Date:   Wed, 12 Jan 2022 18:05:24 +0300
From:   Sergey Shtylyov <s.shtylyov@....ru>
To:     "Rafael J. Wysocki" <rafael@...nel.org>,
        Geert Uytterhoeven <geert@...ux-m68k.org>
CC:     Andrew Lunn <andrew@...n.ch>,
        Uwe Kleine-König <u.kleine-koenig@...gutronix.de>,
        Ulf Hansson <ulf.hansson@...aro.org>,
        Vignesh Raghavendra <vigneshr@...com>,
        KVM list <kvm@...r.kernel.org>, <linux-iio@...r.kernel.org>,
        Linus Walleij <linus.walleij@...aro.org>,
        "Amit Kucheria" <amitk@...nel.org>,
        ALSA Development Mailing List <alsa-devel@...a-project.org>,
        Liam Girdwood <lgirdwood@...il.com>,
        "Guenter Roeck" <groeck@...omium.org>,
        Thierry Reding <thierry.reding@...il.com>,
        "MTD Maling List" <linux-mtd@...ts.infradead.org>,
        Linux I2C <linux-i2c@...r.kernel.org>,
        Miquel Raynal <miquel.raynal@...tlin.com>,
        <linux-phy@...ts.infradead.org>, Jiri Slaby <jirislaby@...nel.org>,
        "David S. Miller" <davem@...emloft.net>,
        Khuong Dinh <khuong@...amperecomputing.com>,
        Florian Fainelli <f.fainelli@...il.com>,
        Matthias Schiffer <matthias.schiffer@...tq-group.com>,
        Joakim Zhang <qiangqing.zhang@....com>,
        Kamal Dasu <kdasu.kdev@...il.com>,
        Lee Jones <lee.jones@...aro.org>,
        "Bartosz Golaszewski" <brgl@...ev.pl>,
        Daniel Lezcano <daniel.lezcano@...aro.org>,
        "Tony Luck" <tony.luck@...el.com>,
        Kishon Vijay Abraham I <kishon@...com>,
        bcm-kernel-feedback-list <bcm-kernel-feedback-list@...adcom.com>,
        "open list:SERIAL DRIVERS" <linux-serial@...r.kernel.org>,
        Jakub Kicinski <kuba@...nel.org>,
        Zhang Rui <rui.zhang@...el.com>,
        Matthias Brugger <matthias.bgg@...il.com>,
        Platform Driver <platform-driver-x86@...r.kernel.org>,
        Linux PWM List <linux-pwm@...r.kernel.org>,
        Robert Richter <rric@...nel.org>,
        "Saravanan Sekar" <sravanhome@...il.com>,
        Corey Minyard <minyard@....org>,
        Linux PM list <linux-pm@...r.kernel.org>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        "John Garry" <john.garry@...wei.com>,
        Peter Korsgaard <peter@...sgaard.com>,
        "William Breathitt Gray" <vilhelm.gray@...il.com>,
        Mark Gross <markgross@...nel.org>,
        "open list:GPIO SUBSYSTEM" <linux-gpio@...r.kernel.org>,
        Alex Williamson <alex.williamson@...hat.com>,
        Mark Brown <broonie@...nel.org>,
        "Borislav Petkov" <bp@...en8.de>,
        Eric Auger <eric.auger@...hat.com>,
        Takashi Iwai <tiwai@...e.com>,
        Jaroslav Kysela <perex@...ex.cz>,
        <openipmi-developer@...ts.sourceforge.net>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Benson Leung <bleung@...omium.org>,
        Pengutronix Kernel Team <kernel@...gutronix.de>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        "open list:EDAC-CORE" <linux-edac@...r.kernel.org>,
        Richard Weinberger <richard@....at>,
        "Mun Yew Tham" <mun.yew.tham@...el.com>,
        Hans de Goede <hdegoede@...hat.com>,
        "Greg Kroah-Hartman" <gregkh@...uxfoundation.org>,
        Yoshihiro Shimoda <yoshihiro.shimoda.uh@...esas.com>,
        Cornelia Huck <cohuck@...hat.com>,
        "Linux MMC List" <linux-mmc@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-spi <linux-spi@...r.kernel.org>,
        Linux-Renesas <linux-renesas-soc@...r.kernel.org>,
        Vinod Koul <vkoul@...nel.org>,
        James Morse <james.morse@....com>,
        Zha Qipeng <qipeng.zha@...el.com>,
        Sebastian Reichel <sre@...nel.org>,
        Niklas Söderlund <niklas.soderlund@...natech.se>,
        "moderated list:ARM/Mediatek SoC..." 
        <linux-mediatek@...ts.infradead.org>,
        Brian Norris <computersforpeace@...il.com>,
        netdev <netdev@...r.kernel.org>
Subject: Re: [PATCH 1/2] platform: make platform_get_irq_optional() optional

