On Sat, Jan 15, 2022 at 9:22 PM Sergey Shtylyov [off-list ref] wrote:

On 1/14/22 11:22 PM, Uwe Kleine-König wrote:

quoted

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...

quoted

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@linux-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