[U-Boot] OT: SD/MMC-over-SPI and speed in linux

[Wolfgang Wegner]

Hi,

On Thu, May 06, 2010 at 04:13:41PM +0200, Alexander Stein wrote:
> 
> I can't say much about actual transfer rates. But i noticed a real slowdown 
> when the filesystem buffer is about to be flushed.
> We are using a COldfire 5484 and AFAIK there is no DMA for SPI, at least we 
> don't use it.

I used M54451EVB/M54455EVB as a starting point, both use eDMA for
SPI by default. Sorry, don't know about MCF547x/548x.

Things are clearing up now: as far as I could see, there are no
real timing-relevant things changed in mmc_spi code, BUT the underlying
SPI driver is crucial.

Setting the same default timing values found in mcf5445x-devices.c in
U-Boot (CONFIG_SYS_DSPI_CTAR0), I can degrade the performance of MMC-SPI
in U-Boot by a factor of 3, approximately.
However, setting the fast default values from U-Boot in the linux kernel
(mcf5445x-devices.c) makes things even worse there, too.

After all, I am sure now it is only the SPI part I have to investigate
further, most probably concerning latency between transfers.

Thanks for the comments and sorry for the noise!

Best regards,
Wolfgang