Defining schemas for Device Tree

[jonsmirl at gmail.com]

On Tue, Jul 30, 2013 at 1:34 PM, Stephen Warren <swarren at wwwdotorg.org> wrote:
> On 07/30/2013 11:29 AM, jonsmirl at gmail.com wrote:
>> On Tue, Jul 30, 2013 at 1:19 PM, Stephen Warren <swarren at wwwdotorg.org> wrote:
>>>
>>> On 07/30/2013 07:14 AM, jonsmirl at gmail.com wrote:
>>>> On Tue, Jul 30, 2013 at 6:25 AM, Mark Brown <broonie at kernel.org> wrote:
>>>>> On Mon, Jul 29, 2013 at 08:30:29PM -0400, jonsmirl at gmail.com wrote:
>>>>>
>>>>>> This...
>>>>>> tx-dma-channel = <&pdma0 7>; /* preliminary */
>>>>>> rx-dma-channel = <&pdma0 6>; /* preliminary */
>>>>>
>>>>>> Probably should be
>>>>>> dmas = <&pdma0 7>,<&pdma0 6>;
>>>>>> dma-names = "tx", "rx";
>>>>>
>>>>> It should be - the latter is the generic DMA binding.  There's a lot of
>>>>> bindings in the kernel that predate that but people are currently
>>>>> working to transfer over, this is one of the examples of instability
>>>>> that everyone is talking about.
>>>>
>>>> Is something similar to this possible in device tree syntax?
>>>> dmas =  <"tx" &pdma0 7>,  <"rx"  &pdma0 6>;
>>>
>>> I /think/ you can physically write that in *.dts, or something very
>>> similar; with the strings outside the <>:
>>>
>>> dmas = "tx", <&pdma0 7>, "rx", <&pdma0 6>;
>>>
>>> However, there's been strong push-back (i.e. doing that has not been
>>> allowed at all) on attempting to mix variable-length strings and
>>> fixed-length/alignment integer cells in the same property. This is
>>> primarily because you then can't ensure that the integer cell data is
>>> aligned in the DTB (dtc and/or the DTB format spec does/requires/allows

The PowerPC FDT doc says that strings are indirected into a string
table. So they aren't variable length in the data structures.

I was wondering how dmas = "tx", "rx" was implemented. It is two
pointers into the indirect block. So this should work, there is no
variable length data to worry about in the structure.

>>> dmas = "tx", <&pdma0 7>, "rx", <&pdma0 6>;


>>
>> Can we turn the strings into string phandles? Then they'd be fixed sized.
>
> I believe that phandles point at nodes not arbitrary data, so no.
>
> Now, could the DTB format be enhanced to allow that? Perhaps, although
> we'd need a different string syntax to enable it, since existing
> bindings would require the current inline-string format. That's getting
> outside the realm of schemas and making active changes to the DT format
> though...
>
>> That might provide a way to internationalize the strings too.
>
> I don't think internationalization is needed; DT is an interface to
> drivers/SW, rather than something end-user-visible.



-- 
Jon Smirl
jonsmirl at gmail.com