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 
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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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