sk_buff & DMA

[Bill]

I am having difficulty DMA'ing into sk_buffs. I have DMA code that
works except when the destination is the data buffer in an sk_buff.
No transfer error is reported in the status register but the sk_buff
data buffer (the destination) is all zeros following the DMA while the
source buffer is nonzero. I am running Linux 2.6.10 on an MPC8248.

[Janaka]

Post by Bill
I am having difficulty DMA'ing into sk_buffs. I have DMA code that
works except when the destination is the data buffer in an sk_buff.
No transfer error is reported in the status register but the sk_buff
data buffer (the destination) is all zeros following the DMA while the
source buffer is nonzero. I am running Linux 2.6.10 on an MPC8248.

These are classic symptoms of caching issues. My guess is because the
DMA HW does the transfer of your data, the processor cache is not
aware of the changes in actual memory. Try manually flushing /
reloading your cache for the destination memory.
BTW "dma_map_single /dma_unmap_single" does not work for some ppc
architectures. So manually flush the cache using "flush_dcache_range"

[Bill]

Post by Janaka

Post by Bill
I am having difficulty DMA'ing into sk_buffs. I have DMA code that
works except when the destination is the data buffer in an sk_buff.
No transfer error is reported in the status register but the sk_buff
data buffer (the destination) is all zeros following the DMA while the
source buffer is nonzero. I am running Linux 2.6.10 on an MPC8248.

These are classic symptoms of caching issues. My guess is because the
DMA HW does the transfer of your data, the processor cache is not
aware of the changes in actual memory. Try manually flushing /
reloading your cache for the destination memory.
BTW "dma_map_single /dma_unmap_single" does not work for some ppc
architectures. So manually flush the cache using "flush_dcache_range"

Thank you for the suggestion. My problem turned out to be the program
I was using to read memory was not able to read from the memory
location of the DMA's destination. When I read the memory location
within my driver I saw that the DMA was successful.