[-next,4/4] spi: mockup: Add documentation

diff --git a/Documentation/spi/index.rst b/Documentation/spi/index.rst
index 06c34ea11bcf..a8f4f5cd0f09 100644
--- a/Documentation/spi/index.rst
+++ b/Documentation/spi/index.rst
@@ -13,6 +13,7 @@  Serial Peripheral Interface (SPI)
    pxa2xx
    spi-lm70llp
    spi-sc18is602
+   spi-mockup
 
 .. only::  subproject and html
 
diff --git a/Documentation/spi/spi-mockup.rst b/Documentation/spi/spi-mockup.rst
new file mode 100644
index 000000000000..b38f44dca785
--- /dev/null
+++ b/Documentation/spi/spi-mockup.rst
@@ -0,0 +1,201 @@ 
+==========
+spi-mockup
+==========
+
+Description
+===========
+
+This module is a very simple fake SPI controller driver. It implements
+a BPF based interface to mockup SPI device.
+
+No hardware is needed nor associated with this module. It will respond
+spi message by BPF program attached to spi_transfer_writeable tracepoint
+by reading from or writing BPF maps.
+
+The typical use-case is like this:
+        1. load this module
+        2. use bpftool to load BPF program
+        3. load the target chip driver module
+
+Example
+=======
+
+This example show how to mock a MTD device by using spi-mockup driver.
+
+Compile your copy of the kernel source. Make sure to configure the spi-mockup
+and the target chip driver as a module. Prepare a dts described the spi-mockup
+device.
+
+::
+
+  /dts-v1/;
+
+  / {
+      spi: spi {
+          compatible = "spi-mockup";
+
+          #address-cells = <1>;
+          #size-cells = <0>;
+      };
+  }
+
+Write a BPF program as device's backup.
+
+::
+
+  #define MCHP23K256_CMD_WRITE_STATUS	0x01
+  #define MCHP23K256_CMD_WRITE		0x02
+  #define MCHP23K256_CMD_READ		0x03
+
+  #define CHIP_REGS_SIZE			0x20000
+
+  #define MAX_CMD_SIZE			4
+
+  struct {
+  	__uint(type, BPF_MAP_TYPE_ARRAY);
+  	__uint(max_entries, CHIP_REGS_SIZE);
+  	__type(key, __u32);
+  	__type(value, __u8);
+  } regs_mtd_mchp23k256 SEC(".maps");
+
+  static unsigned int chip_reg = 0;
+
+  static int spi_transfer_read(struct spi_msg_ctx *msg, unsigned int len)
+  {
+  	int i, key;
+  	u8 *reg;
+
+  	for (i = 0; i < len && i < sizeof(msg->data); i++) {
+  		key = i + chip_reg;
+
+  		reg = bpf_map_lookup_elem(&regs_mtd_mchp23k256, &key);
+  		if (!reg) {
+  			bpf_printk("key %d not exists", key);
+  			return -EINVAL;
+  		}
+
+  		msg->data[i] = *reg;
+  	}
+
+  	return 0;
+  }
+
+  static int spi_transfer_write(struct spi_msg_ctx *msg, unsigned int len)
+  {
+  	u8 opcode = msg->data[0], value;
+  	int i, key;
+
+  	switch (opcode) {
+  	case MCHP23K256_CMD_READ:
+  	case MCHP23K256_CMD_WRITE:
+  		if (len < 2)
+  			return -EINVAL;
+
+  		chip_reg = 0;
+  		for (i = 0; i < MAX_CMD_SIZE && i < len - 1; i++)
+  			chip_reg = (chip_reg << 8) + msg->data[1 + i];
+
+  		return 0;
+  	case MCHP23K256_CMD_WRITE_STATUS:
+  		// ignore write status
+  		return 0;
+  	default:
+  		break;
+  	}
+
+  	for (i = 0; i < len && i < sizeof(msg->data); i++) {
+  		value = msg->data[i];
+  		key = chip_reg + i;
+
+  		if (bpf_map_update_elem(&regs_mtd_mchp23k256, &key, &value,
+  					BPF_EXIST)) {
+  			bpf_printk("key %d not exists", key);
+  			return -EINVAL;
+  		}
+  	}
+
+  	return 0;
+  }
+
+  SEC("raw_tp.w/spi_transfer_writeable")
+  int BPF_PROG(mtd_mchp23k256, struct spi_msg_ctx *msg, u8 chip,
+  	     unsigned int len, u8 tx_nbits, u8 rx_nbits)
+  {
+  	int ret = 0;
+
+  	if (tx_nbits)
+  		ret = spi_transfer_write(msg, len);
+  	else if (rx_nbits)
+  		ret = spi_transfer_read(msg, len);
+
+  	return ret;
+  }
+
+  char LICENSE[] SEC("license") = "GPL";
+
+
+Then boot a qemu instance by the following command:
+
+::
+
+  sudo qemu-system-x86_64 -m 4096 -smp 4 -display none -serial stdio -no-reboot \
+  -enable-kvm -cpu host,migratable=off -dtb mocktest.dtb -snapshot -hda linux.img \
+  -kernel arch/x86/boot/bzImage \
+  -append "earlyprintk=serial root=/dev/sda console=ttyS0"
+
+
+Use bpftool to load the BPF program.
+
+::
+
+  bpftool prog load mtd-mchp23k256.o /sys/fs/bpf/test_prog
+  bpftool perf attach name mtd_mchp23k256 spi_transfer_writeable /sys/fs/bpf/test_perf
+
+
+Load the target chip driver module. This is accomplished by executing the
+following command:
+
+::
+
+  $ echo mchp23k256 0 > /sys/class/spi_master/spi0/new_device
+
+
+The name of the target driver and its chip select were used to instantiate
+the device.
+
+Now, the mchp23k256 MTD device named /dev/mtd0 has been created successfully.
+
+::
+
+  $ ls /sys/bus/spi/devices/spi0.0/mtd/
+  mtd0  mtd0ro
+
+  $ cat /sys/class/mtd/mtd0/name
+  spi0.0
+
+  $ hexdump /dev/mtd0
+  0000000 0000 0000 0000 0000 0000 0000 0000 0000
+  *
+  0008000
+
+  $echo aaaa > /dev/mtd0
+
+  $ hexdump /dev/mtd0
+  0000000 6161 6161 000a 0000 0000 0000 0000 0000
+  0000010 0000 0000 0000 0000 0000 0000 0000 0000
+  *
+  0008000
+
+  $ bpftool map update name mtd_mchp23k256_ key 0 0 0 0 value 0
+
+  $ hexdump /dev/mtd0
+  0000000 6100 6161 000a 0000 0000 0000 0000 0000
+  0000010 0000 0000 0000 0000 0000 0000 0000 0000
+  *
+  0008000
+
+Remove the mockup device by executing the following command:
+
+::
+
+  $echo 0 > /sys/class/spi_master/spi0/delete_device