Re: [PATCH 3/3] eeprom: at25: Split writes in two SPI transfers to optimize DMA

[Arnd Bergmann]

On 10/10/18, Geert Uytterhoeven <geert+renesas@xxxxxxxxx> wrote:
> Currently EEPROM writes are implemented using a single SPI transfer,
> which contains all of command, address, and payload data bytes.
> As some SPI controllers impose limitations on transfers with respect to
> the use of DMA, they may have to fall back to PIO. E.g. DMA may require
> the transfer length to be a multiple of 4 bytes.
>
> Optimize writes for DMA by splitting writes in two SPI transfers:
>   - The first transfer contains command and address bytes,
>   - The second transfer contains the actual payload data, now stored at
>     the start of the (kmalloc() aligned) buffer, to improve payload
>     alignment.
>
> E.g. for a 25LC040 EEPROM with a page size 16 bytes, a 16-byte write
> aligned to the page size was transferred using an 18-byte write.
> After this change, the write is split in a 2-byte and an aligned 16-byte
> write.
>
> Note that EEPROM reads already use a similar scheme, due to the
> different data directions for command and address bytes versus payload
> data.
>
> Signed-off-by: Geert Uytterhoeven <geert+renesas@xxxxxxxxx>

Acked-by: Arnd Bergmann <arnd@xxxxxxxx>