linux-kernel - Re: mmaping a kernel buffer to user space

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Date:	Wed, 1 Nov 2006 19:30:25 +0530
From:	"yogeshwar sonawane" <yogyas@...il.com>
To:	"Guillermo Marcus Martinez" <marcus@...uni-mannheim.de>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: mmaping a kernel buffer to user space

On 11/1/06, Guillermo Marcus Martinez <marcus@...uni-mannheim.de> wrote:
> Rolf Offermanns schrieb:
> > Guillermo Marcus <marcus <at> ti.uni-mannheim.de> writes:
> >> Note: I am using kernel 2.6.9 for these tests, as it is required by my
> >> current setup. Maybe this issue has already been addressed in newer
> >> kernel. If that is the case, please let me know.
> >
> > Have a look at this article:
> >
> > "The evolution of driver page remapping"
> > http://lwn.net/Articles/162860/
> >
> > It should make things clearer.
> >
> > The "API changes in the 2.6 kernel series" page is also a very good read:
> > http://lwn.net/Articles/2.6-kernel-api/
> >
> > HTH,
> > Rolf
>
> Thanks for the links!
>
> Yes, it looks like a step in the right direction. However, the article
> says about vm_insert_page(): "...What it does require is that the page
> be an order-zero allocation obtained for this purpose...", therefore
> making it also unusable for this case (mmaping a pci_alloc_consistent).
>
> I think the limitation (being order zero), is related to the page
> counting, as I understand that for bigger order allocations, only the
> first-page counter is incremented (not every page). If that is a
> problem, I guess I would also see a problem with my workaround, and I
> see none (yet). So I may try in a newer kernel and see if I can use it
> to walk the pages on the mmap without using the nopage().

Setting 'PG_reserved' bit of all allocated pages & then calling
remap_page/pfn_range()
will do the things for 2.6.9.

>
> My suggestion would be to add two functions: pci_map_consistent() and
> dma_map_coherent() to address this issue, and their corresponding
> unmap's. That will make sure all that is needed is done, is a clean and
> consistent with the pci_ and dma_ APIs, and fills a mmap requirement not
> covered by the other functions.
>
> Best wishes,
> Guillermo
>
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