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

[Wolfgang Wegner]

Hi Alexander,

On Thu, May 06, 2010 at 03:33:53PM +0200, Alexander Stein wrote:
[...]
> IIRC 400kBytes/second is the initial speed for SD cards on SPI. The card 
> itself responds which max speed it can handle. Which could be adjusted later 
> (up to 25MHz or even more).

the initial speed is 400 kBits/second, because there may be some (few)
cards that could not handle more.

> I backported this spi_mmc driver to 2.6.10 (yes, even older) and it is slow, 
> nevertheless.  I don't know how SPI is done in u-boot, but maybe the system 
> interrupts in linux like ethernet and timer slow SPI transfer down.

Thanks for the confirmation!
I just found a page from the blackfin linux project:
http://docs.blackfin.uclinux.org/doku.php?id=linux-kernel:drivers:spi_mmc
which suggests better values (using dd with bs=512 should be similar to
what bonnie does in the block tests?). A hint I got from this page was to
disable SPI DMA (which was also enabled on my coldfire), and this improved
the situation to get around 115 kBytes/second.

Strange enough enabling DMA decreases performance, there still seems to
be room for improvement. I will try to see if I can spot differences in
the blackfin kernel that I could back-port - although I already saw there
are many differences when back-porting the locking part.

Best regards,
Wolfgang