Message-ID: <570D7895.7000606@gmail.com>
Date:	Tue, 12 Apr 2016 15:37:09 -0700
From:	Frank Rowand <frowand.list@...il.com>
To:	Rob Herring <robh+dt@...nel.org>
CC:	Tony Lindgren <tony@...mide.com>,
	Grant Likely <grant.likely@...aro.org>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	linux-omap <linux-omap@...r.kernel.org>,
	Nishanth Menon <nm@...com>, Tero Kristo <t-kristo@...com>,
	Tom Rini <trini@...sulko.com>
Subject: Re: [PATCH] of: Add generic handling for hardware incomplete fail
 state

On 4/12/2016 3:20 PM, Rob Herring wrote:
> On Tue, Apr 12, 2016 at 4:41 PM, Frank Rowand <frowand.list@...il.com> wrote:
>> On 4/12/2016 1:34 PM, Tony Lindgren wrote:
>>> Hi,
>>>
>>> * Frank Rowand <frowand.list@...il.com> [160412 13:15]:
>>>> Hi Tony,
>>>>
>>>> I agree with the need for some way of handling the incomplete
>>>> hardware issue.  I like the idea of having a uniform method for
>>>> all nodes.
>>>>
>>>> I am stumbling over what the status property is supposed to convey
>>>> and what the "fail-hw-incomplete" is meant to convey.
>>>>
>>>> The status property is meant to convey the current state of the
>>>> node.
>>>>
>>>> "fail-hw-incomplete" is meant to describe the node implementation,
>>>> saying that some portions of hardware that the driver expects to
>>>> be present do not exist.  If I understood your explanation at ELC
>>>> correctly, an examples of this could be that a uart cell is not
>>>> routed to transmit and receive data pins or the interrupt line
>>>> from the cell is not routed to an interrupt controller.  So the
>>>> node is not useful, but it makes sense to be able to power manage
>>>> the node, turning off power so that it is not wasting power.
>>>
>>> Yes cases like that are common.
>>>
>>>> It seems to me that the info that needs to be conveyed is a
>>>> description of the hardware, stating:
>>>>   - some portions or features of the node are not present and/or
>>>>     are not usable
>>>>   - power management of the node is possible
>>>>
>>>> Status of "fail-sss" is meant to indicate an error was detected in
>>>> the device, and that the error might (or might not) be repairable.
>>>>
>>>> So the difference I see is state vs hardware description.
> 
> The question to ask is are we indicating the "operational status of a
> device"? If yes, that is the definition of status and using it would
> be appropriate.
> 
> IMO, I think we are.

I see the reasoning.  I could go either way, but I lean toward thinking
of it as hardware description.


>>> OK thanks for the clarification. I don't see why "fail-hw-incomplete"
>>> could not be set dynamically during the probe in some cases based
>>> on the SoC revision detection for example. So from that point of
>>> view using status with the "fail-sss" logic would make more sense.
>>
>> If the probe detects that the device should only be power managed
>> based on the SoC revision, then it would simply be one more
>> test added at the top of probe.  The patch would change from:
>>
>>    if (of_device_is_incomplete(pdev->dev.of_node)) {
>>
>> to:
>>
>>    if (of_device_is_incomplete(pdev->dev.of_node) || socrev == XXX) {
> 
> I think Tony meant the bootloader or platform code would do this and
> tweak the DT. We don't have much of a standard API for revision
> checking, so drivers don't check SoC revisions generally.

OK, that makes more sense to me.


>> That code would be the same whether the property involved was
>> status or something else.
>>
>>>
>>>> I would prefer to come up with a new boolean property (with a
>>>> standard name that any node binding could choose to implement)
>>>> that says something like "only power management is available for
>>>> this node, do not attempt to use any other feature of the node".
>>>
>>> Heh that's going to be a long property name :) How about
>>> unusable-incomplete-idle-only :)
>>
>> Or even pm-only.  Maybe I got a little carried away with my
>> verbosity. :)
> 
> I don't think we should define it so narrowly. I think DT just
> indicates the device is in a non-usable state (somewhere between ok
> and disabled) and the driver knows what to do with that information.

My concern is that "non-usable" state is really vague.  I would
prefer that the message (however it is communicated) tells the
driver either what is usable or what is unusable.


>>>> With that change, the bulk of your patch looks good, with
>>>> minor changes:
>>>>
>>>>   __of_device_is_available() would not need to change.
>>>>
>>>>   __of_device_is_incomplete() would change to check the new
>>>>   boolean property.  (And I would suggest renaming it to
>>>>   something that conveys it is ok to power manage the
>>>>   device, but do not do anything else to the device.)
>>>
>>> I'm fine with property too, but the runtime probe fail state
>>> changes worry me a bit with that one.
>>
>> I don't understand what the concern is.  The change I suggested
>> would use exactly the same code for probe as the example patch
>> you provided, but just with a slight name change for the function.
>>
>>
>>> I think Rob also preferred to use the status though while we
>>> chatted at ELC?
>>
>> That is the impression I got too.  We'll have to see if I can
>> convince him otherwise.
> 
> I did, but I'm not wed to it. I think it depends on the question above.
> 
> Rob
> 

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