Re: [RFC 1/2] dma-buf: Introduce dma buffer sharing mechanismch

Daniel Vetter
2011-11-08T18:42:16Z

On Tue, Nov 08, 2011 at 05:55:17PM +0000, Russell King - ARM Linux wrote:
> On Tue, Nov 08, 2011 at 06:42:27PM +0100, Daniel Vetter wrote:
> > Actually I think the importer should get a _mapped_ scatterlist when it
> > calls get_scatterlist. The simple reason is that for strange stuff like
> > memory remapped into e.g. omaps TILER doesn't have any sensible notion of
> > an address in physical memory. For the USB-example I think the right
> > approach is to attach the usb hci to the dma_buf, after all that is the
> > device that will read the data and move over the usb bus to the udl
> > device. Similar for any other device that sits behind a bus that can't do
> > dma (or it doesn't make sense to do dma).
> >
> > Imo if there's a use-case where the client needs to frob the sg_list
> > before calling dma_map_sg, we have an issue with the dma subsystem in
> > general.
>
> Let's clear something up about the DMA API, which I think is causing some
> misunderstanding here. For this purpose, I'm going to talk about
> dma_map_single(), but the same applies to the scatterlist and _page
> variants as well.
>
> dma = dma_map_single(dev, cpuaddr, size, dir);
>
> dev := the device _performing_ the DMA operation. You are quite correct
> that in the case of a USB peripheral device, the device is normally
> the USB HCI device.
>
> dma := dma address to be programmed into 'dev' which corresponds (by some
> means) with 'cpuaddr'. This may not be the physical address due
> to bus offset translations or mappings setup in IOMMUs.
>
> Therefore, it is wrong to talk about a 'physical address' when talking
> about the DMA API.
>
> We can take this one step further. Lets say that the USB HCI is not
> capable of performing memory accesses itself, but it is connected to a
> separate DMA engine device:
>
> mem <---> dma engine <---> usb hci <---> usb peripheral
>
> (such setups do exist, but despite having such implementations I've never
> tried to support it.)
>
> In this case, the dma engine, in response to control signals from the
> USB host controller, will generate the appropriate bus address to access
> memory and transfer the data into the USB HCI device.
>
> So, in this case, the struct device to be used for mapping memory for
> transfers to the usb peripheral is the DMA engine device, not the USB HCI
> device nor the USB peripheral device.

Thanks for the clarification. I think this is another reason why
get_scatterlist should return the sg_list already mapped into the device
address space - it's more consisten with the other dma apis. Another
reason to completely hide everything but mapped addresses is crazy stuff
like this

mem <---> tiling iommu <-+-> gpu
|
+-> scanout engine
|
+-> mpeg decoder

where it doesn't really make sense to talk about the memory backing the
dma buffer because that's smeared all over the place due to tiling. IIRC
for the case of omap these devices can also access memory through other
paths and iommut that don't tile (but just remap like a normal iommu)

-Daniel
--
Daniel Vetter
Mail: daniel@ffwll.ch
Mobile: +41 (0)79 365 57 48