how to access RAM memory by DMA controller?

4
votes

i want to copy data from flash memory to RAM. So how can i set the destination memory address of RAM in DMA controller so that it can use its channel to copy the data from source address(in flash memory) to RAM memory.

I am doing it on x86 arch and it's a 8237 DMA.

1
x86dma
What kind of device is this? Is this a USB or SATA flash memory? - David Schwartz
It's the kernel OS that should take care of managing that... unless there's no kernel. - m0skit0
David :- Ita SDD flash memory from where i have to copy the data(kernel image) and put that in RAM. I have read some tutoials but cudnt get much info how to do mem-to-mem copy (especially setting the destination address. m0skit0 - I am doing this operation by bootloader and there is no kernel so far now as bootloader itself is copying the kernel image from flash to RAM. - user1083212
It is probably out of reach to the DMA controller, not exactly clear from the question. There is no point in using it anyway, DMA is only useful if the processor can do something else while the transfer is taking place. - Hans Passant
@Hans - The processor will be checking checking the rootfs image signature for secure boot while the transfer of kernel image is taking place. Please got thru this page -: elinux.org/DMA_Copy_Of_Kernel_On_Startup Any suggestion now to control RAM by DMA? - user1083212

1 Answers

1
votes

The 8237 DMA controller is capable of making transfers from RAM to RAM, from I/O to RAM, and from RAM to I/O device.

You must take into account that you will only able to copy up to 64KB of data. If you want to copy more than 64KB, you will need more than one transfer operation.

Assuming you are using the same environment that a IBM PC has, and you are able to set the segment address of the destination memory in a separate latch, you will need to:

  1. Normalize your destination address segment:offset so that offset is lower than 16. Normalization is carried this way (DIV is integer division, MOD is modulus):

    normalized_segment = segment + offset DIV 16

    normalized_offset = offset MOD 16

  2. Load the segment value into the high order bits latch belonging to the channel you want to use (the page address register)

  3. Load the offset address into the Start Address Register of the DMA channel you want to use.
  4. Continue with DMA setup (transfer length, mode, interrupt on EOC, etc)