linux-kernel - Re: Why isn't IRQ shared for i2c-ocore

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Message-ID: <CAMuHMdWmqS7G2fzsECW0GOGP7JusPj6TA==ZV3RSJZFnig2wLQ@mail.gmail.com>
Date:	Fri, 8 May 2015 10:03:14 +0200
From:	Geert Uytterhoeven <geert@...ux-m68k.org>
To:	Lee Jones <lee.jones@...aro.org>
Cc:	York Sun <yorksun@...escale.com>,
	Julia Lawall <Julia.Lawall@...6.fr>,
	Linux I2C <linux-i2c@...r.kernel.org>, wolfram@...-dreams.de,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: Why isn't IRQ shared for i2c-ocore

On Fri, May 8, 2015 at 9:27 AM, Lee Jones <lee.jones@...aro.org> wrote:
> On Fri, 08 May 2015, Geert Uytterhoeven wrote:
>> On Fri, May 8, 2015 at 8:57 AM, Lee Jones <lee.jones@...aro.org> wrote:
>> > On Fri, 08 May 2015, Geert Uytterhoeven wrote:
>> >> On Thu, May 7, 2015 at 9:01 AM, Lee Jones <lee.jones@...aro.org> wrote:
>> >> >> I have a follow up question regarding interrupt. I see many I2C bus drivers
>> >> >> request interrupt with flag = 0. Why not using IRQF_SHARED?
>> >> >
>> >> > Probably because that particular IRQ is only used by the I2C
>> >> > Controller.  I'm not exactly sure that you're getting at?  Why do you
>> >> > think it should be shared?  You should only flag it as shared if it
>> >> > is.
>> >>
>> >> However, that's something the driver can't know.
>> >> Sharing interrupts is an integration property. The same IP core may share its
>> >> interrupt on one SoC, and not on another.
>> >
>> > I guess that would depend on the IP.  If this is part of an MFD, you'd
>> > know if you only hand a single interrupt line coming into the chip or
>> > not.  If the IP can be moved around (copy & pasted) into different
>> > chips, then yes, that might change.
>> >
>> > How does one share an interrupt with other drivers if all them don't
>> > know the IRQ is shared thought?
>>
>> All drivers sharing the same interrupt must pass IRQF_SHARED, cfr. the
>> checks in __setup_irq().
>>
>> Traditionally, PC (ISA) drivers didn't share interrupts, as the ISA bus
>> prohibited interrupt sharing.
>> Amiga drivers did.
>>
>> New drivers should support IRQ sharing.
>
> Precisely, which is why I'm confused by:
>
>   "However, that's something the driver can't know."

So better safe than sorry: always use IRQF_SHARED, unless you have
a very good reason not to use it.

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds
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