On 08/20/2013 04:57 AM, Marek Szyprowski wrote:

Hello,

On 8/20/2013 12:17 AM, Stephen Warren wrote:

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On 08/19/2013 04:02 PM, Tomasz Figa wrote:

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Hi Stephen,

On Monday 19 of August 2013 15:49:20 Stephen Warren wrote:

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On 08/19/2013 09:04 AM, Marek Szyprowski wrote:

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This patch adds device tree support for contiguous and reserved

memory

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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

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"additional" nodes should be placed.

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+compatible:    "linux,contiguous-memory-region" - enables binding

of

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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

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+        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

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perhaps "linux,cma-region" since it's not really describing whether

the

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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-

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region". The concept of contiguous memory region is rather OS

independent

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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

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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

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this.

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+*** 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>;

Support for multiple regions is something that need to be discussed
separately. In case of Exynos hardware it is also related to iommu bindings
and configuration, because each busmuster on the internal memory bus has
separate iommu controller. Having each busmaster defined as a separate
node,
enables to put there the associated memory region and/or iommu
controller as
well as dma window properties (in case of limited dma window).

However right now I would like to focus on the simple case (1 device,
1 memory region) and define bindings which can be extended later.
I hope that my current proposal covers this (I will send updated binding
documentation asap) and the initial support for reserved memory can be
merged to -next soon.

I don't believe that's a good approach unless you have at least a
partial idea of how the current bindings will be extended to support
multiple memory regions.

Without a clear idea how that will eventually work, you run a real risk
of not being able to extend the bindings in a 100% backwards-compatible
way, and hence wishing to break DT ABI.