linux-kernel - Re: [PATCH] irqchip: bcm2835: Add FIQ support

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Message-ID: <55AEF6EB.5080403@wwwdotorg.org>
Date:	Tue, 21 Jul 2015 19:50:35 -0600
From:	Stephen Warren <swarren@...dotorg.org>
To:	Noralf Trønnes <noralf@...nnes.org>
CC:	tglx@...utronix.de, jason@...edaemon.net,
	linux-rpi-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH] irqchip: bcm2835: Add FIQ support

On 07/14/2015 05:48 AM, Noralf Trønnes wrote:
> Den 14.07.2015 06:50, skrev Stephen Warren:
>> On 07/11/2015 09:26 AM, Noralf Trønnes wrote:
>>> Den 11.07.2015 06:09, skrev Stephen Warren:
>>>> (Sorry for the slow reply; I was on vacation)
>>>>
>>>> On 06/18/2015 07:32 AM, Noralf Trønnes wrote:
>>>>> Den 18.06.2015 04:26, skrev Stephen Warren:
>>>>>> On 06/12/2015 11:26 AM, Noralf Trønnes wrote:
>>>>>>> Add a duplicate irq range with an offset on the hwirq's so the
>>>>>>> driver can detect that enable_fiq() is used.
>>>>>>> Tested with downstream dwc_otg USB controller driver.
>>>>>> This basically looks OK, but a few comments/thoughts:
>>>>>> b) Doesn't the driver need to refuse some operation (handler
>>>>>> registration, IRQ setup, IRQ enable, ...?) for more than 1 IRQ in the
>>>>>> FIQ range, since the FIQ control register only allows routing 1
>>>>>> IRQ to
>>>>>> FIQ.
>>>>> claim_fiq() protects the FIQ. See d) answer below.
>>>> That assumes the IRQ is "accessed" via the fiq-specific APIs. Since
>>>> this
>>>> patch changes the IRQ domain from having n IRQs to having 2*n IRQs, and
>>>> doesn't do anything special to prevent clients from using IRQs n..2n-1
>>>> via the existing IRQ APIs, it's quite possible the a buggy client
>>>> would.
>>> Yes, but doesn't this apply to all irq use, using the wrong one doesn't
>>> work.
>>> If FIQ's where in more common use, we might have seen a FIQ IRQ flag
>>> instead
>>> of special FIQ irqs.
>>>
>>>> (From another email):
>>>>>>> c) The DT binding needs updating to describe the extra IRQs:
>>>>>>>
>>>>>>>> Documentation/devicetree/bindings/interrupt-controller/brcm,bcm28armctrl-ic.txt
>>>>>>>>
>>>>>>>>
>>>>>> Ok.
>>>>> I have seconds thoughts on this:
>>>>> This patch does not change the DT bindings so I don't see what update
>>>>> I should make. This patch only adds support for the Linux way of
>>>>> handling FIQ's through enable_fiq(). It doesn't change how interrupts
>>>>> are described in the DT.
>>>> The intention of the patch may not be to expand the set of IRQs
>>>> available via DT, but it does in practice. I think you need to add a
>>>> custom of_xlate for the IRQ domain to ensure that only IRQs 0..n-1 can
>>>> be translated from DT, and not IRQs n..2n-1. If you do that, then I
>>>> agree that no DT binding update should be required.
>>> armctrl_xlate() maps to the same hwirqs as before. This patch adds a
>>> new range of hwirqs at the end of the "real" hwirq range.
>>> It's not possible to get to these FIQ shadow hwirqs through DT.
>> What prevents a DT from (incorrectly) referencing the extra hwirqs?
> 
> armctrl_xlate() has these limits:
> 
> if (WARN_ON(intspec[0] >= NR_BANKS))
> if (WARN_ON(intspec[1] >= IRQS_PER_BANK))
> if (WARN_ON(intspec[0] == 0 && intspec[1] >= NR_IRQS_BANK0))
> 
> Thus the maximum values allowed are:
> intspec[0]: (NR_BANKS - 1) = 2
> intspec[1]: (IRQS_PER_BANK - 1) = 31
> 
> This gives a maximum hwirq:
> *out_hwirq = MAKE_HWIRQ(intspec[0], intspec[1]);
> *out_hwirq = (2 << 5) | 31 = 95
> 
> The FIQ shadow hwirq range starts at 96:
> irq = irq_create_mapping(intc.domain, MAKE_HWIRQ(b, i) + NUMBER_IRQS);
> 
> NUMBER_IRQS = MAKE_HWIRQ(NR_BANKS, 0) = 96

Great, thanks for the explanation. That should be fine then.

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