docs: mtd: spi-nor: add sections about flash additions and testing

Add sections about how to propose a new flash addition and about the
minimum testing requirements.

Reviewed-by: Michael Walle <[e-mail]>
Reviewed-by: Pratyush Yadav <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Tudor Ambarus <[e-mail]>

Documentation/driver-api/mtd/spi-nor.rst

···
 initialize the necessary fields for spi_nor{}. Please see
 drivers/mtd/spi-nor/spi-nor.c for detail. Please also refer to spi-fsl-qspi.c
 when you want to write a new driver for a SPI NOR controller.
+
+How to propose a new flash addition
+-----------------------------------
+
+Most SPI NOR flashes comply with the JEDEC JESD216
+Serial Flash Discoverable Parameter (SFDP) standard. SFDP describes
+the functional and feature capabilities of serial flash devices in a
+standard set of internal read-only parameter tables.
+
+The SPI NOR driver queries the SFDP tables in order to determine the
+flash's parameters and settings. If the flash defines the SFDP tables
+it's likely that you won't need a flash entry at all, and instead
+rely on the generic flash driver which probes the flash solely based
+on its SFDP data. All one has to do is to specify the "jedec,spi-nor"
+compatible in the device tree.
+
+There are cases however where you need to define an explicit flash
+entry. This typically happens when the flash has settings or support
+that is not covered by the SFDP tables (e.g. Block Protection), or
+when the flash contains mangled SFDP data. If the later, one needs
+to implement the ``spi_nor_fixups`` hooks in order to amend the SFDP
+parameters with the correct values.
+
+Minimum testing requirements
+-----------------------------
+
+Do all the tests from below and paste them in the commit's comments
+section, after the ``---`` marker.
+
+1) Specify the controller that you used to test the flash and specify
+   the frequency at which the flash was operated, e.g.::
+
+    This flash is populated on the X board and was tested at Y
+    frequency using the Z (put compatible) SPI controller.
+
+2) Dump the sysfs entries and print the md5/sha1/sha256 SFDP checksum::
+
+    root@1:~# cat /sys/bus/spi/devices/spi0.0/spi-nor/partname
+    sst26vf064b
+    root@1:~# cat /sys/bus/spi/devices/spi0.0/spi-nor/jedec_id
+    bf2643
+    root@1:~# cat /sys/bus/spi/devices/spi0.0/spi-nor/manufacturer
+    sst
+    root@1:~# xxd -p /sys/bus/spi/devices/spi0.0/spi-nor/sfdp
+    53464450060102ff00060110300000ff81000106000100ffbf0001180002
+    0001fffffffffffffffffffffffffffffffffd20f1ffffffff0344eb086b
+    083b80bbfeffffffffff00ffffff440b0c200dd80fd810d820914824806f
+    1d81ed0f773830b030b0f7ffffff29c25cfff030c080ffffffffffffffff
+    ffffffffffffffffffffffffffffffffffffffffffffffffffffffffffff
+    ffffffffffffffffffffffffffffffffffffffffffffffffffffffffffff
+    ffffffffffffffffffffffffffffffffffffffffffffffffffffffffffff
+    ffffffffffffffffffffffffffffffffffffffffffffffffffffffffffff
+    ffffffffffffffffffffffffffffffffff0004fff37f0000f57f0000f9ff
+    7d00f57f0000f37f0000ffffffffffffffffffffffffffffffffffffffff
+    ffffffffffffffffffffffffffffffffffffffffffffffffffffffffffff
+    ffffffffffffffffffffffffffffffffffffffffffffffffffffffffffff
+    ffffffffffffffffffffffffffffffffffffffffffffffffffffffffffff
+    ffffffffffffffffffffffffffffffffffffffffffffffffffffffffffff
+    ffffffffffffffffffffffffffffffffffffffffffffffffffffffffffff
+    ffffffffffffffffffffffffffffffffffffffffffffffffffffffffffff
+    ffffffffffffffffffffffffffffffffffffffffffffffffffffffffffff
+    ffffbf2643ffb95ffdff30f260f332ff0a122346ff0f19320f1919ffffff
+    ffffffff00669938ff05013506040232b03072428de89888a585c09faf5a
+    ffff06ec060c0003080bffffffffff07ffff0202ff060300fdfd040700fc
+    0300fefe0202070e
+    root@1:~# sha256sum /sys/bus/spi/devices/spi0.0/spi-nor/sfdp
+    428f34d0461876f189ac97f93e68a05fa6428c6650b3b7baf736a921e5898ed1  /sys/bus/spi/devices/spi0.0/spi-nor/sfdp
+
