memory allocation for DMA (how to make bdbuf use non-cacheable region for SD card read and write reuqest)

[Chan Kim]

Hi, Sebastian, thanks

> It makes no sense to put the buffers in non-cacheable memory. The buffers
> are already cache line aligned, so you can safely use cache flush/invalidate
> operations or use a bounce buffer.

I forgot to finish my reply yesterday. Yes, I was confused. It was the flushing operation I put in not the page table setup that had the good effect.  But I'm still confused about the cache operation. I put "flush" for when a SD command processing(block read or write) has been completed. This somehow seems to make the DMAed data from SD card to 'come to cache' - maybe declaring the cache invalid  or empty' for SD card read. But I don't understand how data from disk cache(bdbuf) goes to real memory for DMA (to SD card) because I fulsh the cache only 'after' the command completion. (just by lucky flush out?, or should I put another cache flush before DMA write to SD card controller? )

Now it takes 2~3 seconds to write 512KB file and 3~4 seconds to copy a file in SD card to another file in the SD card. I have to find the reason for occasional stop during file write or copy.

Chan



보낸 사람 : "Sebastian Huber" <sebastian.huber at embedded-brains.de>
보낸 날짜 : 2015-09-02 22:56:15 ( +09:00 )
받는 사람 : 김찬 <ckim at etri.re.kr>, users at rtems.org <users at rtems.org>
참조 : 
제목 : Re: memory allocation for DMA (how to make bdbuf use non-cacheable region for SD card read and write reuqest)



On 02/09/15 15:33, Chan Kim wrote:
> My question is : how come the buffer used for SD card read or write request from the bdbuf is allocated in cacheable memory? I guess there should be setting that tells bdbuf to use some area that the bsp assigned for DMA(non-cacheable). Or , maybe I should map the buffer address to non-cacheable adderss and copyit myself?(should be slow). What is the correct method?

It makes no sense to put the buffers in non-cacheable memory. The 
buffers are already cache line aligned, so you can safely use cache 
flush/invalidate operations or use a bounce buffer.

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Sebastian Huber, embedded brains GmbH

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