Re: How to get the physical page addresses from a kernel virtual address for DMA SG List?

[Clemens Koller <clemens.koller@anagramm.de>]

Hello, Dick!

Thanks for your help so far!

linux-os (Dick Johnson) wrote:
> You are trying to do it backwards. You need to have your driver
> use get_dma_pages() to acquire pages suitable for DMA. Your
> driver then impliments mmap().

Okay, I have seen that, too. I've seen that some drivers do it the other
way around as I do, but I still try to follow my idea that the
application allocs the memory and the dma / the driver fills it up.
Or are there fundamental problems I get with my approach which I haven't seen yet?

> The user-mode application then mmaps() the dma-able pages into
> its address-space. FYI, the pages may be from anywhere, some
> archs can only DMA to/from memory below 16MB.

My DMA machine (ppc32, mpc8540 cpu) can do the whole 32bit phys address
space so, that's not an issue here.

> The pages do not have
> to be continuous because you will build a scatter-list for
> the DMA engine and you will mmap() the pages so they are
> contiguous to the user.

Yes, only virtual space is contigous. The DMA can do nice
sg_lists and chained sg_lists, so, this should not be a problem,
too.

> Also 400 Megabytes is absurd.

Why?
Actually I am planning to alloc more than 1.5GByte at once,
lock that down, build a big sg_list for all that memory because
I need to _continously_ feed it with data from the DMA. I get
about 200MBytes/sec and I cannot stop in between!

Greets,

Clemens



> On Thu, 4 Aug 2005, Clemens Koller wrote:
>
>
> > Hello!
> >
> > This might be an FAQ - I've got several ideas from googling
> > around for days and reading 'Linux Device Drivers' or
> > 'Understanding The Linux Kernel' which are both really good
> > books. However I am not really sure of how to do it on the latest
> > linux-2.6.
> >
> > I am currently working on a dma driver for a ppc32 system.
> > The idea is that a userspace app allocates a big contigous
> > chunk of memory (i.e. 400MBytes, user virtual mem) and tells
> > my dma's char driver via ioctl the pointer to that memory.
> >
> > In the driver I can now use that (void __user *) casted address
> > as a kernel virtual address, right? It's contigous there and I can
> > do a memcpy() to get data to userspace simliar to a copy_to_user().
> > fine!
> >
> > But I want to setup a scatter/gather DMA list and blow my data
> > directly into the applications physical pages.
> >
> > What's the best way to setup the dma_sg_list?
> > I have checked several things to get the pages and physical addresses
> > but with no real success now:
> > get_user_page()
> > vmalloc_to_page()
> > kvirt_to_bus() (deprecated?)
> > virt_to_phys()
> > virt_to_bus()
> >
> > Or do I need to remap the whole thing before I can get all the pages and
> > physical addresses?
> > remap_page_range()
> > remap_pfn_range()
> > map_user_kiobuf()
> > unmap_kiobuf()
> >
> > Or do I need the direct-io stuff or the block-io?
> > How do I need to alloc the mem in my app? (get_pages()?)
> > How do I need to lock the memory to make sure it's in phys memory? (mlockall()?)
> > Can somebody please put some light on what's _the_ way to do that on the
> > latest 2.6 kernels? I am pretty much confused which functions are current
> > and okay to use to solve my problem.
> > Pointers to some code is also very welcome. But it should be _current_.
> > I've spent already a lot of time reading outdated things. :-(
> >
> > Best regards,
> >
> > Clemens Koller
> > _______________________________
> > R&D Imaging Devices
> > Anagramm GmbH
> > Rupert-Mayer-Str. 45/1
> > 81379 Muenchen
> > Germany
> >
> > http://www.anagramm.de
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>
> Cheers,
> Dick Johnson
> Penguin : Linux version 2.6.12 on an i686 machine (5537.79 BogoMips).
> Warning : 98.36% of all statistics are fiction.
> .
> I apologize for the following. I tried to kill it with the above dot :
>
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