ALSA: Add driver for big Scarlett 4th Gen interfaces

Hi Takashi,

Thanks again! All changes from your feedback on v7 incorporated in v8.

Regards,
Geoffrey.

---
Changes in v8:
- Rename SNDRV_CTL_TLVT_LABELS to SNDRV_CTL_TLVT_FCP_CHANNEL_LABELS
- Make meter levels buffer persistent rather than allocate/free each time
- Add data protection to fcp_meter_tlv_callback()
- Set private->step* and co after allocation of the buffer
- Use ALIGN() rather than open-coding it

---
Changes in v7:
- Add #define SNDRV_CTL_TLVT_LABELS for channel labels
- Add ioctl for setting meter labels and remove TLV write support
- Simplify reinit handling
- Remove more unnecessary casts
- Add #define FCP_SEGMENT_APP_GOLD to remove hardcoded value

---
Changes in v6:
- Move re-initialisation out of fcp_usb() so it's clear there's no infinite recursion
- Update theory of operation and clarify the use of meter TLV metadata
- Rename meter_labels to meter_metadata for clarity
- Remove unnecessary casts in fcp_ioctl_cmd()

---
Changes in v5:
- Remove version/union complexity from init ioctl
- Add documentation to clarify ioctl usage and big picture

---
Changes in v4:
- Use variable-length data arrays in ioctl structs instead of pointers
- Add CAP_SYS_RAWIO requirement to hwdep interface
- Add validation of flash commands to prevent accidental bricking due to erasing/writing the App_Gold segment
- Refactor URB cleanup

---
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    |  120 ++++
 include/uapi/sound/tlv.h    |    2 +
 sound/usb/Makefile          |    1 +
 sound/usb/fcp.c             | 1134 +++++++++++++++++++++++++++++++++++
 sound/usb/fcp.h             |    7 +
 sound/usb/mixer_quirks.c    |    7 +
 sound/usb/mixer_scarlett2.c |    8 +
 8 files changed, 1285 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