Re: [PATCH 1/5] pci: handled return value of platform_get_irq correctly

From: Bjorn Helgaas <helgaas@kernel.org>
To: Linus Walleij <linus.walleij@linaro.org>
Cc: Aman Sharma <amanharitsh123@gmail.com>,
	Thomas Petazzoni <thomas.petazzoni@bootlin.com>,
	Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>,
	Andrew Murray <amurray@thegoodpenguin.co.uk>,
	Ryder Lee <ryder.lee@mediatek.com>,
	Karthikeyan Mitran <m.karthikeyan@mobiveil.co.in>,
	Hou Zhiqiang <Zhiqiang.Hou@nxp.com>,
	Marc Gonzalez <marc.w.gonzalez@free.fr>,
	Mans Rullgard <mans@mansr.com>,
	Matthias Brugger <matthias.bgg@gmail.com>,
	linux-pci <linux-pci@vger.kernel.org>,
	Linux ARM <linux-arm-kernel@lists.infradead.org>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	linux-mediatek@lists.infradead.org, Marc Zyngier <maz@kernel.org>,
	Thomas Gleixner <tglx@linutronix.de>
Subject: Re: [PATCH 1/5] pci: handled return value of platform_get_irq correctly
Date: Thu, 12 Mar 2020 14:02:02 -0500	[thread overview]
Message-ID: <20200312190202.GA110276@google.com> (raw)
In-Reply-To: <CACRpkdYv0U0RmT7snp+UejEXecq4wLkhc11DUniUfGYAgyXC=A@mail.gmail.com>

[+cc Marc, Thomas]

Hi Linus,

On Thu, Mar 12, 2020 at 03:07:58PM +0100, Linus Walleij wrote:
> On Wed, Mar 11, 2020 at 8:19 PM Aman Sharma <amanharitsh123@gmail.com> wrote:
> > Signed-off-by: Aman Sharma <amanharitsh123@gmail.com>
> > ---
> >  drivers/pci/controller/pci-v3-semi.c | 4 ++--
> >  1 file changed, 2 insertions(+), 2 deletions(-)
> >
> > diff --git a/drivers/pci/controller/pci-v3-semi.c b/drivers/pci/controller/pci-v3-semi.c
> > index bd05221f5a22..a5bf945d2eda 100644
> > --- a/drivers/pci/controller/pci-v3-semi.c
> > +++ b/drivers/pci/controller/pci-v3-semi.c
> > @@ -777,9 +777,9 @@ static int v3_pci_probe(struct platform_device *pdev)
> >
> >         /* Get and request error IRQ resource */
> >         irq = platform_get_irq(pdev, 0);
> > -       if (irq <= 0) {
> > +       if (irq < 0) {
> 
> Have you considered:
> https://lwn.net/Articles/470820/
> 
> TL;DR Linus (both of them) are not with you on this.
> 
> And that is why the code is written like this.

I'm not sure I understand you here, so please correct me when I go in
the weeds.  I guess you're saying that platform_get_irq() can return
0 here and we should treat that as an error?

This particular driver seems to be ARM-specific -- does that mean we
need to check for 0 on some arches but not others?  That would
definitely be suboptimal, and that's what I'd like to fix here.

IIUC, in the link you mentioned, Linus T says that "dev->irq == 0"
means we don't have a valid IRQ.  I think that makes sense, but I'm
not sure it follows that 0 must be a sensical return value for
platform_get_irq().  It seems to me that platform_get_irq() ought to
return either a valid IRQ or an error, and the convention for errors
is a negative errno.

In fact, the platform_get_irq() function comment says it returns "IRQ
number on success, negative error number on failure."  If we need to
interpret 0 as an error on some arches, it sounds like something is
wrong in the arch-specific bowels of platform_get_irq().

If platform_get_irq() returns an error, a driver might want to
continue in polled mode without IRQs, in which case it could set its
"dev->irq = 0" to indicate that it has no valid IRQ.  But I think we
might be able to separate that "stored IRQ" situation from the
platform_get_irq() interface.

Bjorn