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

[Nishanth Peethambaran]

Limiting the scope of the product to a particular use-case (720p or
1080p video playback) will be part of firmware mostly OTP-ed in the
SoC. Firmware author could decide the amount of CMA memory needed and
put this as part of DT.
Number of CMA areas and linking them to devices could be done in DT
and allows using the same kernel for any of the combinations.

- Nishanth Peethambaran
  +91-9448074166



On Fri, Feb 15, 2013 at 9:54 PM, Rob Herring <robherring2 at gmail.com> wrote:
> On 02/15/2013 02:33 AM, Sascha Hauer wrote:
>> 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.
>
> I agree this does not belong in DT. As a kernel developer, the DT comes
> from firmware. Can the firmware author decide how much CMA memory is
> needed? I don't think so.
>
> I would suggest a kernel command line parameter instead if that does not
> already exist.
>
> Rob
>
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