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

On Thu, Feb 14, 2013 at 11:08:54PM +0100, Sylwester Nawrocki wrote:

Hi,

On 02/14/2013 10:30 PM, Sascha Hauer wrote:

quoted

On Thu, Feb 14, 2013 at 01:45:26PM +0100, Marek Szyprowski wrote:

...

quoted

quoted

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

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