+   Please dump the SFDP tables using ``xxd -p``. It enables us to do
+   the reverse operation and convert the hexdump to binary with
+   ``xxd -rp``. Dumping the SFDP data with ``hexdump -Cv`` is accepted,
+   but less desirable.
+
+3) Dump debugfs data::
+
+    root@1:~# cat /sys/kernel/debug/spi-nor/spi0.0/capabilities
+    Supported read modes by the flash
+     1S-1S-1S
+      opcode		0x03
+      mode cycles	0
+      dummy cycles	0
+     1S-1S-1S (fast read)
+      opcode		0x0b
+      mode cycles	0
+      dummy cycles	8
+     1S-1S-2S
+      opcode		0x3b
+      mode cycles	0
+      dummy cycles	8
+     1S-2S-2S
+      opcode		0xbb
+      mode cycles	4
+      dummy cycles	0
+     1S-1S-4S
+      opcode		0x6b
+      mode cycles	0
+      dummy cycles	8
+     1S-4S-4S
+      opcode		0xeb
+      mode cycles	2
+      dummy cycles	4
+     4S-4S-4S
+      opcode		0x0b
+      mode cycles	2
+      dummy cycles	4
+
+    Supported page program modes by the flash
+     1S-1S-1S
+      opcode	0x02
+
+    root@1:~# cat /sys/kernel/debug/spi-nor/spi0.0/params
+    name		sst26vf064b
+    id			bf 26 43 bf 26 43
+    size		8.00 MiB
+    write size		1
+    page size		256
+    address nbytes	3
+    flags		HAS_LOCK | HAS_16BIT_SR | SOFT_RESET | SWP_IS_VOLATILE
+
+    opcodes
+     read		0xeb
+      dummy cycles	6
+     erase		0x20
+     program		0x02
+     8D extension	none
+
+    protocols
+     read		1S-4S-4S
+     write		1S-1S-1S
+     register		1S-1S-1S
+
+    erase commands
+     20 (4.00 KiB) [0]
+     d8 (8.00 KiB) [1]
+     d8 (32.0 KiB) [2]
+     d8 (64.0 KiB) [3]
+     c7 (8.00 MiB)
+
+    sector map
+     region (in hex)   | erase mask | flags
+     ------------------+------------+----------
+     00000000-00007fff |     [01  ] |
+     00008000-0000ffff |     [0 2 ] |
+     00010000-007effff |     [0  3] |
+     007f0000-007f7fff |     [0 2 ] |
+     007f8000-007fffff |     [01  ] |
+
+4) Use `mtd-utils <https://git.infradead.org/mtd-utils.git>`__
+   and verify that erase, read and page program operations work fine::
+
+    root@1:~# dd if=/dev/urandom of=./spi_test bs=1M count=2
+    2+0 records in
+    2+0 records out
+    2097152 bytes (2.1 MB, 2.0 MiB) copied, 0.848566 s, 2.5 MB/s
+
+    root@1:~# mtd_debug erase /dev/mtd0 0 2097152
+    Erased 2097152 bytes from address 0x00000000 in flash
+
+    root@1:~# mtd_debug read /dev/mtd0 0 2097152 spi_read
+    Copied 2097152 bytes from address 0x00000000 in flash to spi_read
+
+    root@1:~# hexdump spi_read
+    0000000 ffff ffff ffff ffff ffff ffff ffff ffff
+    *
+    0200000
+
+    root@1:~# sha256sum spi_read
+    4bda3a28f4ffe603c0ec1258c0034d65a1a0d35ab7bd523a834608adabf03cc5  spi_read
+
+    root@1:~# mtd_debug write /dev/mtd0 0 2097152 spi_test
+    Copied 2097152 bytes from spi_test to address 0x00000000 in flash
+
+    root@1:~# mtd_debug read /dev/mtd0 0 2097152 spi_read
+    Copied 2097152 bytes from address 0x00000000 in flash to spi_read
+
+    root@1:~# sha256sum spi*
+    c444216a6ba2a4a66cccd60a0dd062bce4b865dd52b200ef5e21838c4b899ac8  spi_read
+    c444216a6ba2a4a66cccd60a0dd062bce4b865dd52b200ef5e21838c4b899ac8  spi_test
+
+   If the flash comes erased by default and the previous erase was ignored,
+   we won't catch it, thus test the erase again::
+
+    root@1:~# mtd_debug erase /dev/mtd0 0 2097152
+    Erased 2097152 bytes from address 0x00000000 in flash
+
+    root@1:~# mtd_debug read /dev/mtd0 0 2097152 spi_read
+    Copied 2097152 bytes from address 0x00000000 in flash to spi_read
+
+    root@1:~# sha256sum spi*
+    4bda3a28f4ffe603c0ec1258c0034d65a1a0d35ab7bd523a834608adabf03cc5  spi_read
+    c444216a6ba2a4a66cccd60a0dd062bce4b865dd52b200ef5e21838c4b899ac8  spi_test
+
+   Dump some other relevant data::
+
+    root@1:~# mtd_debug info /dev/mtd0
+    mtd.type = MTD_NORFLASH
+    mtd.flags = MTD_CAP_NORFLASH
+    mtd.size = 8388608 (8M)
+    mtd.erasesize = 4096 (4K)
+    mtd.writesize = 1
+    mtd.oobsize = 0
+    regions = 0