Re: PCIe device driver question
From: Robert Hancock
Date: Thu Jul 31 2008 - 14:53:16 EST
V.Radhakrishnan wrote:
My guess there was a bug in your DMA mapping code. I don't think kmap is
what is normally used for this. I think with get_user_pages one usually
takes the returned page pointers to create an SG list and uses
dma_map_sg to create a DMA mapping for them.
Looking at the actual code, I see that I had used kmap() only to obtain
kernel virtual addresses for the array of struct pages obtained from
user space by using get_user_pages.
Subsequently, I had used dma_map_single() and dma_unmap_single() calls
for single buffer calls.
I'm suspicious about this usage, I don't know if that will actually
work. There is a dma_map_page call which is meant for doing a DMA
mapping on a struct page which should likely be used instead.
The code didn't have bugs IMHO since it was used for extensive stress
testing the initial FPGA prototype as well as the final ASIC chip ,
sometimes running for over 4 days non-stop without breaking.
However, using Test Access Points on the board and using a Logic
Analyzer showed that DMA was NOT taking place when RAM > 896 MB was
used. The hardware gurus said that PCI bus cycles just didn't seem to be
taking place when RAM > 896 MB was used as the source OR destination
address.
Are you sure the address being passed to the device was correct in this
case? There should be nothing magical about 896MB from a hardware point
of view, and the kernel in general cannot stop you from DMAing anywhere
you like.
Perhaps this was a problem in the earlier kernel(s) and has since been
rectified ? ( I was using 2.6.15 then ... )
I am just curious since Sanka Piyaratna reported a 'similar' kind of
situation.
Regards
V. Radhakrishnan
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