[U-Boot] [PATCH] sf: Add SPI_FLASH_4BYTE_MODE_ONLY option to support 4-byte mode

[Stefan Roese]

Hi Simon,

On 06.08.2018 17:15, Simon Goldschmidt wrote:
> 
> 
> Stefan Roese <sr at denx.de <mailto:sr at denx.de>> schrieb am Mo., 6. Aug. 
> 2018, 16:34:
> 
>     Some SPI NOR chips only support 4-byte mode addressing. Here the default
>     3-byte mode does not work and leads to incorrect accesses. Setting this
>     option enables the use of such SPI NOR chips, that only support this
>     4-byte mode.
> 
> 
> I think it would make more sense to enable 4-byte mode or 4-byte opcodes 
> on all chips with more than 16 mbyte rather than having to select at 
> compile time.

We need to be careful here. As setting the chip into 4-byte mode
unconditionally (for bigger devices) will very likely cause boot
problems with internal bootROMs expecting 3-byte mode.

As noted in the commit message, this is a very special case, where
the SPI NOR only supports 4-byte opcodes / mode.

Another idea would be to check the 3-byte / 4-byte mode of the SPI
NOR device upon SPI NOR driver loading and use the selected mode
accordingly. This could be done without compile time options but
it would not help in general for users with bigger SPI NOR devices
that support both modes.

Thanks,
Stefan