Date:   Mon, 13 Dec 2021 13:36:46 -0500
From:   Sean Anderson <sean.anderson@...o.com>
To:     Uwe Kleine-König <u.kleine-koenig@...gutronix.de>
Cc:     linux-pwm@...r.kernel.org, devicetree@...r.kernel.org,
        Thierry Reding <thierry.reding@...il.com>,
        linux-arm-kernel@...ts.infradead.org,
        Alvaro Gamez <alvaro.gamez@...ent.com>,
        Lee Jones <lee.jones@...aro.org>, michal.simek@...inx.com,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v11 2/2] pwm: Add support for Xilinx AXI Timer

Hi Uwe,

On 11/24/21 2:37 AM, Uwe Kleine-König wrote:
> Hello Sean,
> 
> On Tue, Nov 23, 2021 at 06:25:36PM -0500, 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_timer/v1_03_a/axi_timer_ds764.pdf
>> 
>> Signed-off-by: Sean Anderson <sean.anderson@...o.com>
>> Acked-by: Michal Simek <michal.simek@...inx.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
> 
> I'm mostly happy with this driver now. Just a few minor comments below.
> 
>> 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;
>> +
> 
> The pwm driver has a comment at the location where #pwm-cells is
> checked. I suggest to add a matching comment here.

OK

>>  	if (initialized)
>>  		return -EINVAL;
>>  
>> diff --git a/drivers/pwm/Kconfig b/drivers/pwm/Kconfig
>> index 21e3b05a5153..cefbf00b4c7e 100644
>> --- a/drivers/pwm/Kconfig
>> +++ b/drivers/pwm/Kconfig
>> @@ -640,4 +640,18 @@ 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 OF_ADDRESS
>> +	depends on COMMON_CLK
>> +	select REGMAP_MMIO
>> +	help
>> +	  PWM driver for Xilinx LogiCORE IP AXI timers. This timer is
>> +	  typically a soft core which may be present in Xilinx FPGAs.
>> +	  This device may also be present in Microblaze soft processors.
>> +	  If you don't have this IP in your design, choose N.
>> +
>> +	  To compile this driver as a module, choose M here: the module
>> +	  will be called pwm-xilinx.
>> +
>>  endif
>> diff --git a/drivers/pwm/Makefile b/drivers/pwm/Makefile
>> index 708840b7fba8..ea785480359b 100644
>> --- a/drivers/pwm/Makefile
>> +++ b/drivers/pwm/Makefile
>> @@ -60,3 +60,4 @@ obj-$(CONFIG_PWM_TWL)		+= pwm-twl.o
>>  obj-$(CONFIG_PWM_TWL_LED)	+= pwm-twl-led.o
>>  obj-$(CONFIG_PWM_VISCONTI)	+= pwm-visconti.o
>>  obj-$(CONFIG_PWM_VT8500)	+= pwm-vt8500.o
>> +obj-$(CONFIG_PWM_XILINX)	+= pwm-xilinx.o
>> diff --git a/drivers/pwm/pwm-xilinx.c b/drivers/pwm/pwm-xilinx.c
>> new file mode 100644
>> index 000000000000..b64735880c4c
>> --- /dev/null
>> +++ b/drivers/pwm/pwm-xilinx.c
>> @@ -0,0 +1,318 @@
>> +// SPDX-License-Identifier: GPL-2.0+
>> +/*
>> + * Copyright (C) 2021 Sean Anderson <sean.anderson@...o.com>
>> + *
>> + * Limitations:
>> + * - When changing both duty cycle and period, we may end up with one cycle
>> + *   with the old duty cycle and the new period. This is because the counters
>> + *   may only be reloaded by first stopping them, or by letting them be
>> + *   automatically reloaded at the end of a cycle. If this automatic reload
>> + *   happens after we set TLR0 but before we set TLR1 then we will have a
>> + *   bad cycle. This could probably be fixed by reading TCR0 just before
>> + *   reprogramming, but I think it would add complexity for little gain.
>> + * - Cannot produce 100% duty cycle by configuring the TLRs. This might be
>> + *   possible by stopping the counters at an appropriate point in the cycle,
>> + *   but this is not (yet) implemented.
>> + * - Only produces "normal" output.
>> + * - Always produces low output if disabled.
>> + */
>> +
>> +#include <clocksource/timer-xilinx.h>
>> +#include <linux/clk.h>
>> +#include <linux/clk-provider.h>
>> +#include <linux/device.h>
>> +#include <linux/module.h>
>> +#include <linux/of.h>
>> +#include <linux/platform_device.h>
>> +#include <linux/pwm.h>
>> +#include <linux/regmap.h>
>> +
>> +/*
