[PATCH v5 1/3] cmd: Add command to display or save Linux PStore dumps

["Frédéric Danis" <frederic.danis@collabora.com>]

This patch adds a new pstore command allowing to display or save ramoops
logs (oops, panic, console, ftrace and user) generated by a previous
kernel crash.
PStore parameters can be set in U-Boot configuration file, or at run-time
using "pstore set" command. Records size should be the same as the ones
used by kernel, and should be a power of 2.
This command allows:
- to display uncompressed logs
- to save compressed or uncompressed logs, compressed logs are saved as a
  compressed stream, it may need some work to be able to decompress it,
  e.g. adding a fake header:
  "printf "\x1f\x8b\x08\x00\x00\x00\x00\x00\x00\x00" |
  cat - dmesg-ramoops-0.enc.z | gzip -dc"
- ECC part is not used to check memory corruption
- only 1st FTrace log is displayed or saved

Signed-off-by: Fr?d?ric Danis <frederic.danis@collabora.com>
Cc: Tom Rini <trini@konsulko.com>
Cc: Heinrich Schuchardt <xypron.glpk@gmx.de>
Cc: Wolfgang Denk <wd@denx.de>
Cc: Heiko Schocher <hs@denx.de>
---
Changes in v5: None
Changes in v4: None

Changes in v3:
- Add default value for PStore memory size
- Remove default value of PStore memory address
- Update config entry helps
- Replace calls to debug() by log_debug()
- Update documentation

Changes in v2:
- Fix 64bit mode build warnings
- Add documentation
- Add function description comments
- Replace calls to pr_debug() by debug()

 cmd/Kconfig    |  71 +++++++
 cmd/Makefile   |   1 +
 cmd/pstore.c   | 505 +++++++++++++++++++++++++++++++++++++++++++++++++
 doc/index.rst  |   7 +
 doc/pstore.rst |  74 ++++++++
 5 files changed, 658 insertions(+)
 create mode 100644 cmd/pstore.c
 create mode 100644 doc/pstore.rst

diff --git a/cmd/Kconfig b/cmd/Kconfig
index 6403bc45a5..cd202bf7fb 100644
--- a/cmd/Kconfig
+++ b/cmd/Kconfig
@@ -1736,6 +1736,77 @@ config CMD_QFW
 	  feature is to allow easy loading of files passed to qemu-system
 	  via -kernel / -initrd
 
+config CMD_PSTORE
+	bool "pstore"
+	help
+	  This provides access to Linux PStore with Rammoops backend. The main
+	  feature is to allow to display or save PStore records.
+
+	  See doc/pstore.rst for more information.
+
+if CMD_PSTORE
+
+config CMD_PSTORE_MEM_ADDR
+	hex "Memory Address"
+	depends on CMD_PSTORE
+	help
+	  Base addr used for PStore ramoops memory, should be identical to
+	  ramoops.mem_address parameter used by kernel
+
+config CMD_PSTORE_MEM_SIZE
+	hex "Memory size"
+	depends on CMD_PSTORE
+	default "0x10000"
+	help
+	  Size of PStore ramoops memory, should be identical to ramoops.mem_size
+	  parameter used by kernel, a power of 2 and larger than the sum of the
+	  record sizes
+
+config CMD_PSTORE_RECORD_SIZE
+	hex "Dump record size"
+	depends on CMD_PSTORE
+	default "0x1000"
+	help
+	  Size of each dump done on oops/panic, should be identical to
+	  ramoops.record_size parameter used by kernel and a power of 2
+	  Must be non-zero
+
+config CMD_PSTORE_CONSOLE_SIZE
+	hex "Kernel console log size"
+	depends on CMD_PSTORE
