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 _______________________________________________ devicetree-discuss mailing list devicetree-discuss@lists.ozlabs.org https://lists.ozlabs.org/listinfo/devicetree-discuss