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

[Pantelis Antoniou]

Hi Guenter,

On May 27, 2014, at 6:21 PM, Guenter Roeck wrote:

> On Tue, May 27, 2014 at 03:24:35PM +0300, Pantelis Antoniou wrote:
>> Hi Grant,
>> 
>> On May 27, 2014, at 3:12 PM, Grant Likely wrote:
>> 
>>> On Mon, 26 May 2014 16:42:44 -0700, Guenter Roeck <linux@xxxxxxxxxxxx> wrote:
>>>> On 05/26/2014 03:36 PM, Sebastian Reichel wrote:
>>>>> Hi,
>>>>> 
>>>>> On Mon, May 26, 2014 at 10:33:03PM +0100, Grant Likely wrote:
>>>>>> After thinking about it more, I think it is very likely that removing
>>>>>> all the overlays is the correct thing to do in the kexec use-case. When
>>>>>> kexec-ing, it makes sense that we'd want the exact same behaviour from
>>>>>> the kexec'ed kernel. That means we want the device drivers to do the
>>>>>> same thing including loading whatever overlays they depend on.
>>>>>> 
>>>>>> If the flattened tree was left applied, then the behaviour becomes
>>>>>> different.
>>>>>> 
>>>>>> I say always remove the overlays unless explicitly told not to, but I'm
>>>>>> struggling to come up with use cases where keeping them applied is
>>>>>> desirable.
>>>>> 
>>>>> I would assume, that I want them applied in most cases. DT describes
>>>>> the hardware. If I kexec into a new kernel I change software, not
>>>>> hardware.
>>>>> 
>>>>> Maybe I'm missing the main purpose of the feature. I currently see
>>>>> two useful usecases for DT overlays:
>>>>> 
>>>>> 1. The dtb the kernel is booted with cannot be changed for some
>>>>>   reason, but the board has additional hardware attached (e.g.
>>>>>   the user added a sensor on the i2c bus)
>>>>> 2. The hardware is changed on the fly (e.g. the user flashed the
>>>>>   FPGA part of a zynq processor), sensors on i2c bus, ...
>>>>> 
>>>>> In both cases the kernel should be booted with the additional
>>>>> overlay information IMHO. Though for the second case it should
>>>>> be possible to remove the "programmed" hardware information
>>>>> somehow.
>>>>> 
>>>> 
>>>> 3. Some hot-plug device or card is inserted or removed.
>>>> 
>>>> I would argue that the kernel should _not_ be booted with the overlay in place.
>>>> Otherwise the code handling overlays would have to have special handling
>>>> for the restart case, which is much more complex than just to re-insert
>>>> the overlay when it is determined that the device or card is still there.
>>> 
>>> Exactly.
>>> 
>> 
>> Looks like we are levitating to the 'remove overlays on kexec' approach.
>> Is that correct?
>> 
> 
> Let's just assume for a minute that this is not the case, and that loaded
> overlays are passed on.
> 
> This would be an interesting challenge for the overlay manager, as it would
> have to handle a number of startup conditions. After all, it can not take
> it as granted that the hardware state did not change after it was stopped
> in the old kernel, and before it was started in the new kernel.
> - overlay loaded, but hardware/device no longer present
>  -> unload overlay
> - overlay loaded, but different hardware present
>  -> unload old overlay, load new one
> - overlay not loaded, hardware present
>  -> load overlay
> - overlay loaded and matches hardware
>  -> do nothing
> 
> In comparison, its task would be quite straightforward if loaded overlays
> are not passed on to the new kernel.
> - If hardware is present, load overlay
> 

Yeah, exactly. The only case where having the applied overlays present on the 
kexec-ed kernel is either some kind of virtualization environment, or an fpga
that takes an awful lot amount of time to re-initializa.

> Ultimately, I seem to be missing something, as I don't really see the benefit
> of passing on the loaded overlay(s) to the new kernel. Activation time, maybe,
> for the most common case (overlay loaded and matches hardware) ?
> 
> Concern though is the other cases, with a mismatch between HW and loaded
> overlays. I am not sure if it is even possible to ensure that there are
> no race conditions if the devicetree is outdated at system startup. Sure,
> that is unlikely to happen in 'normal' operating conditions, but that only
> means that it _will_ happen a few hours after the first customer starts
> playing with the system.
> 

I concur. Accidents will happen, and if it's not a tightly controlled system
breakage is guaranteed.

> Guenter

Regards

-- Pantelis

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