Re: [PATCH 1/2] platform: make platform_get_irq_optional() optional

Uwe Kleine-König
2022-01-18T22:27:27Z

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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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