Current upstream driver reports errors running mtd_oobtest kernel module
test:

root@OpenWrt:/# insmod mtd_test.ko
root@OpenWrt:/# insmod mtd_oobtest.ko dev=5
[ 7023.730584] =================================================
[ 7023.736399] mtd_oobtest: MTD device: 5
[ 7023.740160] mtd_oobtest: MTD device size 3670016, eraseblock size 131072, page size 2048, count of eraseblocks 28, pages per eraseblock 64, OOB size 128
[ 7023.753837] mtd_test: scanning for bad eraseblocks
[ 7023.758636] mtd_test: scanned 28 eraseblocks, 0 are bad
[ 7023.763861] mtd_oobtest: test 1 of 5
[ 7024.042076] mtd_oobtest: writing OOBs of whole device
[ 7024.682069] mtd_oobtest: written up to eraseblock 0
[ 7041.962077] mtd_oobtest: written 28 eraseblocks
[ 7041.966626] mtd_oobtest: verifying all eraseblocks
[ 7041.972276] mtd_oobtest: error @addr[0x0:0x0] 0xff -> 0xe diff 0xf1
[ 7041.978550] mtd_oobtest: error @addr[0x0:0x1] 0xff -> 0x10 diff 0xef
[ 7041.984932] mtd_oobtest: error @addr[0x0:0x2] 0xff -> 0x82 diff 0x7d
[ 7041.991293] mtd_oobtest: error @addr[0x0:0x3] 0xff -> 0x10 diff 0xef
[ 7041.997659] mtd_oobtest: error @addr[0x0:0x4] 0xff -> 0x0 diff 0xff
[ 7042.003942] mtd_oobtest: error @addr[0x0:0x5] 0xff -> 0x8a diff 0x75
[ 7042.010294] mtd_oobtest: error @addr[0x0:0x6] 0xff -> 0x20 diff 0xdf
[ 7042.016659] mtd_oobtest: error @addr[0x0:0x7] 0xff -> 0x1 diff 0xfe
[ 7042.022935] mtd_oobtest: error @addr[0x0:0x8] 0xff -> 0x2e diff 0xd1
[ 7042.029295] mtd_oobtest: error @addr[0x0:0x9] 0xff -> 0x40 diff 0xbf
[ 7042.035661] mtd_oobtest: error @addr[0x0:0xa] 0xff -> 0x0 diff 0xff
[ 7042.041935] mtd_oobtest: error @addr[0x0:0xb] 0xff -> 0x89 diff 0x76
[ 7042.048300] mtd_oobtest: error @addr[0x0:0xc] 0xff -> 0x82 diff 0x7d
[ 7042.054662] mtd_oobtest: error @addr[0x0:0xd] 0xff -> 0x15 diff 0xea
[ 7042.061014] mtd_oobtest: error @addr[0x0:0xe] 0xff -> 0x90 diff 0x6f
[ 7042.067380] mtd_oobtest: error @addr[0x0:0xf] 0xff -> 0x0 diff 0xff
....
[ 7432.421369] mtd_oobtest: error @addr[0x237800:0x36] 0xff -> 0x5f diff 0xa0
[ 7432.428242] mtd_oobtest: error @addr[0x237800:0x37] 0xff -> 0x21 diff 0xde
[ 7432.435118] mtd_oobtest: error: verify failed at 0x237800
[ 7432.440510] mtd_oobtest: error: too many errors
[ 7432.445053] mtd_oobtest: error -1 occurred

The above errors are due to the buggy logic in the 'read cache' available
in airoha_snand_dirmap_read() routine since there are some corner cases
where we are missing data updates. Since we do not get any read/write speed
improvement using the cache (according to the mtd_speedtest kernel
module test), in order to fix the mtd_oobtest test, remove the 'read cache'
in airoha_snand_dirmap_read routine. Now the driver is passing all the
tests available in mtd_test suite.

Fixes: a403997c ("spi: airoha: add SPI-NAND Flash controller driver")
Tested-by: Christian Marangi <ansuelsmth@gmail.com>
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Link: https://patch.msgid.link/20240919-airoha-spi-fixes-v2-1-cb0f0ed9920a@kernel.orgSigned-off-by: Mark Brown <broonie@kernel.org>