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DMA from Local memory to PCI device - DMA Address

Link to this post 07 Oct 09

I am working on a driver that uses PCI DMA transfers from processor local memory (slave memory) to the PCI device. The board that is use is MIPS based & the kernel version is linux 2.6.18.8. For some reason, i am not able to achieve this. Instead, if i use __get_free_pages which allocates pages from main memory, the dma transfer happens. Given below are the steps i followed for each case

Case 1 : (using get_free_pages)
a. allocate memory
__get_free_pages() ---> returns virtual address
b. assign the physical address to dma_handle
*dma_handle = __pa(ret)
c. Return the uncach address to the callee
return UNCAC_ADDR(ret);

With the above steps dma happen and the system works as expected.

Now, in the next case, i replace step a with my custom function which allocates memory from the system local memory (slave memory). All other steps are kept as is.
Case 2 :
a. allocate memory from slave memory
custom_allocate_memory () ---> returns physical address of the slave memory
__va( custom_allocate_memory () ) ---> this will convert the physical address to virtual memory. This is done so as to keep the step b & step c of Case 1 as it is.
b. assign the physical address to dma_handle
*dma_handle = __pa(ret)
c. Return the uncached address to the callee
return UNCAC_ADDR(ret);
In Case2, the DMA doesn't work and returns an error which indicates that the address provided to the DMA is wrong? What could be the probable mistake i am doing here? Is there anything else to be considered while replacing the __get_free_pages() with my custom function?

Can someone give me a pointer of how Case 2 can be acheived?

-Joe

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