Re: [PATCH 0/2] Device Tree support for CMA (Contiguous Memory Allocator)

[Sascha Hauer <s.hauer-bIcnvbaLZ9MEGnE8C9+IrQ@public.gmane.org>]

On Thu, Feb 14, 2013 at 11:08:54PM +0100, Sylwester Nawrocki wrote:
> Hi,
> 
> On 02/14/2013 10:30 PM, Sascha Hauer wrote:
> >On Thu, Feb 14, 2013 at 01:45:26PM +0100, Marek Szyprowski wrote:
> ...
> >>Here is my initial proposal for device tree integration for Contiguous
> >>Memory Allocator. The code is quite straightforward, however I expect
> >>that the memory bindings require some discussion.
> >>
> >>The proposed bindings allows to define contiguous memory regions of
> >>specified base address and size. Then, the defined regions can be
> >>assigned to the given device(s) by adding a property with a phanle to
> >>the defined contiguous memory region. From the device tree perspective
> >>that's all. Once the bindings are added, all the memory allocations from
> >>dma-mapping subsystem will be served from the defined contiguous memory
> >>regions.
> >>
> >
> >I think CMA regions should not be described in the devicetre at all. The
> >devicetree is about hardware description and it should be OS agnostic,
> >but CMA is only a Linux specific implementation detail. It's not even
> >specific to a particular board, it's specific to a particular usecase of
> >a board.
> 
> I disagree. For example, in a multiprocessor system describing the memory
> regions this way allows to assign memory to each subsystem, e.g. shared
> memory, so that the memory region constraints are satisfied.
> 
> CMA just happens to be an implementation of a method of assigning memory
> to each device in Linux. The constraints on the memory are real hardware
> constraints, resulting from a particular subsystem architecture.

If you are talking about DMA controllers which can only access a certain
memory area, then yes, that's a hardware constraint, I'm not sure though
if describing this as CMA in the devicetree is the way to go.

If you are talking about 'on this board I want to have 128MiB for this
device because I'm doing 1080p while on another board 64MiB are enough
because I'm doing 720p', then this is not a hardware constraint.

There may be valid scenarios for putting CMA into the devicetrees, but
doing this also opens the door for abuse of this binding. I for once
don't want to find areas being allocated for CMA in the devicetree for
devices I don't care about. I know I can always exchange a devicetree,
but I think the devicetree should be seen as firmware to a certain
degree.

Sascha

-- 
Pengutronix e.K.                           |                             |
Industrial Linux Solutions                 | http://www.pengutronix.de/  |
Peiner Str. 6-8, 31137 Hildesheim, Germany | Phone: +49-5121-206917-0    |
Amtsgericht Hildesheim, HRA 2686           | Fax:   +49-5121-206917-5555 |