In short:
1) im about to make the mmc_spi driver work on Blackfin uClinux
platforms via the Blackfin SPI master driver.
2) will then apply and evaluate some new ideas to fix concurrent access
on the SPI bus.
Longer:
I made another mmc/sd driver a few years ago so i have had this battle
before vs the odd mmc/sd spi protocol implementation and shaky spi
master drivers. Progress and modifications so far, all to mmc_spi.c host
driver:
1) Disabled spi DMA(could be dma_map_page not working correctly vs
bfin-uclinux, will look into this deeper soon)
2) Disabled spi crc checks, CRC is one off(0xabcd vs 0xabce ie.) on many
read blocks. Data seem correct anyways, checked by md5 on files etc.
3) Ignored transmission data-response byte (allways 0xff for me) but
data get to the card anyways. md5 verified.
4) Implement one-time low-level retry. It increase stability immensley.
I can format, copy files and do any heavy action vs the card. Without
it, no go. I implemented it like this:
in mmc_data_do()
retried_once = 0;
retry:
if (direction == DMA_TO_DEVICE)
status = mmc_spi_writeblock(host, t);
else
status = mmc_spi_readblock(host, t);
/* one low-level retry */
if (status < 0 && !retried_once) {
retried_once = 1;
goto retry;
}
I know i did the same to my old spi_mmc driver to improve stability.
However, i always got the feeling this was not the right way to do it.
Books and others told med to end_request with a status != 0 and let
block layer resubmit the request(eventually) for me to retry. Whats the
correct policy for this? Whitout my retry-fix i cant write more than a
couple of 100 sectors until i get a sector error like:
end_request: I/O error, dev mmcblk0, sector 160
Why is this sector not resumited to lower levels to try again? Since the
sector number that failed was different allways a retry would probably
work, its a communication error not an actual bad sector.
Without fix, the sector fail ratio is somewhere around 1:100 to 1:1000,
wich is not very good, my fix is hiding a common low level SPI error.
But even with a perfect SPI driver i would really like to have a retry
policy somewhere when doing data transfers, in this case it actually
squares the fail ratio to make it 1:10000 to 1:Mil. Where should that
policy be implemented? I never got a good answer to this the last time i
asked around.
Thanks.
/Hans
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