Message-ID: <5294D692.3080100@ti.com>
Date:	Tue, 26 Nov 2013 22:42:50 +0530
From:	Sekhar Nori <nsekhar@...com>
To:	Grygorii Strashko <grygorii.strashko@...com>,
	Prabhakar Lad <prabhakar.csengg@...il.com>,
	Linus Walleij <linus.walleij@...aro.org>
CC:	LKML <linux-kernel@...r.kernel.org>,
	DLOS <davinci-linux-open-source@...ux.davincidsp.com>,
	LAK <linux-arm-kernel@...ts.infradead.org>,
	<linux-gpio@...r.kernel.org>, <devicetree@...r.kernel.org>,
	<linux-doc@...r.kernel.org>, Rob Herring <rob.herring@...xeda.com>,
	Pawel Moll <pawel.moll@....com>,
	Mark Rutland <mark.rutland@....com>,
	Stephen Warren <swarren@...dotorg.org>,
	Ian Campbell <ijc+devicetree@...lion.org.uk>,
	Rob Landley <rob@...dley.net>,
	Grant Likely <grant.likely@...aro.org>
Subject: Re: [PATCH v6 4/6] gpio: davinci: add OF support

On Tuesday 26 November 2013 06:03 PM, Grygorii Strashko wrote:
> On 11/25/2013 01:00 PM, Sekhar Nori wrote:
>> On Thursday 21 November 2013 11:45 PM, Prabhakar Lad wrote:
>>> From: KV Sujith <sujithkv@...com>
>>>
>>> This patch adds OF parser support for davinci gpio
>>> driver and also appropriate documentation in gpio-davinci.txt
>>> located at Documentation/devicetree/bindings/gpio/.
>>>
>>> Acked-by: Linus Walleij <linus.walleij@...aro.org>
>>> Acked-by: Rob Herring <rob.herring@...xeda.com>
>>> Signed-off-by: KV Sujith <sujithkv@...com>
>>> Signed-off-by: Philip Avinash <avinashphilip@...com>
>>> [prabhakar.csengg@...il.com: simplified the OF code, removed
>>> 		unnecessary DT property and also simplified
>>> 		the commit message]
>>> Signed-off-by: Lad, Prabhakar <prabhakar.csengg@...il.com>
>>> ---
>>>   .../devicetree/bindings/gpio/gpio-davinci.txt      |   41 ++++++++++++++
>>>   drivers/gpio/gpio-davinci.c                        |   57 ++++++++++++++++++--
>>>   2 files changed, 95 insertions(+), 3 deletions(-)
>>>   create mode 100644 Documentation/devicetree/bindings/gpio/gpio-davinci.txt
>>>
>>> diff --git a/Documentation/devicetree/bindings/gpio/gpio-davinci.txt b/Documentation/devicetree/bindings/gpio/gpio-davinci.txt
>>> new file mode 100644
>>> index 0000000..a2e839d
>>> --- /dev/null
>>> +++ b/Documentation/devicetree/bindings/gpio/gpio-davinci.txt
>>> @@ -0,0 +1,41 @@
>>> +Davinci GPIO controller bindings
>>> +
>>> +Required Properties:
>>> +- compatible: should be "ti,dm6441-gpio"
>>> +
>>> +- reg: Physical base address of the controller and the size of memory mapped
>>> +       registers.
>>> +
>>> +- gpio-controller : Marks the device node as a gpio controller.
>>> +
>>> +- interrupt-parent: phandle of the parent interrupt controller.
>>> +
>>> +- interrupts: Array of GPIO interrupt number. Only banked or unbanked IRQs are
>>> +	      supported at a time.
>>
>> If this is true..
>>
>>> +
>>> +- ti,ngpio: The number of GPIO pins supported.
>>> +
>>> +- ti,davinci-gpio-unbanked: The number of GPIOs that have an individual interrupt
>>> +		             line to processor.
>>
>> .. then why do you need to maintain this separately? Number of elements
>> in interrupts property should give you this answer, no?
>>
>> There can certainly be devices (past and future) which use a mixture of
>> banked and unbanked IRQs. So a binding which does not take care of this
>> is likely to change in future and that is a problem since it brings in
>> backward compatibility of the binding into picture.
>>
>> The right thing would be to define the DT node per-bank similar to what
>> is done on OMAP rather than for all banks together. That way there can
>> be a separate property which determines whether that bank supports
>> direct-mapped or banked IRQs (or that could be inferred if the number of
>> tuples in the interrupts property is more than one).
> 
> Number of IRQ can't be simply used to determine type of IRQ - need to handle IRQ names,
> because each bank(32 gpios) may have up to 2 banked IRQs (one per 16 GPIO).

Okay. That's why I inserted that comment in parenthesis :)

> 
> Few things here:
> - The mixed banked/unbanked functionality has never been supported before. 

True. I actually misread the driver before.

> - The Davinci GPIO IP is different from OMAP and has common
>   control registers for all banks.

Well the only common register I can see is BINTEN - bank interrupt
enable. This register can simply be initialized to enable interrupts
from all banks possible as until the rising and falling edge triggers
are programmed, there wont be any actual interrupts generated.

> - The proposed approach is more less easy to implement for DT case, but for not-DT
>   case - the platform data will need to be changed significantly (.
>   So, from this point of view, that would be a big change (actually the total driver rewriting).

Well, I certainly don't think its a complete driver re-write. It will
take a bit of effort agreed, but I think the driver will also come out a
lot cleaner.

Honestly, I am not so much worried about the kernel code here. Its the
bindings I am worried about. Once the bindings go in assuming there will
never be banked and unbanked GPIO IRQs on the same SoC, changing them to
do something else will be very painful with the need to keep backward
compatibility and support for both semantics.

That said, because there is no present hardware which needs both banked
and unbanked at the same time, I wont press for this to be done endlessly.

> Do you have any thoughts about how it can be done in a simpler way?

I don't know if there is a "simpler" way, but I don't think there is too
much effort too. I leave it to those implementing it though.

> 
> Actually, the same was proposed by Linus, but we've tried avoid such huge rework -
> by switching to one irq_domain per all banks for example.

I didn't really read that proposal from Linus so if two people
independently suggested the same thing, there must be something worth
considering there :)

Thanks,
Sekhar
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