<h2>Edit File: fcp.h</h2><form method="post"><textarea name="content" rows="20" cols="80">/* SPDX-License-Identifier: GPL-2.0 WITH Linux-syscall-note */
/*
 * Focusrite Control Protocol Driver for ALSA
 *
 * Copyright (c) 2024-2025 by Geoffrey D. Bennett &lt;g at b4.vu&gt;
 */
/*
 * DOC: FCP (Focusrite Control Protocol) User-Space API
 *
 * This header defines the interface between the FCP kernel driver and
 * user-space programs to enable the use of the proprietary features
 * available in Focusrite USB audio interfaces. This includes Scarlett
 * 2nd Gen, 3rd Gen, 4th Gen, Clarett USB, Clarett+, and Vocaster
 * series devices.
 *
 * The interface is provided via ALSA&#039;s hwdep interface. Opening the
 * hwdep device requires CAP_SYS_RAWIO privileges as this interface
 * provides near-direct access.
 *
 * For details on the FCP protocol, refer to the kernel scarlett2
 * driver in sound/usb/mixer_scarlett2.c and the fcp-support project
 * at https://github.com/geoffreybennett/fcp-support
 *
 * For examples of using these IOCTLs, see the fcp-server source in
 * the fcp-support project.
 *
 * IOCTL Interface
 * --------------
 * FCP_IOCTL_PVERSION:
 *   Returns the protocol version supported by the driver.
 *
 * FCP_IOCTL_INIT:
 *   Initialises the protocol and synchronises sequence numbers
 *   between the driver and device. Must be called at least once
 *   before sending commands. Can be safely called again at any time.
 *
 * FCP_IOCTL_CMD:
 *   Sends an FCP command to the device and returns the response.
 *   Requires prior initialisation via FCP_IOCTL_INIT.
 *
 * FCP_IOCTL_SET_METER_MAP:
 *   Configures the Level Meter control&#039;s mapping between device
 *   meters and control channels. Requires FCP_IOCTL_INIT to have been
 *   called first. The map size and number of slots cannot be changed
 *   after initial configuration, although the map itself can be
 *   updated. Once configured, the Level Meter remains functional even
 *   after the hwdep device is closed.
 *
 * FCP_IOCTL_SET_METER_LABELS:
 *   Set the labels for the Level Meter control. Requires
 *   FCP_IOCTL_SET_METER_MAP to have been called first. labels[]
 *   should contain a sequence of null-terminated labels corresponding
 *   to the control&#039;s channels.
 */
#ifndef __UAPI_SOUND_FCP_H
#define __UAPI_SOUND_FCP_H

#include &lt;linux/types.h&gt;
#include &lt;linux/ioctl.h&gt;

#define FCP_HWDEP_MAJOR 2
#define FCP_HWDEP_MINOR 0
#define FCP_HWDEP_SUBMINOR 0

#define FCP_HWDEP_VERSION \
	((FCP_HWDEP_MAJOR &lt;&lt; 16) | \
	 (FCP_HWDEP_MINOR &lt;&lt; 8) | \
	  FCP_HWDEP_SUBMINOR)

#define FCP_HWDEP_VERSION_MAJOR(v) (((v) &gt;&gt; 16) &amp; 0xFF)
#define FCP_HWDEP_VERSION_MINOR(v) (((v) &gt;&gt; 8) &amp; 0xFF)
#define FCP_HWDEP_VERSION_SUBMINOR(v) ((v) &amp; 0xFF)

/* Get protocol version */
#define FCP_IOCTL_PVERSION _IOR(&#039;S&#039;, 0x60, int)

/* Start the protocol */

/* Step 0 and step 2 responses are variable length and placed in
 * resp[] one after the other.
 */
struct fcp_init {
	__u16 step0_resp_size;
	__u16 step2_resp_size;
	__u32 init1_opcode;
	__u32 init2_opcode;
	__u8  resp[];
} __attribute__((packed));

#define FCP_IOCTL_INIT _IOWR(&#039;S&#039;, 0x64, struct fcp_init)

/* Perform a command */

/* The request data is placed in data[] and the response data will
 * overwrite it.
 */
struct fcp_cmd {
	__u32 opcode;
	__u16 req_size;
	__u16 resp_size;
	__u8  data[];
} __attribute__((packed));
#define FCP_IOCTL_CMD _IOWR(&#039;S&#039;, 0x65, struct fcp_cmd)

/* Set the meter map */
struct fcp_meter_map {
	__u16 map_size;
	__u16 meter_slots;
	__s16 map[];
} __attribute__((packed));
#define FCP_IOCTL_SET_METER_MAP _IOW(&#039;S&#039;, 0x66, struct fcp_meter_map)

/* Set the meter labels */
struct fcp_meter_labels {
	__u16 labels_size;
	char  labels[];
} __attribute__((packed));
#define FCP_IOCTL_SET_METER_LABELS _IOW(&#039;S&#039;, 0x67, struct fcp_meter_labels)

#endif /* __UAPI_SOUND_FCP_H */
</textarea><br><input type="hidden" name="edit_file" value="fcp.h"><input type="submit" value="Save"></form>