On 1/12/22 5:41 PM, Rafael J. Wysocki wrote:

[...]
>>>> If an optional IRQ is not present, drivers either just ignore it (e.g.
>>>> for devices that can have multiple interrupts or a single muxed IRQ),
>>>> or they have to resort to polling. For the latter, fall-back handling
>>>> is needed elsewhere in the driver.
>>>> 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.
>>>
>>> The *_optional() functions return an error code if there has been a
>>> real error which should be reported up the call stack. This excludes
>>> whatever error code indicates the requested resource does not exist,
>>> which can be -ENODEV etc. If the device does not exist, a magic cookie
>>> is returned which appears to be a valid resources but in fact is
>>> not. So the users of these functions just need to check for an error
>>> code, and fail the probe if present.
>>
>> Agreed.
>>
>> Note that in most (all?) other cases, the return type is a pointer
>> (e.g. to struct clk), and NULL is the magic cookie.
>>
>>> You seems to be suggesting in binary return value: non-zero
>>> (available) or zero (not available)
>>
>> Only in case of success. In case of a real failure, an error code
>> must be returned.
>>
>>> This discards the error code when something goes wrong. That is useful
>>> information to have, so we should not be discarding it.
>>
>> No, the error code must be retained in case of failure.
>>
>>> IRQ don't currently have a magic cookie value. One option would be to
>>> add such a magic cookie to the subsystem. Otherwise, since 0 is
>>> invalid, return 0 to indicate the IRQ does not exist.
>>
>> Exactly. And using 0 means the similar code can be used as for other
>> subsystems, where NULL would be returned.
>>
>> The only remaining difference is the "dummy cookie can be passed
>> to other functions" behavior.  Which is IMHO a valid difference,
>> as unlike with e.g. clk_prepare_enable(), you do pass extra data to
>> request_irq(), and sometimes you do need to handle the absence of
>> the interrupt using e.g. polling.
>>
>>> The request for a script checking this then makes sense. However, i
>>> don't know how well coccinelle/sparse can track values across function
>>> calls. They probably can check for:
>>>
>>>    ret = irq_get_optional()
>>>    if (ret < 0)
>>>       return ret;
>>>
>>> A missing if < 0 statement somewhere later is very likely to be an
>>> error. A comparison of <= 0 is also likely to be an error. A check for
>>>> 0 before calling any other IRQ functions would be good. I'm
>>> surprised such a check does not already existing in the IRQ API, but
>>> there are probably historical reasons for that.
>>
>> There are still a few platforms where IRQ 0 does exist.
> 
> Not just a few even.  This happens on a reasonably recent x86 PC:
> 
> rafael@...tch:~/work/linux-pm> head -2 /proc/interrupts
>            CPU0       CPU1       CPU2       CPU3       CPU4       CPU5
>   0:         10          0          0          0          0          0
>  IR-IO-APIC    2-edge
> timer

   IIRC Linus has proclaimed that IRQ0 was valid for the i8253 driver (living in
arch/x86/); IRQ0 only was frowned upon when returned by platform_get_irq() and its
ilk.

MBR, Sergey

Powered by blists - more mailing lists