[PATCH v6 3/4] drivers: of: add initialization code for dma reserved memory

[Stephen Warren]

On 08/19/2013 04:02 PM, Tomasz Figa wrote:
> Hi Stephen,
> 
> On Monday 19 of August 2013 15:49:20 Stephen Warren wrote:
>> On 08/19/2013 09:04 AM, Marek Szyprowski wrote:
>>> This patch adds device tree support for contiguous and reserved memory
>>> regions defined in device tree.
>>>
>>> diff --git a/Documentation/devicetree/bindings/memory.txt
>>> b/Documentation/devicetree/bindings/memory.txt
>>>
>>> +*** Reserved memory regions ***
>>> +
>>> +In /memory/reserved-memory node one can create additional nodes
>>
>> s/additional/child/ or s/additional/sub/ would make it clearer where the
>> "additional" nodes should be placed.
>>
>>> +compatible:	"linux,contiguous-memory-region" - enables binding 
> of
>>> this
>>> +		region to Contiguous Memory Allocator (special region for
>>> +		contiguous memory allocations, shared with movable system
>>> +		memory, Linux kernel-specific), alternatively if
>>> +		"reserved-memory-region" - compatibility is defined, given
>>> +		region is assigned for exclusive usage for by the 
> respective
>>> +		devices
>>
>> "alternatively" makes it sound like the two compatible values are
>> mutually-exclusive. Perhaps make this a list, like:
>>
>> ----------
>> compatible: One or more of:
>>
>> 	- "linux,contiguous-memory-region" - enables binding of this
>> 	  region to Contiguous Memory Allocator (special region for
>> 	  contiguous memory allocations, shared with movable system
>> 	  memory, Linux kernel-specific).
>> 	- "reserved-memory-region" - compatibility is defined, given
>> 	  region is assigned for exclusive usage for by the respective
>> 	  devices.
>> ----------
>>
>> "linux,contiguous-memory-region" is already long enough, but I'd
>> slightly bikeshed towards "linux,contiguous-memory-allocator-region", or
>> perhaps "linux,cma-region" since it's not really describing whether the
>> memory is contiguous (at the level of /memory, each chunk of memory is
>> contiguous...)
> 
> I'm not really sure if we need the "linux" prefix for "contiguous-memory-
> region". The concept of contiguous memory region is rather OS independent 
> and tells us that memory allocated from this region will be contiguous. 
> IMHO any OS is free to implement its own contiguous memory allocation 
> method, without being limited to Linux CMA.
> 
> Keep in mind that rationale behind those contiguous regions was that there 
> are devices that require buffers contiguous in memory to operate 
> correctly.
> 
> But this is just nitpicking and I don't really have any strong opinion on 
> this.
> 
>>> +*** Device node's properties ***
>>> +
>>> +Once regions in the /memory/reserved-memory node have been defined,
>>> they +can be assigned to device nodes to enable respective device
>>> drivers to +access them. The following properties are defined:
>>> +
>>> +memory-region = <&phandle_to_defined_region>;
>>
>> I think the naming of that property should more obviously match this
>> binding and/or compatible value; perhaps cma-region or
>> contiguous-memory-region?
> 
> This property is not CMA-specific. Any memory region can be given using 
> the memory-region property and used to allocate buffers for particular 
> device.

OK, that makes sense.

What I'm looking for is some way to make it obvious that when you see a
"memory-region" property in a node, you go look at the "memory.txt"
rather than the DT binding for the node where that property appears.
Right now, it doesn't seem that obvious to me.

I think the real issue here is that my brief reading of memory.txt
implies that arbitrary device nodes can include the memory-region
property without the node's own binding having to document it; the
property name is essentially "forced into" all other bindings.

I think instead, memory.txt should say:

----------
** Device node's properties ***

Once regions in the /memory/reserved-memory node have been defined, they
may be referenced by other device nodes. Bindings that wish to reference
memory regions should explicitly document their use of the following
property:

memory-region = <&phandle_to_defined_region>;

This property indicates that the device driver should use the memory
region pointed by the given phandle.
----------

Also, what if a device needs multiple separate memory regions? Perhaps a
GPU is forced to allocate displayable surfaces from addresses 0..32M and
textures/off-screen-render-targets from 256M..384M or something whacky
like that. In that case, we could either:

a) Adjust memory.txt to allow multiple entries in memory-regions, and
add an associated memory-region-names property.

or:

b) Adjust memory.txt not to mention any specific property names, but
simply mention that other DT nodes can refer to define memory regions by
phandle, and leave it up to individual bindings to define which property
they use to reference the memory regions, perhaps with memory.txt
providing a recommendation of memory-region for the simple case, but
perhaps also allowing a custom case, e.g.:

display-memory-region = <&phandl1e1>;
texture-memory-region = <&phahndle2>;