Date:   Wed, 19 Jan 2022 18:34:53 +0300
From:   Sergey Shtylyov <s.shtylyov@....ru>
To:     Uwe Kleine-König <u.kleine-koenig@...gutronix.de>
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Subject: Re: [PATCH 1/2] platform: make platform_get_irq_optional() optional

Hello!

On 1/19/22 1:26 AM, Uwe Kleine-König wrote:

[...]
>>>>>>> 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?

   Yes, they follow from the platform_get_irq_optional() changing the sense of its result...
 
[...]
>>> 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

   MIPS has read-only register 0 (containing 0 :-)) which should simplify things. But
I'd have to check the actual object code... yeah, MIPS has a branching instruction that
compares 2 registers and branches on the result'; with -ENXIO you'd have to load an
immediate value into a register first... 

> 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

   Mhm, do we really use -O3 to build the kernel?

> 	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

   Hm, 2 bytes only -- perhaps Thumb mode?

> i686-linux-gnu-gcc		|         0000002a |         0000002a

   Expected.

> mips64el-linux-gnuabi64-gcc	| 0000000000000054 | 000000000000005c

   That's even more than expected -- 64-bit mode used?

> powerpc-linux-gnu-gcc		|         00000058 |         00000058

   Well, they say

> s390x-linux-gnu-gcc		| 000000000000002e | 0000000000000030
> x86_64-linux-gnu-gcc		| 0000000000000022 | 0000000000000022

   Again, as expected...

> So you save some bytes indeed.

   I see you have a lot of spare time (unlike me!). :-)

>>> 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:

   I think they do...

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

   Mhm, how is this connected with my patch? :-/

>  - 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?

   I have dropped this file from my (to be posted yet) v2... sorry that it
took so long...

>  - 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?!

  That's debatable... but anyway it's a matter of a separate (follow up)
patch...

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

   Same as above...

>  - 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?

   This doesn't matter much really but can change to 0, if you want... :-)

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

   Again, matter of a separate patch (I don't have the guily hardware anymore
but I guess Geert could help with that).

> I stopped here.

   Note that most of your complaints are about the existing driver logic --
which my patch just couldn't deal with... I currently don't have the bandwidth
for addressing all your complaints; some (more obvious) I'm goiing to address
as the time permits, the draft patches have been done already...

> It seems quite common that drivers assume a value < 0 returned by
> platform_get_irq means not-found

   Of course, before this patch -ENXIO meant IRQ-not-found, many drivers
don't bother to filter it out separately (for simplicity?).

> and don't care for -EPROBE_DEFER (what else can happen?).

   Hm, I haven't really seen a lot the probe dererral mishandling in the code
touched by at least my patch #1...

> Changing a relevant function in that mess seems unfortunate here :-\

   You seem to have some spare time and I'm getting distracted contrariwise...
want to help? :-)

> Best regards
> Uwe

MBR, Sergey

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