linux-kernel - Re: [PATCH v1] gpio: keystone: add dsp gpio controller driver

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Date:	Thu, 24 Jul 2014 13:52:51 -0500
From:	Suman Anna <s-anna@...com>
To:	Jassi Brar <jaswinder.singh@...aro.org>,
	Santosh Shilimkar <santosh.shilimkar@...com>
CC:	Linus Walleij <linus.walleij@...aro.org>,
	Grygorii Strashko <grygorii.strashko@...com>,
	Alexander Shiyan <shc_work@...l.ru>,
	Alexandre Courbot <gnurou@...il.com>,
	"linux-gpio@...r.kernel.org" <linux-gpio@...r.kernel.org>,
	<ivan.khoronzhuk@...com>,
	Muralidharan Karicheri <m-karicheri2@...com>,
	Rob Herring <robh+dt@...nel.org>,
	Kumar Gala <galak@...eaurora.org>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v1] gpio: keystone: add dsp gpio controller driver

Hi,

On 07/24/2014 01:12 PM, Jassi Brar wrote:
> On 24 July 2014 22:52, Santosh Shilimkar <santosh.shilimkar@...com> wrote:
>> On Thursday 24 July 2014 01:19 PM, Jassi Brar wrote:
>>> On 23 July 2014 20:40, Linus Walleij <linus.walleij@...aro.org> wrote:
>>>> On Wed, Jul 16, 2014 at 12:43 PM, Grygorii Strashko
>>>> <grygorii.strashko@...com> wrote:
>>>>
>>>>> From: Murali Karicheri <m-karicheri2@...com>
>>>>>
>>>>> On Keystone SOCs, ARM host can send interrupts to DSP cores using the
>>>>> DSP GPIO controller IP. Each DSP GPIO controller provides 28 IRQ signals for
>>>>> each DSP core. This is one of the component used by the IPC mechanism used
>>>>> on Keystone SOCs.
>>>>>
>>>>> Keystone 2 DSP GPIO controller has specific features:
>>>>> - each GPIO can be configured only as output pin;
>>>>> - setting GPIO value to 1 causes IRQ generation on target DSP core;
>>>>> - reading pin value returns 0 - if IRQ was handled or 1 - IRQ is still
>>>>>   pending.
>>>>>
>>>>> Signed-off-by: Murali Karicheri <m-karicheri2@...com>
>>>>> Signed-off-by: Grygorii Strashko <grygorii.strashko@...com>
>>>>
>>>> Pardon me. How is this GENERAL PURPOSE Input/Output?
>>>>
>>>> It seems very very much SPECIAL PURPOSE to me, it's like
>>>> you're just shoehorning some IPC mechanism into the GPIO
>>>> subsystem, and this may be because the datasheet calls it
>>>> GPIO when it's not.
>>>>
>>>> What other stuff than DSP is connected to these lines, and is it
>>>> really even external lines? Aren't these just polysilicon rails
>>>> pretty much hammered to be used by the DSP and nothing else.
>>>>
>>>> What is the difference between this and a mailbox IRQ line
>>>> and the kind of stuff handled by drivers/mailbox?
>>>>
>>>> I'd like Suman and Jassi to have a look at this to see if it's
>>>> actually a mailbox before we proceed.
>>>>
>>> The controller seems like most others, only incapable of reading
>>> signals (output only).
>>>  The userspace driving those signals to communicate with a DSP isn't
>>> enough to call it a mailbox usecase, because on a different board the
>>> userspace may drive those signals to control LEDs :)
>>>
>> Exactly !!
>> And that was my point. Thanks for echo.
>>
> Yeah but if the AP and DSP are within the same package (i.e, the
> 'pins' can't be used for any other purpose on any board), one might
> sell it as a mailbox. However, since the mailbox protocol driver would
> be in userspace, I think it is justified to expose that as GPIO
> otherwise we'll have to add another interface for userspace to control
> the DSP.

If all these pins or a group of pins are collectively being used to
denote a message to the DSP, then I can see the argument for this being
a mailbox platform driver. It then probably would need to be
supplemented by some other protocol driver to expose the necessary
functionality to userspace. Otherwise, I agree with Jassi in general.

That said, having two different drivers for different GPIO IP instances
within the same SoC doesn't make sense. There should be a single driver
for all the GPIO IPs in keystone, output/input only are properties of
the instance and the driver should handle that IMHO.

regards
Suman
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