+	default "0x1000"
+	help
+	  Size of kernel console log, should be identical to
+	  ramoops.console_size parameter used by kernel and a power of 2
+	  Must be non-zero
+
+config CMD_PSTORE_FTRACE_SIZE
+	hex "FTrace log size"
+	depends on CMD_PSTORE
+	default "0x1000"
+	help
+	  Size of ftrace log, should be identical to ramoops.ftrace_size
+	  parameter used by kernel and a power of 2
+
+config CMD_PSTORE_PMSG_SIZE
+	hex "User space message log size"
+	depends on CMD_PSTORE
+	default "0x1000"
+	help
+	  Size of user space message log, should be identical to
+	  ramoops.pmsg_size parameter used by kernel and a power of 2
+
+config CMD_PSTORE_ECC_SIZE
+	int "ECC size"
+	depends on CMD_PSTORE
+	default "0"
+	help
+	if non-zero, the option enables ECC support and specifies ECC buffer
+	size in bytes (1 is a special value, means 16 bytes ECC), should be
+	identical to ramoops.ramoops_ecc parameter used by kernel
+
+endif
+
 source "cmd/mvebu/Kconfig"
 
 config CMD_TERMINAL
diff --git a/cmd/Makefile b/cmd/Makefile
index f1dd513a4b..06d7ad7375 100644
--- a/cmd/Makefile
+++ b/cmd/Makefile
@@ -110,6 +110,7 @@ obj-$(CONFIG_CMD_PCI) += pci.o
 endif
 obj-$(CONFIG_CMD_PINMUX) += pinmux.o
 obj-$(CONFIG_CMD_PMC) += pmc.o
+obj-$(CONFIG_CMD_PSTORE) += pstore.o
 obj-$(CONFIG_CMD_PXE) += pxe.o pxe_utils.o
 obj-$(CONFIG_CMD_WOL) += wol.o
 obj-$(CONFIG_CMD_QFW) += qfw.o
diff --git a/cmd/pstore.c b/cmd/pstore.c
new file mode 100644
index 0000000000..4e4d70d604
--- /dev/null
+++ b/cmd/pstore.c
@@ -0,0 +1,505 @@
+// SPDX-License-Identifier: GPL-2.0+
+/*
+ * Copyright ? 2019 Collabora Ltd
+ */
+
+#include <config.h>
+#include <common.h>
+#include <fs.h>
+#include <mapmem.h>
+#include <memalign.h>
+#include <part.h>
+
+struct persistent_ram_buffer {
+	u32    sig;
+	u32    start;
+	u32    size;
+	u8     data[0];
+};
+
+#define PERSISTENT_RAM_SIG (0x43474244) /* DBGC */
+#define RAMOOPS_KERNMSG_HDR "===="
+
+#define PSTORE_TYPE_DMESG 0
+#define PSTORE_TYPE_CONSOLE 2
+#define PSTORE_TYPE_FTRACE 3
+#define PSTORE_TYPE_PMSG 7
+#define PSTORE_TYPE_ALL 255
+
+static phys_addr_t pstore_addr = CONFIG_CMD_PSTORE_MEM_ADDR;
+static phys_size_t pstore_length = CONFIG_CMD_PSTORE_MEM_SIZE;
+static unsigned int pstore_record_size = CONFIG_CMD_PSTORE_RECORD_SIZE;
+static unsigned int pstore_console_size = CONFIG_CMD_PSTORE_CONSOLE_SIZE;
+static unsigned int pstore_ftrace_size = CONFIG_CMD_PSTORE_FTRACE_SIZE;
+static unsigned int pstore_pmsg_size = CONFIG_CMD_PSTORE_PMSG_SIZE;
+static unsigned int pstore_ecc_size = CONFIG_CMD_PSTORE_ECC_SIZE;
+static unsigned int buffer_size;
+
+ /**
+  * pstore_read_kmsg_hdr() - Check kernel header and get compression flag if
+  *                          available.
+  * @buffer: Kernel messages buffer.
+  * @compressed: Returns TRUE if kernel buffer is compressed, else FALSE.
+  *
+  * Check if buffer starts with a kernel header of the form:
+  *   ====<secs>.<nsecs>[-<compression>]\n
+  * If <compression> is equal to 'C' then the buffer is compressed, else iter
+  * should be 'D'.
+  *
+  * Return: Length of kernel header.
