On 02/15/2013 09:33 AM, Sascha Hauer wrote:
On Thu, Feb 14, 2013 at 11:08:54PM +0100, Sylwester Nawrocki wrote:
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.
My context was a multiprocessor SoC, where one of the processors is somehow application specific (e.g. camera ISP) and can be considered as a slave subsystem, running it's firmware and sharing system memory with the main processor.
The slave subsystem can have insane constraints on the memory region where its firmware is located and which it also uses as its generic purpose RAM.
While the region assigned to such a slave device would also likely include a memory for its DMA buffers, the main concern here was an allocation of a working memory for the slave processor.
Perhaps the device tree is not a perfect place for defining exact memory regions. If we happen to decide against it, then I guess some set of properties would be needed, that would allow CMA to assign appropriate memory region(s) to a device.
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.
No, that's not something I was referring to.
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 device tree should be seen as firmware to a certain degree.
I agree. You're likely aware of it, I just want to remind that the CMA memory can be used by the OS in a normal way, until a device driver actually allocates and uses it.