MMC + DMA too fast for CPU?

[Admar Schoonen]

Hi all,

I have an LPC3250 (ARM926, ~ 200 MHz) with eMMC running Linux 3.12.20. By default, it runs the MMC (with MMCI driver) in PIO mode and I need to set the MMC clock very low ( ~ 400 kHz or lower) to get reliable operation. This gives me about 200 KB/s throughput.

I have enabled DMA with the patch from http://lists.infradead.org/pipermail/linux-arm-kernel/2013-January/143995.html and some small modifications here and there and MMC now seems to run fine at 1.5 MHz. This gives me a throughput of about 660 KB/s.

If I clock the MMC higher than that, I get lots of errors (ETIMEDOUT, EIO etc) when executing the mmc_tests. Could that be caused by the CPU being too slow to handle the interrupts?

And is this performance of 660 KB/s indeed what could be expected of such a CPU? Or must there be something else that is limiting the system?

Thanks in advance and please keep me on CC as I am not subscribed to this list.

Kind regards,

Admar Schoonen 
Engineer 

www.prodrive-technologies.com 

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