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

[Marek Szyprowski <m.szyprowski@samsung.com>]

Hello,

I'm sorry for the late reply, I must have missed this mail...

On Tuesday, November 08, 2011 7:43 PM Daniel Vetter wrote:

> 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)

I really don't get why you want to force the exporter to map the buffer into
clients dma address space. Only the client device might know all the quirks
required to do this correctly. Exporter should only provide a scatter-list 
with the memory that belongs to the exported buffer (might be pinned). How
do you want to solve the following case - the gpu hardware from your diagram
and a simple usb webcam with generic driver. The application would like to
export a buffer from the webcam to scanout engine. How the generic webcam 
driver might know HOW to set up the tiller to create correct mappings for 
the GPU/scanout? IMHO only a GPU driver is capable of doing that assuming
it got just a scatter list from the webcam driver.

Best regards
-- 
Marek Szyprowski
Samsung Poland R&D Center



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