[U-Boot] [PATCH] Davinci: SPI performance enhancements

[Paulraj, Sandeep]

> The following restructuring and optimisations increase the SPI
> read performance from 1.3MiB/s (on da850) to 2.87MiB/s (on da830):
> 
> Remove continual revaluation of driver state from the core of the
> copy loop. State can not change during the copy loop, so it is
> possible to move these evaluations to before the copy loop.
> 
> Cost is more code space as loop variants are required for each set
> of possible configurations. The loops are simpler however, so the
> extra is only 128bytes on da830 with CONFIG_SPI_HALF_DUPLEX
> defined.
> 
> Unrolling the first copy loop iteration allows the TX buffer to be
> pre-loaded reducing SPI clock starvation.
> 
> Unrolling the last copy loop iteration removes testing for the
> final loop iteration every time round the loop.
> 
> Using the RX buffer empty flag as a transfer throttle allows the
> assumption that it is always safe to write to the TX buffer, so
> polling of TX buffer full flag can be removed.
> 
> Signed-off-by: Nick Thompson <nick.thompson at ge.com>
> ---
> da850 and da830 are similar devices. The SPI module is common to
> both, but da850 uses DDR and da830 uses SDRAM. The EVM's might
> not actually be comparable, but they appear to be at least similar.
> 
> The speed was tested with a 8MiB transfer from SPI FLASH using:
> 
> sf read 0xc0008000 0 0x800000
> 
>  drivers/spi/davinci_spi.c |  195 +++++++++++++++++++++++++++++-----------

Applied to u-boot-ti/next