Hello,
On Thu, Mar 05, 2020 at 01:22:32PM +0000, Paul Barker wrote:
> I recently ran into an issue using the pwm-fan driver with an inverted
> PWM output on iMX6.
>
> The fan is defined in the device tree as follows:
>
> fan0: pwm-fan {
> compatible = "pwm-fan";
> pwms = <&pwm2 0 25000 PWM_POLARITY_INVERTED>;
> ...
> }
>
> In pwm_imx27_probe() the support for a third `flags` argument in a pwm
> reference is enabled:
>
> imx->chip.of_xlate = of_pwm_xlate_with_flags;
> imx->chip.of_pwm_n_cells = 3;
>
> However, the flag is ignored and the output is not inverted.
>
> By adding some prints I saw that when of_pwm_xlate_with_flags() is
> called, args->args_count is 2 instead of 3.
>
> Looking at the definition of the pwm device itself in imx6qdl.dtsi I
> can see that the number of cells in a pwm reference is set to 2 not 3:
>
> pwm2: pwm@2084000 {
> #pwm-cells = <2>;
> ...
> };
>
> That seems to be preventing a third argument from being passed.
>
> I can change `#pwm-cells` to <3> and then everything works for my
> device but I'm not sure that is the correct solution for everyone. That
> would require all pwm references on iMX6 devices to use 3 cells. The
> code in of_pwm_xlate_with_flags() seems to be built to handle either 2
> or 3 argument cells but I can't see any way to allow this choice in the
> device tree.
>
> If the solution is to set `#pwm-cells` to <3> I'm happy to send a patch
> which does this and updates all pwm references in device trees which
> include `imx6dql.dtsi`. Before I do that I'd like to know that it's the
> correct approach though.
>
> For context I've confirmed this is the case in Linux 5.4 and that the
> relevant files haven't changed between that release and 5.6.0-rc4.
I think changing that is fine. However you'd have to care that all
in-tree users that rely on #pwm-cells = <2> are fixed accordingly.
I'd do: add #pwm-cells = <3> in the cpu.dtsi and then adapt all
machine.dts to add #pwm-cells = <2> until there are no more changes to
the generated files compared to the current state.
Best regards
Uwe
--
Pengutronix e.K. | Uwe Kleine-König |
Industrial Linux Solutions | https://www.pengutronix.de/ |
