Re: [PATCH v4 2/8] OF: Introduce DT overlay support.

[Pantelis Antoniou]

Hi Grant,

On May 26, 2014, at 2:23 PM, Grant Likely wrote:

> On Mon, 26 May 2014 12:57:32 +0200, Geert Uytterhoeven <geert@xxxxxxxxxxxxxx> wrote:
>> Hi Grant,
>> 
>> On Mon, May 26, 2014 at 12:48 PM, Grant Likely
>> <grant.likely@xxxxxxxxxxxx> wrote:
>>> On Tue, 20 May 2014 09:38:49 +0200, Geert Uytterhoeven <geert@xxxxxxxxxxxxxx> wrote:
>>>> On Tue, May 20, 2014 at 7:50 AM, Grant Likely <grant.likely@xxxxxxxxxxxx> wrote:
>>>>>> Why has the overlay system been designed for plugging and unpluging whole
>>>>>> overlays?
>>>>>> That means the kernel has to remember the full stack, causing issues with
>>>>>> e.g. kexec.
>>>>> 
>>>>> Mostly so that drivers don't see any difference in the livetree data
>>>>> structure. It also means that userspace sees a single representation of
>>>>> the hardware at any given time.
>>>> 
>>>> Sorry, I don't follow the argument about the "single representation of the
>>>> hardware".
>>> 
>>> Er, s/of the hardware/of the tree/. Right now the overlay design
>>> modifies the live tree which at the same time modifies the tree
>>> representation in /sys/firmware/devicetree. If the design was changed to
>>> keep the overlay logically separate, then I would think we want to
>>> expose that information to usespace also. In fact, I think we would need
>>> to for usecases like kexec.
>> 
>> OK, so it does modify the real tree, and doesn't keep the actual overlays.
>> 
>> I was under the impression the overlay stack was also kept in memory, to allow
>> reversal, so there was a misunderstanding.
>> 
>> Hence for kexec, the tree in /sys/firmware/devicetree can just be passed
>> to the new kernel, as that's the current representation of the hardware?
> 
> Heeheehee. We're back where we started. The original question is whether
> or not that is a valid approach. If the overlay represents something
> that can be hot plugged/unplugged, then passing it through to the second
> kernel would be the wrong thing to do. If it was a permenant addition,
> then it probably doesn't need to be removed.
> 
> We do actually keep the overlay info in memory for the purpose of
> removal exactly so we can support hot unbinding of devices and drivers
> that make use of overlays.
> 

We can support either method. I am not feeling any wiser about which one should be
the default TBH, so what about exporting a property and let the platform 
figure out which is more appropriate?

> g.
> 

Regards

-- Pantelis

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