>> + * The following functions are "common" to drivers for this device, and may be
>> + * exported at a future date.
>> + */
>> +u32 xilinx_timer_tlr_cycles(struct xilinx_timer_priv *priv, u32 tcsr,
>> +			    u64 cycles)
>> +{
>> +	WARN_ON(cycles < 2 || cycles - 2 > priv->max);
>> +
>> +	if (tcsr & TCSR_UDT)
>> +		return cycles - 2;
>> +	return priv->max - cycles + 2;
>> +}
>> +
>> +unsigned int xilinx_timer_get_period(struct xilinx_timer_priv *priv,
>> +				     u32 tlr, u32 tcsr)
>> +{
>> +	u64 cycles;
>> +
>> +	if (tcsr & TCSR_UDT)
>> +		cycles = tlr + 2;
>> +	else
>> +		cycles = (u64)priv->max - tlr + 2;
>> +
>> +	/* cycles has a max of 2^32 + 2 */
>> +	return DIV64_U64_ROUND_UP(cycles * NSEC_PER_SEC,
>> +				  clk_get_rate(priv->clk));
>> +}
>> +
>> +/*
>> + * The idea here is to capture whether the PWM is actually running (e.g.
>> + * because we or the bootloader set it up) and we need to be careful to ensure
>> + * we don't cause a glitch. According to the data sheet, to enable the PWM we
>> + * need to
>> + *
>> + * - Set both timers to generate mode (MDT=1)
>> + * - Set both timers to PWM mode (PWMA=1)
>> + * - Enable the generate out signals (GENT=1)
>> + *
>> + * In addition,
>> + *
>> + * - The timer must be running (ENT=1)
>> + * - The timer must auto-reload TLR into TCR (ARHT=1)
>> + * - We must not be in the process of loading TLR into TCR (LOAD=0)
>> + * - Cascade mode must be disabled (CASC=0)
>> + *
>> + * If any of these differ from usual, then the PWM is either disabled, or is
>> + * running in a mode that this driver does not support.
>> + */
>> +#define TCSR_PWM_SET (TCSR_GENT | TCSR_ARHT | TCSR_ENT | TCSR_PWMA)
>> +#define TCSR_PWM_CLEAR (TCSR_MDT | TCSR_LOAD)
>> +#define TCSR_PWM_MASK (TCSR_PWM_SET | TCSR_PWM_CLEAR)
>> +
>> +struct xilinx_pwm_device {
>> +	struct pwm_chip chip;
>> +	struct xilinx_timer_priv priv;
>> +};
>> +
>> +static inline struct xilinx_timer_priv
>> +*xilinx_pwm_chip_to_priv(struct pwm_chip *chip)
>> +{
>> +	return &container_of(chip, struct xilinx_pwm_device, chip)->priv;
>> +}
>> +
>> +static bool xilinx_timer_pwm_enabled(u32 tcsr0, u32 tcsr1)
>> +{
>> +	return ((TCSR_PWM_MASK | TCSR_CASC) & tcsr0) == TCSR_PWM_SET &&
>> +		(TCSR_PWM_MASK & tcsr1) == TCSR_PWM_SET;
>> +}
>> +
>> +static int xilinx_pwm_apply(struct pwm_chip *chip, struct pwm_device *unused,
>> +			    const struct pwm_state *state)
>> +{
>> +	struct xilinx_timer_priv *priv = xilinx_pwm_chip_to_priv(chip);
>> +	u32 tlr0, tlr1, tcsr0, tcsr1;
>> +	u64 period_cycles, duty_cycles;
>> +	unsigned long rate;
>> +
>> +	if (state->polarity != PWM_POLARITY_NORMAL)
>> +		return -EINVAL;
>> +
>> +	/*
>> +	 * To be representable by TLR, cycles must be between 2 and
>> +	 * priv->max + 2. To enforce this we can reduce the duty
>> +	 * cycle, but we may not increase it.
> 
> s/duty cycle/period/

replaced with "cycles", since this applies to both

>> +	 */
>> +	rate = clk_get_rate(priv->clk);
>> +	/* Avoid overflow */
>> +	period_cycles = min_t(u64, state->period, ULONG_MAX * NSEC_PER_SEC);
>> +	period_cycles = mul_u64_u32_div(period_cycles, rate, NSEC_PER_SEC);
>> +	/* Clamp it for Uwe */
> 
> Hmm, not sure this comment is understandable in the long term.

Incorporated into the above

>> +	period_cycles = min_t(u64, period_cycles, priv->max + 2);
>> +	if (period_cycles < 2)
>> +		return -ERANGE;
>> +
>> +	/* Same thing for duty cycles */
> 
> s/duty cycles/duty cycle/, also for the variable name.

It is named this way to match period_cycles. "duty_cycle_cycles" seems rather redundant.

>> +	duty_cycles = min_t(u64, state->duty_cycle, ULONG_MAX * NSEC_PER_SEC);
>> +	duty_cycles = mul_u64_u32_div(duty_cycles, rate, NSEC_PER_SEC);
>> +	duty_cycles = min_t(u64, duty_cycles, priv->max + 2);

--Sean

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