Re: [PATCH v11 2/2] pwm: Add support for Xilinx AXI Timer

[Sean Anderson <sean.anderson@seco.com>]

On 12/14/21 4:08 AM, Mubin Usman Sayyed wrote:
> Hi Sean,
> 
>> -----Original Message-----
>> From: Michal Simek <monstr@monstr.eu>
>> Sent: Tuesday, December 14, 2021 1:38 PM
>> To: Sean Anderson <sean.anderson@seco.com>; linux-
>> pwm@vger.kernel.org; devicetree@vger.kernel.org; Thierry Reding
>> <thierry.reding@gmail.com>
>> Cc: linux-arm-kernel@lists.infradead.org; Alvaro Gamez
>> <alvaro.gamez@hazent.com>; Lee Jones <lee.jones@linaro.org>; Uwe
>> Kleine-König <u.kleine-koenig@pengutronix.de>; Michal Simek
>> <michals@xilinx.com>; linux-kernel@vger.kernel.org; Mubin Usman Sayyed
>> <MUBINUSM@xilinx.com>
>> Subject: Re: [PATCH v11 2/2] pwm: Add support for Xilinx AXI Timer
>> 
>> +Mubin
>> 
>> On 11/24/21 00:25, Sean Anderson wrote:
>> > This adds PWM support for Xilinx LogiCORE IP AXI soft timers commonly
>> > found on Xilinx FPGAs. At the moment clock control is very basic: we
>> > just enable the clock during probe and pin the frequency. In the
>> > future, someone could add support for disabling the clock when not in use.
>> >
>> > Some common code has been specially demarcated. While currently only
>> > used by the PWM driver, it is anticipated that it may be split off in
>> > the future to be used by the timer driver as well.
>> >
>> > This driver was written with reference to Xilinx DS764 for v1.03.a [1].
>> >
>> > [1]
>> >
>> https://www.xilinx.com/support/documentation/ip_documentation/axi_tim
>> e
>> > r/v1_03_a/axi_timer_ds764.pdf
>> >
>> > Signed-off-by: Sean Anderson <sean.anderson@seco.com>
>> > Acked-by: Michal Simek <michal.simek@xilinx.com>
>> > ---
>> >
>> > Changes in v11:
>> > - Add comment about why we test for #pwm-cells
>> > - Clarify comment on generate out signal
>> > - Rename pwm variables to xilinx_pwm
>> > - Round like Uwe wants...
>> > - s/xilinx_timer/xilinx_pwm/ for non-common functions
>> >
>> > Changes in v10:
>> > - Fix compilation error in timer driver
>> >
>> > Changes in v9:
>> > - Refactor "if { return } else if { }" to "if { return } if { }"
>> > - Remove drivers/clocksource/timer-xilinx-common.c from MAINTAINERS
>> > - Remove xilinx_timer_common_init and integrate it into
>> > xilinx_timer_probe
>> >
>> > Changes in v8:
>> > - Drop new timer driver; it has been deferred for future series
>> >
>> > Changes in v7:
>> > - Add dependency on OF_ADDRESS
>> > - Fix period_cycles calculation
>> > - Fix typo in limitations
>> >
>> > Changes in v6:
>> > - Capitalize error messages
>> > - Don't disable regmap locking to allow inspection of registers via
>> >    debugfs
>> > - Prevent overflow when calculating period_cycles
>> > - Remove enabled variable from xilinx_pwm_apply
>> > - Swap order of period_cycle range comparisons
>> >
>> > Changes in v5:
>> > - Allow non-zero #pwm-cells
>> > - Correctly set duty_cycle in get_state when TLR0=TLR1
>> > - Elaborate on limitation section
>> > - Perform some additional checks/rounding in apply_state
>> > - Remove xlnx,axi-timer-2.0 compatible string
>> > - Rework duty-cycle and period calculations with feedback from Uwe
>> > - Switch to regmap to abstract endianness issues
>> > - Use more verbose error messages
>> >
>> > Changes in v4:
>> > - Don't use volatile in read/write replacements. Some arches have it and