+  */
+static int pstore_read_kmsg_hdr(char *buffer, bool *compressed)
+{
+	char *ptr = buffer;
+	*compressed = false;
+
+	if (strncmp(RAMOOPS_KERNMSG_HDR, ptr, strlen(RAMOOPS_KERNMSG_HDR)) != 0)
+		return 0;
+
+	ptr += strlen(RAMOOPS_KERNMSG_HDR);
+
+	ptr = strchr(ptr, '\n');
+	if (!ptr)
+		return 0;
+
+	if (ptr[-2] == '-' && ptr[-1] == 'C')
+		*compressed = true;
+
+	return ptr - buffer + 1;
+}
+
+/**
+ * pstore_get_buffer() - Get unwrapped record buffer
+ * @sig: Signature to check
+ * @buffer: Buffer containing wrapped record
+ * @size: wrapped record size
+ * @dest: Buffer used to store unwrapped record
+ *
+ * The record starts with <signature><start><size> header.
+ * The signature is 'DBGC' for all records except for Ftrace's record(s) wich
+ * use LINUX_VERSION_CODE ^ 'DBGC'.
+ * Use 0 for @sig to prevent checking signature.
+ * Start and size are 4 bytes long.
+ *
+ * Return: record's length
+ */
+static u32 pstore_get_buffer(u32 sig, phys_addr_t buffer, u32 size, char *dest)
+{
+	struct persistent_ram_buffer *prb =
+		(struct persistent_ram_buffer *)map_sysmem(buffer, size);
+	u32 dest_size;
+
+	if (sig == 0 || prb->sig == sig) {
+		if (prb->size == 0) {
+			log_debug("found existing empty buffer\n");
+			return 0;
+		}
+
+		if (prb->size > size) {
+			log_debug("found existing invalid buffer, size %u, start %u\n",
+			          prb->size, prb->start);
+			return 0;
+		}
+	} else {
+		log_debug("no valid data in buffer (sig = 0x%08x)\n", prb->sig);
+		return 0;
+	}
+
+	log_debug("found existing buffer, size %u, start %u\n",
+	          prb->size, prb->start);
+
+	memcpy(dest, &prb->data[prb->start], prb->size - prb->start);
+	memcpy(dest + prb->size - prb->start, &prb->data[0], prb->start);
+
+	dest_size = prb->size;
+	unmap_sysmem(prb);
+
+	return dest_size;
+}
+
+/**
+ * pstore_init_buffer_size() - Init buffer size to largest record size
+ *
+ * Records, console, FTrace and user logs can use different buffer sizes.
+ * This function allows to retrieve the biggest one.
+ */
+static void pstore_init_buffer_size(void)
+{
+	if (pstore_record_size > buffer_size)
+		buffer_size = pstore_record_size;
+
+	if (pstore_console_size > buffer_size)
+		buffer_size = pstore_console_size;
+
+	if (pstore_ftrace_size > buffer_size)
+		buffer_size = pstore_ftrace_size;
+
+	if (pstore_pmsg_size > buffer_size)
+		buffer_size = pstore_pmsg_size;
+}
+
+/**
+ * pstore_set() - Initialize PStore settings from command line arguments
+ * @cmdtp: Command data struct pointer
+ * @flag: Command flag
+ * @argc: Command-line argument count
+ * @argv: Array of command-line arguments
+ *
+ * Set pstore reserved memory info, starting at 'addr' for 'len' bytes.
+ * Default length for records is 4K.
+ * Mandatory arguments:
+ * - addr: ramoops starting address
+ * - len: ramoops total length
+ * Optional arguments:
+ * - record-size: size of one panic or oops record ('dump' type)
+ * - console-size: size of the kernel logs record
+ * - ftrace-size: size of the ftrace record(s), this can be a single record or
+ *                divided in parts based on number of CPUs
+ * - pmsg-size: size of the user space logs record
+ * - ecc-size: enables/disables ECC support and specifies ECC buffer size in
+ *             bytes (0 disables it, 1 is a special value, means 16 bytes ECC)
+ *
+ * Return: zero on success, CMD_RET_USAGE in case of misuse and negative
+ * on error.
