Date:   Mon, 17 Jan 2022 12:49:23 +0100
From:   Uwe Kleine-König <u.kleine-koenig@...gutronix.de>
To:     Geert Uytterhoeven <geert@...ux-m68k.org>
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Subject: Re: [PATCH 1/2] platform: make platform_get_irq_optional() optional

On Mon, Jan 17, 2022 at 11:35:52AM +0100, Geert Uytterhoeven wrote:
> Hi Uwe,
> 
> On Mon, Jan 17, 2022 at 10:24 AM Uwe Kleine-König
> <u.kleine-koenig@...gutronix.de> wrote:
> > On Mon, Jan 17, 2022 at 09:41:42AM +0100, Geert Uytterhoeven wrote:
> > > On Sat, Jan 15, 2022 at 9:22 PM Sergey Shtylyov <s.shtylyov@....ru> wrote:
> > > > On 1/14/22 11:22 PM, Uwe Kleine-König wrote:
> > > > > You have to understand that for clk (and regulator and gpiod) NULL is a
> > > > > valid descriptor that can actually be used, it just has no effect. So
> > > > > this is a convenience value for the case "If the clk/regulator/gpiod in
> > > > > question isn't available, there is nothing to do". This is what makes
> > > > > clk_get_optional() and the others really useful and justifies their
> > > > > existence. This doesn't apply to platform_get_irq_optional().
> > > >
> > > >    I do understand that. However, IRQs are a different beast with their
> > > > own justifications...
> > >
> > > > > clk_get_optional() is sane and sensible for cases where the clk might be
> > > > > absent and it helps you because you don't have to differentiate between
> > > > > "not found" and "there is an actual resource".
> > > > >
> > > > > The reason for platform_get_irq_optional()'s existence is just that
> > > > > platform_get_irq() emits an error message which is wrong or suboptimal
> > > >
> > > >    I think you are very wrong here. The real reason is to simplify the
> > > > callers.
> > >
> > > Indeed.
> >
> > The commit that introduced platform_get_irq_optional() said:
> >
> >         Introduce a new platform_get_irq_optional() that works much like
> >         platform_get_irq() but does not output an error on failure to
> >         find the interrupt.
> >
> > So the author of 8973ea47901c81a1912bd05f1577bed9b5b52506 failed to
> > mention the real reason? Or look at
> > 31a8d8fa84c51d3ab00bf059158d5de6178cf890:
> >
> >         [...] use platform_get_irq_optional() to get second/third IRQ
> >         which are optional to avoid below error message during probe:
> >         [...]
> >
> > Look through the output of
> >
> >         git log -Splatform_get_irq_optional
> >
> > to find several more of these.
> 
> Commit 8973ea47901c81a1 ("driver core: platform: Introduce
> platform_get_irq_optional()") and the various fixups fixed the ugly
> printing of error messages that were not applicable.
> In hindsight, probably commit 7723f4c5ecdb8d83 ("driver core:
> platform: Add an error message to platform_get_irq*()") should have
> been reverted instead, until a platform_get_irq_optional() with proper
> semantics was introduced.

ack.

> But as we were all in a hurry to kill the non-applicable error
> message, we went for the quick and dirty fix.
> 
> > Also I fail to see how a caller of (today's) platform_get_irq_optional()
> > is simpler than a caller of platform_get_irq() given that there is no
> > semantic difference between the two. Please show me a single
> > conversion from platform_get_irq to platform_get_irq_optional that
> > yielded a simplification.
> 
> That's exactly why we want to change the latter to return 0 ;-)

OK. So you agree to my statement "The reason for
platform_get_irq_optional()'s existence is just that platform_get_irq()
emits an error message [...]". Actually you don't want to oppose but
say: It's unfortunate that the silent variant of platform_get_irq() took
the obvious name of a function that could have an improved return code
semantic.

So my suggestion to rename todays platform_get_irq_optional() to
platform_get_irq_silently() and then introducing
platform_get_irq_optional() with your suggested semantic seems
intriguing and straigt forward to me.

