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Message-ID: <CAMuHMdXVbRudGs69f9ZzaP1PXhteDNZiXA658eMFAwP4nr9r3w@mail.gmail.com> Date: Mon, 17 Jan 2022 09:41:42 +0100 From: Geert Uytterhoeven <geert@...ux-m68k.org> To: Sergey Shtylyov <s.shtylyov@....ru> Cc: Uwe Kleine-König <u.kleine-koenig@...gutronix.de>, Andrew Lunn <andrew@...n.ch>, Ulf Hansson <ulf.hansson@...aro.org>, Vignesh Raghavendra <vigneshr@...com>, KVM list <kvm@...r.kernel.org>, "Rafael J. Wysocki" <rafael@...nel.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>, Andy Shevchenko <andriy.shevchenko@...ux.intel.com>, Jaroslav Kysela <perex@...ex.cz>, 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>, openipmi-developer@...ts.sourceforge.net, "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>, Kishon Vijay Abraham I <kishon@...com>, "open list:SERIAL DRIVERS" <linux-serial@...r.kernel.org>, Yoshihiro Shimoda <yoshihiro.shimoda.uh@...esas.com>, bcm-kernel-feedback-list <bcm-kernel-feedback-list@...adcom.com>, Zhang Rui <rui.zhang@...el.com>, 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>, Liam Girdwood <lgirdwood@...il.com>, 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>, Hans de Goede <hdegoede@...hat.com>, Alex Williamson <alex.williamson@...hat.com>, Mark Brown <broonie@...nel.org>, Borislav Petkov <bp@...en8.de>, Sebastian Reichel <sre@...nel.org>, Jakub Kicinski <kuba@...nel.org>, Matthias Brugger <matthias.bgg@...il.com>, Takashi Iwai <tiwai@...e.com>, platform-driver-x86@...r.kernel.org, Benson Leung <bleung@...omium.org>, Linux ARM <linux-arm-kernel@...ts.infradead.org>, linux-edac@...r.kernel.org, Tony Luck <tony.luck@...el.com>, Mun Yew Tham <mun.yew.tham@...el.com>, Eric Auger <eric.auger@...hat.com>, Greg Kroah-Hartman <gregkh@...uxfoundation.org>, "open list:GPIO SUBSYSTEM" <linux-gpio@...r.kernel.org>, 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>, Pengutronix Kernel Team <kernel@...gutronix.de>, Richard Weinberger <richard@....at>, Niklas Söderlund <niklas.soderlund@...natech.se>, linux-mediatek@...ts.infradead.org, Brian Norris <computersforpeace@...il.com>, netdev@...r.kernel.org Subject: Re: [PATCH 1/2] platform: make platform_get_irq_optional() optional 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. 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. Consider a device with multiple clock inputs, some of them optional, where the device driver has to find, select and configure a suitable clock to operate at a certain clock frequency. The driver can call clk_get_rate(NULL) fine, but will always receive a zero rate, so it has to check for this (regardless of this being a dummy clock or not, because this could be an unpopulated clock crystal, which would be described in DT as a (present) fixed-rate clock with clock-frequency = <0>). For configuring the clock rate, the driver does need to check explicitly for the presence of a dummy clock, as clk_set_rate(NULL, rate) returns 0 ("success"), while obviously it didn't do anything, and thus configuring the device to use that clock would cause breakage. You can check if the clock is a real clock or a dummy using "if (clk) ...". And you'd use the same pattern with platform_irq_get_optional() if it would return 0 if the IRQ was not found: "if (irq) ...". Gr{oetje,eeting}s, Geert -- Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68k.org In personal conversations with technical people, I call myself a hacker. But when I'm talking to journalists I just say "programmer" or something like that. -- Linus Torvalds
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