Re: [PATCH] firmware: Add device tree binding for coreboot

[Rob Herring]

On Mon, Jun 16, 2014 at 3:43 PM, Stephen Warren <swarren@xxxxxxxxxxxxx> wrote:
> On 06/16/2014 07:30 AM, Rob Herring wrote:
>> On Fri, Jun 13, 2014 at 4:58 PM, Julius Werner <jwerner@xxxxxxxxxxxx> wrote:
> ...
>>> Rob Herring wrote:
>>>> Don't you need need to keep the kernel from allocating this memory by
>>>> using one of the reserved memory mechanisms? The recently added one
>>>> should be able to specific what the memory is reserved for IIRC.
>>>
>>> Our bootloader is carving the location out of the /memory node and
>>> adding it to the device tree reserve map. As far as I know, that only
>>> contains a list of raw start and size entries. At any rate, I think
>>> it's useful (and in line with other bindings) to add a more explicit
>>> node like this (if only to make it easier accessible through
>>> /proc/device-tree).
>>
>> Understand there are 3 different memory reservation bindings. The
>> original "/memreserve/" method is indeed limited. What I think you
>> should use is the binding documented in
>> Documentation/devicetree/bindings/reserved-memory/reserved-memory.txt.
>> So you could do something like this:
>>
>>         reserved-memory {
>>                 #address-cells = <1>;
>>                 #size-cells = <1>;
>>                 ranges;
>>
>>                 /* global autoconfigured region for contiguous allocations */
>>                 linux,cma {
>>                         compatible = "shared-dma-pool";
>>                         reusable;
>>                         size = <0x4000000>;
>>                         alignment = <0x2000>;
>>                         linux,cma-default;
>>                 };
>>
>>                 coreboot_reserved: coreboot@fdfea000 {
>>                       compatible = "coreboot";
>>                       reg = <0xfdfea000 0x264>,
>>                           <0xfdfea000 0x16000>;
>>                 };
>>
>
> I thought that the /reserved-memory node was more so that the
> (preferred?) location of firmware images or data buffers used by HW
> accelerators could be communicated to the kernel. This feels like pure data.

That was the initial use, but I think this case fits well. PPC may
have some other uses already. I haven't looked.

> The coreboot binding seems to be more about defining an interface to a
> particular firmware (this feels like semantics), which as a side-effect
> needs to communicate the location of certain data.

The DT is only defining it's presence and location. Knowledge of what
the data is is (optionally) in the kernel.

> If /reserved-memory is used to communicate the address of the memory
> regions, I think we still need a /firmware/coreboot node to indicate the
> semantics of the reserved memory region, and point at the phandle of the
> region. As such, it seems simpler just to put the addresses in the
> coreboot node's reg property. The only exception I see to that argument
> is if putting the region in /reserved-memory automatically carves that
> region out of the memory the kernel will allocate from. That would
> simplify the bootloader, since it wouldn't have to fiddle with the
> /memory node and do the carveout itself.

The semantics are defined by a compatible property which the
/reserved-memory binding has. Knowledge of what is in that range is in
the kernel, not the DT. And yes, either /memreserve/ or
/reserved-memory is needed to prevent kernel allocations.

I'm not tied to /reserved-memory, but I just want a consistent way to
describe random blocks of static data in memory. DMI tables are
another similar example. We've already got some scattered usage of
/firmware. There's also /psci for PSCI. UEFI is using /chosen for its
tables (probably for good reason since the tables are needed early and
need to be simple to parse).

Rob
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