Date:   Tue, 18 Jan 2022 23:26:06 +0100
From:   Uwe Kleine-König <u.kleine-koenig@...gutronix.de>
To:     Sergey Shtylyov <s.shtylyov@....ru>
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Subject: Re: [PATCH 1/2] platform: make platform_get_irq_optional() optional

On Tue, Jan 18, 2022 at 11:21:45PM +0300, Sergey Shtylyov wrote:
> Hello!
> 
> On 1/17/22 11:47 AM, Uwe Kleine-König wrote:
> 
> [...]
> >>>>>>>>> To me it sounds much more logical for the driver to check if an
> >>>>>>>>> optional irq is non-zero (available) or zero (not available), than to
> >>>>>>>>> sprinkle around checks for -ENXIO. In addition, you have to remember
> >>>>>>>>> that this one returns -ENXIO, while other APIs use -ENOENT or -ENOSYS
> >>>>>>>>> (or some other error code) to indicate absence. I thought not having
> >>>>>>>>> to care about the actual error code was the main reason behind the
> >>>>>>>>> introduction of the *_optional() APIs.
> >>>>>>>
> >>>>>>>> No, the main benefit of gpiod_get_optional() (and clk_get_optional()) is
> >>>>>>>> that you can handle an absent GPIO (or clk) as if it were available.
> >>>>>>
> >>>>>>    Hm, I've just looked at these and must note that they match 1:1 with
> >>>>>> platform_get_irq_optional(). Unfortunately, we can't however behave the
> >>>>>> same way in request_irq() -- because it has to support IRQ0 for the sake
> >>>>>> of i8253 drivers in arch/...
> >>>>>
> >>>>> Let me reformulate your statement to the IMHO equivalent:
> >>>>>
> >>>>> 	If you set aside the differences between
> >>>>> 	platform_get_irq_optional() and gpiod_get_optional(),
> >>>>
> >>>>    Sorry, I should make it clear this is actually the diff between a would-be
> >>>> platform_get_irq_optional() after my patch, not the current code...
> >>>
> >>> The similarity is that with your patch both gpiod_get_optional() and
> >>> platform_get_irq_optional() return NULL and 0 on not-found. The relevant
> >>> difference however is that for a gpiod NULL is a dummy value, while for
> >>> irqs it's not. So the similarity is only syntactically, but not
> >>> semantically.
> >>
> >>    I have noting to say here, rather than optional IRQ could well have a different
> >> meaning than for clk/gpio/etc.
> >>
> >> [...]
> >>>>> However for an interupt this cannot work. You will always have to check
> >>>>> if the irq is actually there or not because if it's not you cannot just
> >>>>> ignore that. So there is no benefit of an optional irq.
> >>>>>
> >>>>> Leaving error message reporting aside, the introduction of
> >>>>> platform_get_irq_optional() allows to change
> >>>>>
> >>>>> 	irq = platform_get_irq(...);
> >>>>> 	if (irq < 0 && irq != -ENXIO) {
> >>>>> 		return irq;
> >>>>> 	} else if (irq >= 0) {
> >>>>
> >>>>    Rather (irq > 0) actually, IRQ0 is considered invalid (but still returned).
> >>>
> >>> This is a topic I don't feel strong for, so I'm sloppy here. If changing
> >>> this is all that is needed to convince you of my point ...
> >>
> >>    Note that we should absolutely (and first of all) stop returning 0 from platform_get_irq()
> >> on a "real" IRQ0. Handling that "still good" zero absolutely doesn't scale e.g. for the subsystems
> >> (like libata) which take 0 as an indication that the polling mode should be used... We can't afford
> >> to be sloppy here. ;-)
> > 
> > Then maybe do that really first?
> 
>    I'm doing it first already:
> 
> https://lore.kernel.org/all/5e001ec1-d3f1-bcb8-7f30-a6301fd9930c@omp.ru/
> 
>    This series is atop of the above patch...

Ah, I missed that (probably because I didn't get the cover letter).

> > I didn't recheck, but is this what the
> > driver changes in your patch is about?
> 
>    Partly, yes. We can afford to play with the meaning of 0 after the above patch.

But the changes that are in patch 1 are all needed?
 
