Re: [PATCH v12 2/2] pwm: Add support for Xilinx AXI Timer

[Sean Anderson]

On 1/31/22 11:40 AM, Sean Anderson wrote:
> 
> 
> On 1/31/22 9:10 AM, Uwe Kleine-König wrote:
>> Hello,
>> 
>> first of all: Sorry for taking so long for the next review round.
>> 
>>> diff --git a/arch/microblaze/kernel/timer.c b/arch/microblaze/kernel/timer.c
>>> index f8832cf49384..26c385582c3b 100644
>>> --- a/arch/microblaze/kernel/timer.c
>>> +++ b/arch/microblaze/kernel/timer.c
>>> @@ -251,6 +251,10 @@ static int __init xilinx_timer_init(struct device_node *timer)
>>>  	u32 timer_num = 1;
>>>  	int ret;
>>>  
>>> +	/* If this property is present, the device is a PWM and not a timer */
>>> +	if (of_property_read_bool(timer, "#pwm-cells"))
>>> +		return 0;
>>> +
>>>  	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..b4d93e8812c6
>>> --- /dev/null
>>> +++ b/drivers/pwm/pwm-xilinx.c
>>> @@ -0,0 +1,319 @@
>>> +// SPDX-License-Identifier: GPL-2.0+
>>> +/*
>>> + * Copyright (C) 2021 Sean Anderson <sean.anderson@xxxxxxxx>
>>> + *
>>> + * 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 */
>> 
>> If you add "... so the multiplication doesn't overflow." it becomes more
>> obvious why this comment is there.
>> 
>>> +	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 cycles, but we may
>>> +	 * not increase them. Caveat emptor: while this does result in more
>>> +	 * predictable rounding, it may also result in a completely different
>>> +	 * duty cycle (% high time) than what was requested.
>>> +	 */
>>> +	rate = clk_get_rate(priv->clk);
>>> +	/* Avoid overflow */
>>> +	period_cycles = min_t(u64, state->period, ULONG_MAX * NSEC_PER_SEC);
>> 
>> on a 64 bit platform ULONG_MAX * NSEC_PER_SEC doesn't fit into an u64
>> ... I think if you replace ULONG_MAX by U32_MAX it works as intended.
>> 
>>> +	period_cycles = mul_u64_u32_div(period_cycles, rate, NSEC_PER_SEC);
>>> +	period_cycles = min_t(u64, period_cycles, priv->max + 2);
>>> +	if (period_cycles < 2)
>>> +		return -ERANGE;
>>> +
>>> +[...]
>>> +static void xilinx_pwm_get_state(struct pwm_chip *chip,
>>> +				 struct pwm_device *unused,
>>> +				 struct pwm_state *state)
>>> +{
>>> +	struct xilinx_timer_priv *priv = xilinx_pwm_chip_to_priv(chip);
>>> +	u32 tlr0, tlr1, tcsr0, tcsr1;
>>> +
>>> +	regmap_read(priv->map, TLR0, &tlr0);
>>> +	regmap_read(priv->map, TLR1, &tlr1);
>>> +	regmap_read(priv->map, TCSR0, &tcsr0);
>>> +	regmap_read(priv->map, TCSR1, &tcsr1);
>>> +	state->period = xilinx_timer_get_period(priv, tlr0, tcsr0);
>>> +	state->duty_cycle = xilinx_timer_get_period(priv, tlr1, tcsr1);
>>> +	state->enabled = xilinx_timer_pwm_enabled(tcsr0, tcsr1);
>>> +	state->polarity = PWM_POLARITY_NORMAL;
>>> +
>>> +	/* 100% duty cycle results in constant low output */
>>> +	if (state->period == state->duty_cycle)
>> 
>> There is a corner case: It can happen that
>> xilinx_timer_get_period(priv, tlr0, tcsr0) ==
>> xilinx_timer_get_period(priv, tlr1, tcsr1) but not tlr0 == tlr1.
>> 
>> This only happens for clkrate > 1000000000, but given that the fix is
>> cheap (i.e. check tlr0 == tlr1 instead of state->period ==
>> state->duty_cycle) I'd suggest to do that.
>

This is intentional. xilinx_timer_get_period abstracts over whether UDT
is set or not. I will fix this when you find me this hardware
implemented with a 1GHz clock.

--Sean