+ */
+static int pstore_set(cmd_tbl_t *cmdtp, int flag,  int argc,
+		      char * const argv[])
+{
+	if (argc < 3)
+		return CMD_RET_USAGE;
+
+	/* Address is specified since argc > 2
+	 */
+	pstore_addr = simple_strtoul(argv[1], NULL, 16);
+
+	/* Length is specified since argc > 2
+	 */
+	pstore_length = simple_strtoul(argv[2], NULL, 16);
+
+	if (argc > 3)
+		pstore_record_size = simple_strtoul(argv[3], NULL, 16);
+
+	if (argc > 4)
+		pstore_console_size = simple_strtoul(argv[4], NULL, 16);
+
+	if (argc > 5)
+		pstore_ftrace_size = simple_strtoul(argv[5], NULL, 16);
+
+	if (argc > 6)
+		pstore_pmsg_size = simple_strtoul(argv[6], NULL, 16);
+
+	if (argc > 7)
+		pstore_ecc_size = simple_strtoul(argv[7], NULL, 16);
+
+	if (pstore_length < (pstore_record_size + pstore_console_size
+			     + pstore_ftrace_size + pstore_pmsg_size)) {
+		printf("pstore <len> should be larger than the sum of all records sizes\n");
+		pstore_length = 0;
+	}
+
+	log_debug("pstore set done: start 0x%08llx - length 0x%llx\n",
+	          (unsigned long long)pstore_addr,
+	          (unsigned long long)pstore_length);
+
+	return 0;
+}
+
+/**
+ * pstore_print_buffer() - Print buffer
+ * @type: buffer type
+ * @buffer: buffer to print
+ * @size: buffer size
+ *
+ * Print buffer type and content
+ */
+static void pstore_print_buffer(char *type, char *buffer, u32 size)
+{
+	u32 i = 0;
+
+	printf("**** %s\n", type);
+	while (i < size && buffer[i] != 0) {
+		putc(buffer[i]);
+		i++;
+	}
+}
+
+/**
+ * pstore_display() - Display existing records in pstore reserved memory
+ * @cmdtp: Command data struct pointer
+ * @flag: Command flag
+ * @argc: Command-line argument count
+ * @argv: Array of command-line arguments
+ *
+ * A 'record-type' can be given to only display records of this kind.
+ * If no 'record-type' is given, all valid records are dispayed.
+ * 'record-type' can be one of 'dump', 'console', 'ftrace' or 'user'. For 'dump'
+ * and 'ftrace' types, a 'nb' can be given to only display one record.
+ *
+ * Return: zero on success, CMD_RET_USAGE in case of misuse and negative
+ * on error.
+ */
+static int pstore_display(cmd_tbl_t *cmdtp, int flag,  int argc,
+			  char * const argv[])
+{
+	int type = PSTORE_TYPE_ALL;
+	phys_addr_t ptr;
+	char *buffer;
+	u32 size;
+	int header_len = 0;
+	bool compressed;
+
+	if (argc > 1) {
+		if (!strcmp(argv[1], "dump"))
+			type = PSTORE_TYPE_DMESG;
+		else if (!strcmp(argv[1], "console"))
+			type = PSTORE_TYPE_CONSOLE;
+		else if (!strcmp(argv[1], "ftrace"))
+			type = PSTORE_TYPE_FTRACE;
+		else if (!strcmp(argv[1], "user"))
+			type = PSTORE_TYPE_PMSG;
+		else
+			return CMD_RET_USAGE;
+	}
+
+	if (pstore_length == 0) {
+		printf("Please set PStore configuration\n");
+		return CMD_RET_USAGE;
+	}
+
+	if (buffer_size == 0)
+		pstore_init_buffer_size();
+
+	buffer = malloc_cache_aligned(buffer_size);
+
+	if (type == PSTORE_TYPE_DMESG || type == PSTORE_TYPE_ALL) {
+		ptr = pstore_addr;
+		phys_addr_t ptr_end = ptr + pstore_length - pstore_pmsg_size
+				- pstore_ftrace_size - pstore_console_size;
+
+		if (argc > 2) {
+			ptr += simple_strtoul(argv[2], NULL, 10)
+				* pstore_record_size;
+			ptr_end = ptr + pstore_record_size;
+		}
+
+		while (ptr < ptr_end) {
+			size = pstore_get_buffer(PERSISTENT_RAM_SIG, ptr,
+						 pstore_record_size, buffer);
+			ptr += pstore_record_size;
+
+			if (size == 0)
+				continue;
+
+			header_len = pstore_read_kmsg_hdr(buffer, &compressed);