> > After some more thoughts I wonder if your focus isn't to align
> > platform_get_irq_optional to (clk|gpiod|regulator)_get_optional, but to
> > simplify return code checking. Because with your change we have:
> > 
> >  - < 0 -> error
> >  - == 0 -> no irq
> >  - > 0 -> irq
> 
>    Mainly, yes. That's why the code examples were given in the description.
> 
> > For my part I'd say this doesn't justify the change, but at least I
> > could better life with the reasoning. If you start at:
> > 
> > 	irq = platform_get_irq_optional(...)
> > 	if (irq < 0 && irq != -ENXIO)
> > 		return irq
> > 	else if (irq > 0)
> > 		setup_irq(irq);
> > 	else
> > 		setup_polling()
> > 
> > I'd change that to
> > 
> > 	irq = platform_get_irq_optional(...)
> > 	if (irq > 0) /* or >= 0 ? */
> 
>    Not >= 0, no...
> 
> > 		setup_irq(irq)
> > 	else if (irq == -ENXIO)
> > 		setup_polling()
> > 	else
> > 		return irq
> > 
> > This still has to mention -ENXIO, but this is ok and checking for 0 just
> > hardcodes a different return value.
> 
>    I think comparing with 0 is simpler (and shorter) than with -ENXIO, if you
> consider the RISC CPUs, like e.g. MIPS...

Hmm, I don't know MIPS good enough to judge. So I created a small C
file:

	$ cat test.c
	#include <errno.h>

	int platform_get_irq_optional(void);
	void a(void);

	int func_0()
	{
		int irq = platform_get_irq_optional();

		if (irq == 0)
			a();
	}

	int func_enxio()
	{
		int irq = platform_get_irq_optional();

		if (irq == -ENXIO)
			a();
	}

With some cross compilers as provided by Debian doing

	$CC -c -O3 test.c
	nm --size-sort test.o

I get:

  compiler			|  size of func_0  | size of func_enxio
================================+==================|====================
aarch64-linux-gnu-gcc		| 0000000000000024 | 0000000000000028
arm-linux-gnueabi-gcc		|         00000018 |         00000018
arm-linux-gnueabihf-gcc		|         00000010 |         00000012
i686-linux-gnu-gcc		|         0000002a |         0000002a
mips64el-linux-gnuabi64-gcc	| 0000000000000054 | 000000000000005c
powerpc-linux-gnu-gcc		|         00000058 |         00000058
s390x-linux-gnu-gcc		| 000000000000002e | 0000000000000030
x86_64-linux-gnu-gcc		| 0000000000000022 | 0000000000000022

So you save some bytes indeed.

> > Anyhow, I think if you still want to change platform_get_irq_optional
> > you should add a few patches converting some drivers which demonstrates
> > the improvement for the callers.
> 
>    Mhm, I did include all the drivers where the IRQ checks have to be modified,
> not sure what else you want me to touch...

I somehow expected that the changes that are now necessary (or possible)
to callers makes them prettier somehow. Looking at your patch again:

 - drivers/counter/interrupt-cnt.c
   This one is strange in my eyes because it tests the return value of
   gpiod_get_optional against NULL :-(

 - drivers/edac/xgene_edac.c
   This one just wants a silent irq lookup and then throws away the
   error code returned by platform_get_irq_optional() to return -EINVAL.
   Not so nice, is it?

 - drivers/gpio/gpio-altera.c
   This one just wants a silent irq lookup. And maybe it should only
   goto skip_irq if the irq was not found, but on an other error code
   abort the probe?!

 - drivers/gpio/gpio-mvebu.c
   Similar to gpio-altera.c: Wants a silent irq and improved error
   handling.

 - drivers/i2c/busses/i2c-brcmstb.c
   A bit ugly that we now have dev->irq == 0 if the irq isn't available,
   but if requesting the irq failed irq = -1 is used?

 - drivers/mmc/host/sh_mmcif.c
   Broken error handling. This one wants to abort on irq[1] < 0 (with
   your changed semantic).

I stopped here.

It seems quite common that drivers assume a value < 0 returned by
platform_get_irq means not-found and don't care for -EPROBE_DEFER (what
else can happen?) Changing a relevant function in that mess seems
unfortunate here :-\

Best regards
Uwe

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

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