[U-Boot] [PATCH V6] sf: Turn SPI flash chip into 3-Byte address mode

[Stefan Roese <sr@denx.de>]

Jagan,

On 13.08.2015 13:45, Jagan Teki wrote:
>>> Please correct me if I'm wrong, but AFAIU this BAR thing
>>> (CONFIG_SPI_FLASH_BAR) doesn't support to address e.g. a 64MiB SPI flash
>>> contiguously. Only 16MiB areas. So for example its not possible to put
>>> UBI/UBIFS in such a big partition.
>
> Stefan,
>
> No, BAR is accessing flash > 16MiB in 3-byte addressing mode, for your
> example of
> 64MiB flash, it can grouped into 4 16MiB regions means 4 bank vales
> bank0 to bank3
>
> Based on the sf read/erase/write flash offsets, that particular bank
> will enable an do
> the relevant operations.
>
> In-spite of having 4 byte addressing operations BAR should do exactly
> same with existing
> 3-byte addressing.

Okay, thanks for the explanation. If this really is the case, and this 
BAR support will seamlessly enable the access to the complete flash 
area, then the 4-byte mode should really not be necessary.

I'm wondering though about 2 things:

a) Do all SPI NOR flash chips support this BAR mode?

b) If yes, why isn't BAR enabled per default?

c) Why doesn't the Linux kernel use this BAR mode?

Thanks,
Stefan