+			if (header_len == 0) {
+				log_debug("no valid kernel header\n");
+				continue;
+			}
+
+			if (compressed) {
+				printf("Compressed buffer, display not available\n");
+				continue;
+			}
+
+			pstore_print_buffer("Dump", buffer + header_len,
+					    size - header_len);
+		}
+	}
+
+	if (type == PSTORE_TYPE_CONSOLE || type == PSTORE_TYPE_ALL) {
+		ptr = pstore_addr + pstore_length - pstore_pmsg_size
+			- pstore_ftrace_size - pstore_console_size;
+		size = pstore_get_buffer(PERSISTENT_RAM_SIG, ptr,
+					 pstore_console_size, buffer);
+		if (size != 0)
+			pstore_print_buffer("Console", buffer, size);
+	}
+
+	if (type == PSTORE_TYPE_FTRACE || type == PSTORE_TYPE_ALL) {
+		ptr = pstore_addr + pstore_length - pstore_pmsg_size
+		- pstore_ftrace_size;
+		/* The FTrace record(s) uses LINUX_VERSION_CODE ^ 'DBGC'
+		 * signature, pass 0 to pstore_get_buffer to prevent
+		 * checking it
+		 */
+		size = pstore_get_buffer(0, ptr, pstore_ftrace_size, buffer);
+		if (size != 0)
+			pstore_print_buffer("FTrace", buffer, size);
+	}
+
+	if (type == PSTORE_TYPE_PMSG || type == PSTORE_TYPE_ALL) {
+		ptr = pstore_addr + pstore_length - pstore_pmsg_size;
+		size = pstore_get_buffer(PERSISTENT_RAM_SIG, ptr,
+					 pstore_pmsg_size, buffer);
+		if (size != 0)
+			pstore_print_buffer("User", buffer, size);
+	}
+
+	free(buffer);
+
+	return 0;
+}
+
+/**
+ * pstore_save() - Save existing records from pstore reserved memory
+ * @cmdtp: Command data struct pointer
+ * @flag: Command flag
+ * @argc: Command-line argument count
+ * @argv: Array of command-line arguments
+ *
+ * the records are saved under 'directory path', which should already exist,
+ * to partition 'part' on device type 'interface' instance 'dev'
+ * Filenames are automatically generated, depending on record type, like in
+ * /sys/fs/pstore under Linux
+ *
+ * Return: zero on success, CMD_RET_USAGE in case of misuse and negative
+ * on error.
+ */
+static int pstore_save(cmd_tbl_t *cmdtp, int flag,  int argc,
+		       char * const argv[])
+{
+	phys_addr_t ptr, ptr_end;
+	char *buffer;
+	char *save_argv[6];
+	char addr[19], length[19];
+	char path[256];
+	u32 size;
+	unsigned int index;
+	int header_len = 0;
+	bool compressed;
+
+	if (argc < 4)
+		return CMD_RET_USAGE;
+
+	if (pstore_length == 0) {
+		printf("Please set PStore configuration\n");
+		return CMD_RET_USAGE;
+	}
+
+	if (buffer_size == 0)
+		pstore_init_buffer_size();
+
+	buffer = malloc_cache_aligned(buffer_size);
+	sprintf(addr, "0x%p", buffer);
+
+	save_argv[0] = argv[0];
+	save_argv[1] = argv[1];
+	save_argv[2] = argv[2];
+	save_argv[3] = addr;
+	save_argv[4] = path;
+	save_argv[5] = length;
+
+	/* Save all Dump records */
+	ptr = pstore_addr;
+	ptr_end = ptr + pstore_length - pstore_pmsg_size - pstore_ftrace_size
+				- pstore_console_size;
+	index = 0;
+	while (ptr < ptr_end) {
+		size = pstore_get_buffer(PERSISTENT_RAM_SIG, ptr,
+					 pstore_record_size, buffer);
+		ptr += pstore_record_size;
+
+		if (size == 0)
+			continue;
+
+		header_len = pstore_read_kmsg_hdr(buffer, &compressed);
+		if (header_len == 0) {
+			log_debug("no valid kernel header\n");
+			continue;
+		}
+
+		sprintf(addr, "0x%08lx", (ulong)map_to_sysmem(buffer + header_len));
+		sprintf(length, "0x%X", size - header_len);
+		sprintf(path, "%s/dmesg-ramoops-%u%s", argv[3], index,
+			compressed ? ".enc.z" : "");
+		do_save(cmdtp, flag, 6, save_argv, FS_TYPE_ANY);