Another thought: platform_get_irq emits an error message for all
problems. Wouldn't it be consistent to let platform_get_irq_optional()
emit an error message for all problems but "not found"?
Alternatively remove the error printk from platform_get_irq().

> > So you need some more effort to convince me of your POV.
> >
> > > Even for clocks, you cannot assume that you can always blindly use
> > > the returned dummy (actually a NULL pointer) to call into the clk
> > > API.  While this works fine for simple use cases, where you just
> > > want to enable/disable an optional clock (clk_prepare_enable() and
> > > clk_disable_unprepare()), it does not work for more complex use cases.
> >
> > Agreed. But for clks and gpiods and regulators the simple case is quite
> > usual. For irqs it isn't.
> 
> It is for devices that can have either separate interrupts, or a single
> multiplexed interrupt.
> 
> The logic in e.g. drivers/tty/serial/sh-sci.c and
> drivers/spi/spi-rspi.c could be simplified and improved (currently
> it doesn't handle deferred probe) if platform_get_irq_optional()
> would return 0 instead of -ENXIO.

Looking at sh-sci.c the irq handling logic could be improved even
without a changed platform_get_irq_optional():

diff --git a/drivers/tty/serial/sh-sci.c b/drivers/tty/serial/sh-sci.c
index 968967d722d4..c7dc9fb84844 100644
--- a/drivers/tty/serial/sh-sci.c
+++ b/drivers/tty/serial/sh-sci.c
@@ -2873,11 +2873,13 @@ static int sci_init_single(struct platform_device *dev,
 	 * interrupt ID numbers, or muxed together with another interrupt.
 	 */
 	if (sci_port->irqs[0] < 0)
-		return -ENXIO;
+		return sci_port->irqs[0];
 
-	if (sci_port->irqs[1] < 0)
+	if (sci_port->irqs[1] == -ENXIO)
 		for (i = 1; i < ARRAY_SIZE(sci_port->irqs); i++)
 			sci_port->irqs[i] = sci_port->irqs[0];
+	else if (sci_port->irqs[1] < 0)
+		return sci_port->irqs[1];
 
 	sci_port->params = sci_probe_regmap(p);
 	if (unlikely(sci_port->params == NULL))

And then the code flow is actively irritating. sci_init_single() copies
irqs[0] to all other irqs[i] and then sci_request_irq() loops over the
already requested irqs and checks for duplicates. A single place that
identifies the exact set of required irqs would already help a lot.

Also for spi-rspi.c I don't see how platform_get_irq_byname_optional()
returning 0 instead of -ENXIO would help. Please talk in patches.

Preferably first simplify in-driver logic to make the conversion to the
new platform_get_irq_optional() actually reviewable.

> > And if you cannot blindly use the dummy, then you're not the targetted
> > caller of *_get_optional() and should better use *_get() and handle
> > -ENODEV explicitly.
> 
> No, because the janitors tend to consolidate error message handling,
> by moving the printing up, inside the *_get() methods.  That's exactly
> what happened here.

This is in my eyes the root cause of the issues at hand. Moving the
error message handling into a get function is only right for most of the
callers. So the more conservative approach would be to introduce a noisy
variant of the get function and convert all users that benefit
separately while the unreviewed callers and those that don't want an
error message can happily continue to use the silent variant.

> So there are three reasons: because the absence of an optional IRQ
> is not an error, and thus that should not cause (a) an error code
> to be returned, and (b) an error message to be printed, and (c)
> because it can simplify the logic in device drivers.

I don't agree to (a). If the value signaling not-found is -ENXIO or 0
(or -ENODEV) doesn't matter much. I wouldn't deviate from the return
code semantics of platform_get_irq() just for having to check against 0
instead of -ENXIO. Zero is then just another magic value.
(c) still has to be proven, see above.

> Commit 8973ea47901c81a1 ("driver core: platform: Introduce
> platform_get_irq_optional()") fixed (b), but didn't address (a) and
> (c).

Yes, it fixed (b) and picked a bad name for that.

Best regards
Uwe

-- 
Pengutronix e.K.                           | Uwe Kleine-König            |
Industrial Linux Solutions                 | https://www.pengutronix.de/ |

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