[RFC][PATCH] mtd: spi: Set CONFIG_SF_DEFAULT_MODE default to 0

[Tom Rini]

On Tue, Sep 14, 2021 at 10:10:22PM +0200, Marek Vasut wrote:
> On 9/14/21 9:19 PM, Tom Rini wrote:
> > On Tue, Sep 14, 2021 at 08:28:24PM +0200, Marek Vasut wrote:
> > 
> > > Before e2e95e5e254 ("spi: Update speed/mode on change") most systems
> > > silently defaulted to SF bus mode 0. Now the mode is always updated,
> > > which causes breakage. It seems most SF which are used as boot media
> > > operate in bus mode 0, so switch that as the default.
> > > 
> > > This should fix booting at least on Altera SoCFPGA, ST STM32, Xilinx
> > > ZynqMP, NXP iMX and Rockchip SoCs, which recently ran into trouble
> > > with mode 3. Marvell Kirkwood and Xilinx microblaze need to be checked
> > > as those might need mode 3.
> > > 
> > > Signed-off-by: Marek Vasut <marex at denx.de>
> > > Cc: Aleksandar Gerasimovski <aleksandar.gerasimovski at hitachi-powergrids.com>
> > > Cc: Andreas Biessmann <andreas at biessmann.org>
> > > Cc: Eugen Hristev <eugen.hristev at microchip.com>
> > > Cc: Michal Simek <michal.simek at xilinx.com>
> > > Cc: Patrice Chotard <patrice.chotard at foss.st.com>
> > > Cc: Patrick Delaunay <patrick.delaunay at foss.st.com>
> > > Cc: Peng Fan <peng.fan at nxp.com>
> > > Cc: Siew Chin Lim <elly.siew.chin.lim at intel.com>
> > > Cc: Tom Rini <trini at konsulko.com>
> > > Cc: Valentin Longchamp <valentin.longchamp at hitachi-powergrids.com>
> > > Cc: Vignesh Raghavendra <vigneshr at ti.com>
> > 
> > So, some background.  With commit 88e34e5ff76b ("spl: replace
> > CONFIG_SPL_SPI_* with CONFIG_SF_DEFAULT_*") CONFIG_SF_DEFAULT_MODE and
> > related moved from having their default value of SPI_MODE_3 (which
> > evaluates to 3) be defined in common/cmd_sf.c if not otherwise defined,
> > to include/spi_flash.h, and a more visible global default than it was
> > before.  At that time, the following platforms either set or implied
> > SPI_MODE_3 should be used:
> > alt am335x_boneblack am335x_boneblack_vboot am335x_evm_norboot
> > am335x_evm_nor am335x_evm am335x_evm_spiboot am335x_evm_usbspl
> > am43xx_evm am43xx_evm_qspiboot at91sam9n12ek_mmc at91sam9n12ek_nandflash
> > at91sam9n12ek_spiflash at91sam9x5ek_dataflash at91sam9x5ek_mmc
> > at91sam9x5ek_nandflash at91sam9x5ek_spiflash atstk1004 bf506f-ezkit
> > bf537-stamp cam_enc_4xx coreboot-x86 d2net_v2 da830evm da850_am18xxevm
> > da850evm_direct_nor da850evm dra7xx_evm dra7xx_evm_qspiboot
> > dra7xx_evm_uart3 draco dreamplug dxr2 ea20 ecovec ethernut5 gplugd
> > inetspace_v2 k2e_evm k2hk_evm kmcoge5un km_kirkwood_128m16
> > km_kirkwood_pci km_kirkwood kmnusa kmsugp1 kmsuv31 koelsch lager lschlv2
> > lsxhl M53017EVB M54455EVB mgcoge3un mv88f6281gtw_ge net2big_v2
> > netspace_lite_v2 netspace_max_v2 netspace_mini_v2 netspace_v2
> > nios2-generic pcm051_rev1 pcm051_rev3 portl2 pxm2 rut sama5d3xek_mmc
> > sama5d3xek_nandflash sama5d3xek_spiflash sh7785lcr tseries_spi vf610twr
> > zynq_zc770_xm010
> > 
> > Of those platforms, I have handy dra7xx_evm (also am335x_evm but that
> > needs flipping some DIP switches) which has SPI flash by default.  With
> > current tip of tree, I did "sf probe 0 76800000 3" and a few cycles of
> > reading the whole of flash and crc32'ing it, and getting the same result
> > back.  So, SPI_MODE_3 seems correct / function here, and likely so on
> > the rest of the TI platforms listed above.
> > 
> > With commit 14453fbfadc2 ("Convert CONFIG_SF_DEFAULT_* to Kconfig") we
> > migrate these values to Kconfig, and a quick spot-check shows that yes,
> > the migration did not change any values.
> > 
> > A big change between 88e34e5ff76b and 14453fbfadc2 is that a ton of
> > platforms have been added (it was 5 years, after all) and those
> > platforms seem to be where the problems reside.  I'm not sure if or why
> > SPI_MODE_3 doesn't work on so many new platforms, but I would prefer to
> > see "default 0 if ARCH_SOCFPGA || ARCH_STM32 || .." as needed to switch
> > the default to SPI_MODE_0 if there's not a more fundamental problem to
> > solve on some platforms such that SPI_MODE_3 could / should be enabled.
> 
> I rather suspect mode 3 is special case for atmel/am335x and co. and maybe
> marvell kirkwood from the list above. So shouldn't we default to 0 and
> special-case the two or three instead ?

I guess my question boils down to why is that the right default?  My
argument that 3 is the right default is that it's what it was in
introduction back in b6368467e6a9 ("SPI Flash: Add "sf" command").  But
references to something else such as the Linux Kernel defaults to mode 0
or some SPI specs saying you should default to mode 0 or something else
would be much appreciated.

-- 
Tom
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