linux-kernel - Re: PCIe device driver question

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Date:	Fri, 01 Aug 2008 02:17:45 +0530
From:	"V.Radhakrishnan" <rk@...-labs.com>
To:	Robert Hancock <hancockr@...w.ca>
Cc:	Sanka Piyaratna <cesanka@...oo.com>,
	Alan Cox <alan@...rguk.ukuu.org.uk>,
	linux-kernel@...r.kernel.org
Subject: Re: PCIe device driver question

Thanks Robert !

Unfortunately, I cannot test this with recent kernels, since this was a
WiMAX card based on a CardBus inteface  and the Toshiba laptop I have
has only 512 MB RAM ( though I have PCs with more RAM, the PCs don't
have a CardBus interface ).

BRgds

RK


On Thu, 2008-07-31 at 12:52 -0600, Robert Hancock wrote:
> 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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