The aspeed-smc can have multiple SPI devices attached to it in the
device tree.  If one of the devices is missing or failing the entire
probe will fail and all MTD devices under the controller will be
removed.  On OpenBMC this results in a kernel panic due to missing
rootfs:

[    0.538774] aspeed-smc 1e620000.spi: Using 50 MHz SPI frequency
[    0.540471] aspeed-smc 1e620000.spi: w25q01jv-iq (131072 Kbytes)
[    0.540750] aspeed-smc 1e620000.spi: CE0 window [ 0x20000000 - 0x28000000 ] 128MB
[    0.540943] aspeed-smc 1e620000.spi: CE1 window [ 0x28000000 - 0x2c000000 ] 64MB
[    0.541143] aspeed-smc 1e620000.spi: read control register: 203b0041
[    0.581442] 5 fixed-partitions partitions found on MTD device bmc
[    0.581625] Creating 5 MTD partitions on "bmc":
[    0.581854] 0x000000000000-0x0000000e0000 : "u-boot"
[    0.584472] 0x0000000e0000-0x000000100000 : "u-boot-env"
[    0.586468] 0x000000100000-0x000000a00000 : "kernel"
[    0.588465] 0x000000a00000-0x000006000000 : "rofs"
[    0.590552] 0x000006000000-0x000008000000 : "rwfs"
[    0.592605] aspeed-smc 1e620000.spi: Using 50 MHz SPI frequency
[    0.592801] aspeed-smc 1e620000.spi: unrecognized JEDEC id bytes: 00 00 00 00 00 00
[    0.593039] Deleting MTD partitions on "bmc":
[    0.593175] Deleting u-boot MTD partition
[    0.637929] Deleting u-boot-env MTD partition
[    0.829527] Deleting kernel MTD partition
[    0.856902] Freeing initrd memory: 1032K
[    0.866428] Deleting rofs MTD partition
[    0.906264] Deleting rwfs MTD partition
[    0.986628] aspeed-smc 1e620000.spi: Aspeed SMC probe failed -2
[    0.986929] aspeed-smc: probe of 1e620000.spi failed with error -2
...
[    2.936719] /dev/mtdblock: Can't open blockdev
mount: mounting /dev/mtdblock on run/initramfs/ro failed: No such file or directory
[    2.963030] MTD: Couldn't look up '/dev/mtdblock': -2
mount: mounting /dev/mtdblock on run/initramfs/rw failed: No such file or directory

Mounting read-write /dev/mtdblock filesystem failed.  Please fix and run
	mount /dev/mtdblock run/initramfs/rw -t jffs2 -o rw
or perform a factory reset with the clean-rwfs-filesystem option.
Fatal error, triggering kernel panic!
[    3.013047] Kernel panic - not syncing: Attempted to kill init! exitcode=0x00000100

Many BMC designs have two flash chips so that they can handle a hardware
failure of one of them.  If one chip failed, it doesn't do any good to
have redundancy if they all get removed anyhow.

Improve the resilience of the probe function to handle one of the
children being missing or failed.  Only in the case where all children
fail to probe should the controller be failed out.
Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20211229143334.297305-1-patrick@stwcx.xyz