+		index++;
+	}
+
+	sprintf(addr, "0x%08lx", (ulong)map_to_sysmem(buffer));
+
+	/* Save Console record */
+	size = pstore_get_buffer(PERSISTENT_RAM_SIG, ptr, pstore_console_size,
+				 buffer);
+	if (size != 0) {
+		sprintf(length, "0x%X", size);
+		sprintf(path, "%s/console-ramoops-0", argv[3]);
+		do_save(cmdtp, flag, 6, save_argv, FS_TYPE_ANY);
+	}
+	ptr += pstore_console_size;
+
+	/* Save FTrace record(s)
+	 * The FTrace record(s) uses LINUX_VERSION_CODE ^ 'DBGC' signature,
+	 * pass 0 to pstore_get_buffer to prevent checking it
+	 */
+	size = pstore_get_buffer(0, ptr, pstore_ftrace_size, buffer);
+	if (size != 0) {
+		sprintf(length, "0x%X", size);
+		sprintf(path, "%s/ftrace-ramoops-0", argv[3]);
+		do_save(cmdtp, flag, 6, save_argv, FS_TYPE_ANY);
+	}
+	ptr += pstore_ftrace_size;
+
+	/* Save Console record */
+	size = pstore_get_buffer(PERSISTENT_RAM_SIG, ptr, pstore_pmsg_size,
+				 buffer);
+	if (size != 0) {
+		sprintf(length, "0x%X", size);
+		sprintf(path, "%s/pmsg-ramoops-0", argv[3]);
+		do_save(cmdtp, flag, 6, save_argv, FS_TYPE_ANY);
+	}
+
+	free(buffer);
+
+	return 0;
+}
+
+static cmd_tbl_t cmd_pstore_sub[] = {
+	U_BOOT_CMD_MKENT(set, 8, 0, pstore_set, "", ""),
+	U_BOOT_CMD_MKENT(display, 3, 0, pstore_display, "", ""),
+	U_BOOT_CMD_MKENT(save, 4, 0, pstore_save, "", ""),
+};
+
+static int do_pstore(cmd_tbl_t *cmdtp, int flag, int argc, char * const argv[])
+{
+	cmd_tbl_t *c;
+
+	if (argc < 2)
+		return CMD_RET_USAGE;
+
+	/* Strip off leading argument */
+	argc--;
+	argv++;
+
+	c = find_cmd_tbl(argv[0], cmd_pstore_sub, ARRAY_SIZE(cmd_pstore_sub));
+
+	if (!c)
+		return CMD_RET_USAGE;
+
+	return c->cmd(cmdtp, flag, argc, argv);
+}
+
+U_BOOT_CMD(pstore, 10, 0, do_pstore,
+	   "Manage Linux Persistent Storage",
+	   "set <addr> <len> [record-size] [console-size] [ftrace-size] [pmsg_size] [ecc-size]\n"
+	   "- Set pstore reserved memory info, starting at 'addr' for 'len' bytes.\n"
+	   "  Default length for records is 4K.\n"
+	   "  'record-size' is the size of one panic or oops record ('dump' type).\n"
+	   "  'console-size' is the size of the kernel logs record.\n"
+	   "  'ftrace-size' is the size of the ftrace record(s), this can be a single\n"
+	   "  record or divided in parts based on number of CPUs.\n"
+	   "  'pmsg-size' is the size of the user space logs record.\n"
+	   "  'ecc-size' enables/disables ECC support and specifies ECC buffer size in\n"
+	   "  bytes (0 disables it, 1 is a special value, means 16 bytes ECC).\n"
+	   "pstore display [record-type] [nb]\n"
+	   "- Display existing records in pstore reserved memory. A 'record-type' can\n"
+	   "  be given to only display records of this kind. 'record-type' can be one\n"
+	   "  of 'dump', 'console', 'ftrace' or 'user'. For 'dump' and 'ftrace' types,\n"
+	   "  a 'nb' can be given to only display one record.\n"
+	   "pstore save <interface> <dev[:part]> <directory-path>\n"
+	   "- Save existing records in pstore reserved memory under 'directory path'\n"
+	   "  to partition 'part' on device type 'interface' instance 'dev'.\n"
+	   "  Filenames are automatically generated, depending on record type, like\n"
+	   "  in /sys/fs/pstore under Linux.\n"
+	   "  The 'directory-path' should already exist.\n"
+);
diff --git a/doc/index.rst b/doc/index.rst
index cd98be6cc5..c556cdb607 100644
--- a/doc/index.rst
+++ b/doc/index.rst
@@ -98,6 +98,13 @@ Android-specific features available in U-Boot.
