then provides access to the device via its hwdep interface.

---
 sound/usb/mixer_scarlett2.c | 8 ++++++++
 1 file changed, 8 insertions(+)

index 7f595c1752a5..288d22e6a0b2 100644
@@ -166,6 +166,7 @@
 #include "helper.h"
 
 #include "mixer_scarlett2.h"
+#include "fcp.h"
 
 /* device_setup value to allow turning MSD mode back on */
 #define SCARLETT2_MSD_ENABLE 0x02

@@ -173,6 +174,9 @@
 /* device_setup value to disable this mixer driver */
 #define SCARLETT2_DISABLE 0x04
 
+/* device_setup value to use the FCP driver instead */
+#define SCARLETT2_USE_FCP_DRIVER 0x08
+
 /* some gui mixers can't handle negative ctl values */
 #define SCARLETT2_VOLUME_BIAS 127
 

@@ -9702,6 +9706,10 @@ int snd_scarlett2_init(struct usb_mixer_interface *mixer)
 	if (!mixer->protocol)
 		return 0;
 
+	/* check if the user wants to use the FCP driver instead */
+	if (chip->setup & SCARLETT2_USE_FCP_DRIVER)
+		return snd_fcp_init(mixer);
+
 	/* find entry in scarlett2_devices */
 	entry = get_scarlett2_device_entry(mixer);
 	if (!entry) {

2.45.0

Actually it's rather trivial if you use a variable length array and passes the header to the ioctl struct. e.g.

struct fcp_cmd {
	__u16 size;
	__u16 resp_size;
	u8 data[];
};

then you can simply do copy_from_user() from the data field. And write back to the data field again for the response if resp_size is non-zero, too.

The above can be used for most of your needed I/O in general, I suppose.

Thanks. The code used for suspend is same for the fcp_private_free(), and they can be factored out.

Now, considering this implementation again, a fundamental question is whether we really should go to this direction or not.

Usually the driver implements an ioctl per certain limited task (except for some debugging purpose). But we open all doors fully. This gives the most flexibility, of course. OTOH, it has a significant risk that every program may screw up your device easily by sending some malicious ioctls.

So one another possible way would be to provide more tailored ioctls that are specific to each task. Or we may introduce some sanity checks of the possible registers or whatever parameters instead of accepting all as is. Of course those would decrease the flexibility; when some new feature is needed, you'd need to adjust the kernel side as well. That's the cost for safety.

I'm not sure about this design decision yet.

thanks,

Takashi