linux-kernel - Re: [PATCH] of: Add generic handling for hardware incomplete fail state

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Message-ID: <570D6B7A.3050203@gmail.com>
Date:	Tue, 12 Apr 2016 14:41:14 -0700
From:	Frank Rowand <frowand.list@...il.com>
To:	Tony Lindgren <tony@...mide.com>
CC:	Grant Likely <grant.likely@...aro.org>,
	Rob Herring <robh+dt@...nel.org>, devicetree@...r.kernel.org,
	linux-kernel@...r.kernel.org, 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 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.
> 
> 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) {

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. :)


>> 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.


> Regards,
> 
> Tony
>