linux-kernel - Re: [PATCH 2/2] pwm: Add support for Xilinx AXI Timer

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Message-ID: <20210504161334.tzylwyiz4k2tcztq@pengutronix.de>
Date:   Tue, 4 May 2021 18:13:34 +0200
From:   Uwe Kleine-König <u.kleine-koenig@...gutronix.de>
To:     Sean Anderson <sean.anderson@...o.com>,
        Rob Herring <robh@...nel.org>
Cc:     Michal Simek <michal.simek@...inx.com>, linux-pwm@...r.kernel.org,
        devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org,
        Lee Jones <lee.jones@...aro.org>,
        Thierry Reding <thierry.reding@...il.com>
Subject: Re: [PATCH 2/2] pwm: Add support for Xilinx AXI Timer

Hello Sean,

[Adding Rob to the list of recipents, as he for sure has a valuable
opinion on this matter.]

On Tue, May 04, 2021 at 11:57:20AM -0400, Sean Anderson wrote:
> On 5/4/21 8:32 AM, Michal Simek wrote:
> > On 5/4/21 10:51 AM, Uwe Kleine-König wrote:
> >> On Mon, May 03, 2021 at 05:44:13PM -0400, Sean Anderson wrote:
> >>> This adds PWM support for Xilinx LogiCORE IP AXI soft timers commonly
> >>> found on Xilinx FPGAs. There is another driver for this device located
> >>> at arch/microblaze/kernel/timer.c, but it is only used for timekeeping.
> >>> This driver was written with reference to Xilinx DS764 for v1.03.a [1].
> >>>
> >>> [1] https://www.xilinx.com/support/documentation/ip_documentation/axi_timer/v1_03_a/axi_timer_ds764.pdf
> >>>
> >>> Signed-off-by: Sean Anderson <sean.anderson@...o.com>
> >>> ---
> >>>
> >>>   arch/arm64/configs/defconfig |   1 +
> >>>   drivers/pwm/Kconfig          |  11 ++
> >>>   drivers/pwm/Makefile         |   1 +
> >>>   drivers/pwm/pwm-xilinx.c     | 322 +++++++++++++++++++++++++++++++++++
> >>>   4 files changed, 335 insertions(+)
> >>>   create mode 100644 drivers/pwm/pwm-xilinx.c
> >>>
> >>> diff --git a/arch/arm64/configs/defconfig b/arch/arm64/configs/defconfig
> >>> index 08c6f769df9a..81794209f287 100644
> >>> --- a/arch/arm64/configs/defconfig
> >>> +++ b/arch/arm64/configs/defconfig
> >>> @@ -1083,6 +1083,7 @@ CONFIG_PWM_SAMSUNG=y
> >>>   CONFIG_PWM_SL28CPLD=m
> >>>   CONFIG_PWM_SUN4I=m
> >>>   CONFIG_PWM_TEGRA=m
> >>> +CONFIG_PWM_XILINX=m
> >>>   CONFIG_SL28CPLD_INTC=y
> >>>   CONFIG_QCOM_PDC=y
> >>>   CONFIG_RESET_IMX7=y
> >>
> >> I think this should go into a separate patch once this driver is
> >> accepted. This can then go via the ARM people.
> >>
> >>> diff --git a/drivers/pwm/Kconfig b/drivers/pwm/Kconfig
> >>> index d3371ac7b871..01e62928f4bf 100644
> >>> --- a/drivers/pwm/Kconfig
> >>> +++ b/drivers/pwm/Kconfig
> >>> @@ -628,4 +628,15 @@ config PWM_VT8500
> >>>   	  To compile this driver as a module, choose M here: the module
> >>>   	  will be called pwm-vt8500.
> >>>
> >>> +config PWM_XILINX
> >>> +	tristate "Xilinx AXI Timer PWM support"
> >>> +	depends on !MICROBLAZE
> >>
> >> I don't understand this dependency.
> >
> > The dependency is clear here because microblaze has already driver for
> > this timer here arch/microblaze/kernel/timer.c.

Then at least add a comment.

> > And that's exactly pointing to the way how this should be done.
> > IP itself is single or dual timer and in case of dual timer you can
> > select if there is pwm output and use it for PWM generation.
> >
> > It means it is timer with PMW together.
> > I didn't have a time but Uwe likely knows this better how to design it.
> >
> > I see that gpio-mvebu driver instantiate pwm driver. Maybe that's the
> > way to go.
> 
> I think drivers/clocksource/samsung_pwm_timer.c and
> drivers/pwm/pwm-samsung.c provide another example for how to go about
> this.

I recently had a similar problem (with code that isn't (yet) in
mainline), where a timer can be used as a counter. I chose to change the
compatible. Transferred to this example this would mean to use e.g.

	static const struct of_device_id xilinx_pwm_of_match[] = {
		{ .compatible = "xlnx,xps-timer-pwm-1.00.a" },
		...
	};

and if you want to use the hardware as a PWM, you overwrite the
compatible in your machine.dts.

Not sure however that this is nice enough to be accepted by the dt
people?!

Best regards
Uwe

-- 
Pengutronix e.K.                           | Uwe Kleine-König            |
Industrial Linux Solutions                 | https://www.pengutronix.de/ |

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