>> >    some don't.
>> > - Put common timer properties into their own struct to better reuse
>> >    code.
>> > - Remove references to properties which are not good enough for Linux.
>> >
>> > Changes in v3:
>> > - Add clockevent and clocksource support
>> > - Remove old microblaze driver
>> > - Rewrite probe to only use a device_node, since timers may need to be
>> >    initialized before we have proper devices. This does bloat the code a bit
>> >    since we can no longer rely on helpers such as dev_err_probe. We also
>> >    cannot rely on device resources being free'd on failure, so we must free
>> >    them manually.
>> > - We now access registers through xilinx_timer_(read|write). This allows us
>> >    to deal with endianness issues, as originally seen in the microblaze
>> >    driver. CAVEAT EMPTOR: I have not tested this on big-endian!
>> >
>> > Changes in v2:
>> > - Add comment describing device
>> > - Add comment explaining why we depend on !MICROBLAZE
>> > - Add dependencies on COMMON_CLK and HAS_IOMEM
>> > - Cast dividends to u64 to avoid overflow
>> > - Check for over- and underflow when calculating TLR
>> > - Check range of xlnx,count-width
>> > - Don't compile this module by default for arm64
>> > - Don't set pwmchip.base to -1
>> > - Ensure the clock is always running when the pwm is registered
>> > - Remove debugfs file :l
>> > - Rename TCSR_(SET|CLEAR) to TCSR_RUN_(SET|CLEAR)
>> > - Report errors with dev_error_probe
>> > - Set xilinx_pwm_ops.owner
>> > - Use NSEC_TO_SEC instead of defining our own
>> > - Use TCSR_RUN_MASK to check if the PWM is enabled, as suggested by
>> > Uwe
>> >
>> >   MAINTAINERS                        |   6 +
>> >   arch/microblaze/kernel/timer.c     |   3 +
>> >   drivers/pwm/Kconfig                |  14 ++
>> >   drivers/pwm/Makefile               |   1 +
>> >   drivers/pwm/pwm-xilinx.c           | 318 +++++++++++++++++++++++++++++
>> >   include/clocksource/timer-xilinx.h |  91 +++++++++
>> >   6 files changed, 433 insertions(+)
>> >   create mode 100644 drivers/pwm/pwm-xilinx.c
>> >   create mode 100644 include/clocksource/timer-xilinx.h
>> >
>> > diff --git a/MAINTAINERS b/MAINTAINERS index
>> > 5250298d2817..b2b3ce106e99 100644
>> > --- a/MAINTAINERS
>> > +++ b/MAINTAINERS
>> > @@ -20897,6 +20897,12 @@ F:	drivers/misc/Makefile
>> >   F:	drivers/misc/xilinx_sdfec.c
>> >   F:	include/uapi/misc/xilinx_sdfec.h
>> >
>> > +XILINX PWM DRIVER
>> > +M:	Sean Anderson <sean.anderson@seco.com>
>> > +S:	Maintained
>> > +F:	drivers/pwm/pwm-xilinx.c
>> > +F:	include/clocksource/timer-xilinx.h
>> > +
>> >   XILINX UARTLITE SERIAL DRIVER
>> >   M:	Peter Korsgaard <jacmet@sunsite.dk>
>> >   L:	linux-serial@vger.kernel.org
>> > diff --git a/arch/microblaze/kernel/timer.c
>> > b/arch/microblaze/kernel/timer.c index f8832cf49384..dea34a3d4aa4
>> > 100644
>> > --- a/arch/microblaze/kernel/timer.c
>> > +++ b/arch/microblaze/kernel/timer.c
>> > @@ -251,6 +251,9 @@ static int __init xilinx_timer_init(struct device_node
>> *timer)
>> >   	u32 timer_num = 1;
>> >   	int ret;
>> >
>> > +	if (of_property_read_bool(timer, "#pwm-cells"))
>> > +		return 0;
> [Mubin]:  Can you please return  -ENODEV here,  PWM driver would not be probed if return value is 0.

This needs to return 0 so that timer_probe doesn't spuriously announce the "error".

--Sean