linux-kernel - Re: [PATCH v2 2/3] irqchip: irq-mips-gic: Provide function to map GIC user section

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Message-ID: <561F73D6.8040300@imgtec.com>
Date:	Thu, 15 Oct 2015 10:37:26 +0100
From:	Qais Yousef <qais.yousef@...tec.com>
To:	Thomas Gleixner <tglx@...utronix.de>,
	Marc Zyngier <marc.zyngier@....com>
CC:	Markos Chandras <markos.chandras@...tec.com>,
	<linux-mips@...ux-mips.org>, Alex Smith <alex.smith@...tec.com>,
	Jason Cooper <jason@...edaemon.net>,
	<linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 2/3] irqchip: irq-mips-gic: Provide function to map GIC
 user section

On 10/12/2015 11:16 AM, Thomas Gleixner wrote:
> On Mon, 12 Oct 2015, Marc Zyngier wrote:
>> On 12/10/15 10:40, Markos Chandras wrote:
>>> From: Alex Smith <alex.smith@...tec.com>
>>>
>>> The GIC provides a "user-mode visible" section containing a mirror of
>>> the counter registers which can be mapped into user memory. This will
>>> be used by the VDSO time function implementations, so provide a
>>> function to map it in.
> <SNIP>
>   
>> This looks much better than the previous version (though I cannot find
>> the two other patches on LKML just yet).
> Yes, it looks better. But I really have to ask the question why we are
> trying to pack the world and somemore into an irq chip driver. We
> already have the completely misplaced gic_read_count() there.

This code has a bad history. It was scattered all over the place in arch 
code. Andrew Bresticker did a good job cleaning it up and moved it to 
this irqchip driver.

     https://lkml.org/lkml/2014/9/18/487
     https://lkml.org/lkml/2014/10/20/481

>
> While I understand that all of this is in the GIC block at least
> according to the documentation, technically it's different hardware
> blocks. And logically its different as well.

Yes but they're exposed through the same register interface.

>
> So why not describe the various blocks (interrupt controller, timer,
> shadow timer) as separate entities in the device tree and let each
> subsystem look them up on their own. This cross subsystem hackery is
> just horrible and does not buy anything except merge dependencies and
> other avoidable hassle.

There's a mips-gic-timer driver in drivers/clocksource. But in device 
tree it's a subnode of the irqchip driver.

http://lxr.free-electrons.com/source/drivers/clocksource/mips-gic-timer.c
http://lxr.free-electrons.com/source/Documentation/devicetree/bindings/interrupt-controller/mips-gic.txt

>
> Thoughts?
>

It could be refactored but the DT binding already specifies the GIC 
timer as a subnode of GIC. Exposing this usermode register is the only 
thing left in the register set that GIC driver wasn't dealing with.

Little gain in changing all of this now I think?

Thanks,
Qais
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