 
    android/index
 
+Command line
+------------
+.. toctree::
+   :maxdepth: 2
+
+   pstore.rst
+
 Indices and tables
 ==================
 
diff --git a/doc/pstore.rst b/doc/pstore.rst
new file mode 100644
index 0000000000..90a8e1c2cb
--- /dev/null
+++ b/doc/pstore.rst
@@ -0,0 +1,74 @@
+.. SPDX-License-Identifier: GPL-2.0+
+
+PStore command
+==============
+
+Design
+------
+
+Linux PStore and Ramoops modules (Linux config options PSTORE and PSTORE_RAM)
+allow to use memory to pass data from the dying breath of a crashing kernel to
+its successor. This command allows to read those records from U-Boot command
+line.
+
+Ramoops is an oops/panic logger that writes its logs to RAM before the system
+crashes. It works by logging oopses and panics in a circular buffer. Ramoops
+needs a system with persistent RAM so that the content of that area can survive
+after a restart.
+
+Ramoops uses a predefined memory area to store the dump.
+
+Ramoops parameters can be passed as kernel parameters or through Device Tree,
+i.e.::
+ ramoops.mem_address=0x30000000 ramoops.mem_size=0x100000 ramoops.record_size=0x2000 ramoops.console_size=0x2000 memmap=0x100000$0x30000000
+
+The same values should be set in U-Boot to be able to retrieve the records.
+This values can be set at build time in U-Boot configuration file, or at runtime.
+
+The PStore configuration parameters are:
+
+======================= ==========
+ Name                   Default
+======================= ==========
+CMD_PSTORE_MEM_ADDR
+CMD_PSTORE_MEM_SIZE     0x10000
+CMD_PSTORE_RECORD_SIZE  0x1000
+CMD_PSTORE_CONSOLE_SIZE 0x1000
+CMD_PSTORE_FTRACE_SIZE  0x1000
+CMD_PSTORE_PMSG_SIZE    0x1000
+CMD_PSTORE_ECC_SIZE     0
+======================= ==========
+
+Records sizes should be a power of 2.
+The memory size and the record/console size must be non-zero.
+
+Multiple 'dump' records can be stored in the memory reserved for PStore.
+The memory size has to be larger than the sum of the record sizes, i.e.::
+ MEM_SIZE >= RECORD_SIZE * n + CONSOLE_SIZE + FTRACE_SIZE + PMSG_SIZE
+
+Usage
+-----
+
+Generate kernel crash
+~~~~~~~~~~~~~~~~~~~~~
+
+For test purpose, you can generate a kernel crash by setting reboot timeout to
+10 seconds and trigger a panic::
+ $ sudo sh -c "echo 1 > /proc/sys/kernel/sysrq"
+ $ sudo sh -c "echo 10 > /proc/sys/kernel/panic"
+ $ sudo sh -c "echo c > /proc/sysrq-trigger"
+
+Retrieve logs in U-Boot
+~~~~~~~~~~~~~~~~~~~~~~~
+
+First of all, unless PStore parameters as been set during U-Boot configuration
+and match kernel ramoops parameters, it needs to be set using 'pstore set', e.g.::
+ => pstore set 0x30000000 0x100000 0x2000 0x2000
+
+Then all available dumps can be displayed
+using::
+ => pstore display
+
+Or saved to an existing directory in an Ext2 or Ext4 partition, e.g. on root
+directory of 1st partition of the 2nd MMC::
+ => pstore save mmc 1:1 /
-- 
2.18.0