Re: [PATCH v10 3/3] pwm: Add support for Xilinx AXI Timer

From: Uwe Kleine-König
Date: Thu Nov 18 2021 - 04:28:36 EST


Hello,

On Fri, Nov 12, 2021 at 01:55:04PM -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@xxxxxxxx>
> ---
>
> 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 | 311 +++++++++++++++++++++++++++++
> include/clocksource/timer-xilinx.h | 91 +++++++++
> 6 files changed, 426 insertions(+)
> create mode 100644 drivers/pwm/pwm-xilinx.c
> create mode 100644 include/clocksource/timer-xilinx.h
>
> diff --git a/MAINTAINERS b/MAINTAINERS
> index 3b79fd441dde..6f0f57c041c4 100644
> --- a/MAINTAINERS
> +++ b/MAINTAINERS
> @@ -20614,6 +20614,12 @@ 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@xxxxxxxx>
> +S: Maintained
> +F: drivers/pwm/pwm-xilinx.c
> +F: include/clocksource/timer-xilinx.h
> +
> XILINX UARTLITE SERIAL DRIVER
> M: Peter Korsgaard <jacmet@xxxxxxxxxx>
> L: linux-serial@xxxxxxxxxxxxxxx
> 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;
> +
> if (initialized)
> return -EINVAL;
>
> diff --git a/drivers/pwm/Kconfig b/drivers/pwm/Kconfig
> index aa29841bbb79..47f25237754f 100644
> --- a/drivers/pwm/Kconfig
> +++ b/drivers/pwm/Kconfig
> @@ -638,4 +638,18 @@ 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..d79ef202d62f
> --- /dev/null
> +++ b/drivers/pwm/pwm-xilinx.c
> @@ -0,0 +1,311 @@
> +// 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 */
> + return DIV64_U64_ROUND_CLOSEST(cycles * NSEC_PER_SEC,
> + clk_get_rate(priv->clk));

Please round up here.

> +}
> +
> +/*
> + * 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.
> + */
> + rate = clk_get_rate(priv->clk);
> + /* Prevent overflow by clamping to the worst case of rate */

I wouldn't have called this "worst case of rate", maybe better use
"maximal rate"?

> + period_cycles = min_t(u64, state->period, ULONG_MAX * NSEC_PER_SEC);
> + period_cycles = mul_u64_u32_div(period_cycles, rate, NSEC_PER_SEC);
> + if (period_cycles < 2 || period_cycles - 2 > priv->max)
> + return -ERANGE;

if period_cycles - 2 > priv->max the right reaction is to do

period_cycles = priv->max + 2

.

> + duty_cycles = mul_u64_u32_div(state->duty_cycle, rate, NSEC_PER_SEC);

duty_cycle needs sanitation in case period was reduced and duty_cycle is
bigger now than period.

> + /*
> + * If we specify 100% duty cycle, we will get 0% instead, so decrease
> + * the duty cycle count by one.
> + */
> + if (period_cycles == duty_cycles)
> + duty_cycles--;
> +
> + /* Round down to 0% duty cycle for unrepresentable duty cycles */
> + if (duty_cycles < 2)
> + duty_cycles = period_cycles;
> +
> + regmap_read(priv->map, TCSR0, &tcsr0);
> + regmap_read(priv->map, TCSR1, &tcsr1);
> + tlr0 = xilinx_timer_tlr_cycles(priv, tcsr0, period_cycles);
> + tlr1 = xilinx_timer_tlr_cycles(priv, tcsr1, duty_cycles);
> + regmap_write(priv->map, TLR0, tlr0);
> + regmap_write(priv->map, TLR1, tlr1);
> +
> + if (state->enabled) {
> + /*
> + * If the PWM is already running, then the counters will be
> + * reloaded at the end of the current cycle.
> + */
> + if (!xilinx_timer_pwm_enabled(tcsr0, tcsr1)) {
> + /* Load TLR into TCR */
> + regmap_write(priv->map, TCSR0, tcsr0 | TCSR_LOAD);
> + regmap_write(priv->map, TCSR1, tcsr1 | TCSR_LOAD);
> + /* Enable timers all at once with ENALL */
> + tcsr0 = (TCSR_PWM_SET & ~TCSR_ENT) | (tcsr0 & TCSR_UDT);
> + tcsr1 = TCSR_PWM_SET | TCSR_ENALL | (tcsr1 & TCSR_UDT);
> + regmap_write(priv->map, TCSR0, tcsr0);
> + regmap_write(priv->map, TCSR1, tcsr1);
> + }
> + } else {
> + regmap_write(priv->map, TCSR0, 0);
> + regmap_write(priv->map, TCSR1, 0);
> + }
> +
> + return 0;
> +}
> +
> +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)
> + state->duty_cycle = 0;
> +}
> +
> +static const struct pwm_ops xilinx_pwm_ops = {
> + .apply = xilinx_pwm_apply,
> + .get_state = xilinx_pwm_get_state,
> + .owner = THIS_MODULE,
> +};
> +
> +static const struct regmap_config xilinx_pwm_regmap_config = {
> + .reg_bits = 32,
> + .reg_stride = 4,
> + .val_bits = 32,
> + .val_format_endian = REGMAP_ENDIAN_LITTLE,
> + .max_register = TCR1,
> +};
> +
> +static int xilinx_timer_probe(struct platform_device *pdev)
> +{
> + int ret;
> + struct device *dev = &pdev->dev;
> + struct device_node *np = dev->of_node;
> + struct xilinx_timer_priv *priv;
> + struct xilinx_pwm_device *pwm;

The name "pwm" is usually reserved for struct pwm_device pointers. A
typical name for this would be xlnxpwm or ddata.

> + u32 pwm_cells, one_timer, width;
> + void __iomem *regs;
> +
> + ret = of_property_read_u32(np, "#pwm-cells", &pwm_cells);
> + if (ret == -EINVAL)
> + return -ENODEV;

A comment about why this is done would be great.

> + if (ret)
> + return dev_err_probe(dev, ret, "could not read #pwm-cells\n");
> +
> + pwm = devm_kzalloc(dev, sizeof(*pwm), GFP_KERNEL);
> + if (!pwm)
> + return -ENOMEM;
> + platform_set_drvdata(pdev, pwm);
> + priv = &pwm->priv;
> +
> + regs = devm_platform_ioremap_resource(pdev, 0);
> + if (IS_ERR(regs))
> + return PTR_ERR(regs);
> +
> + priv->map = devm_regmap_init_mmio(dev, regs,
> + &xilinx_pwm_regmap_config);
> + if (IS_ERR(priv->map))
> + return dev_err_probe(dev, PTR_ERR(priv->map),
> + "Could not create regmap\n");
> +
> + ret = of_property_read_u32(np, "xlnx,one-timer-only", &one_timer);
> + if (ret)
> + return dev_err_probe(dev, ret,
> + "Could not read xlnx,one-timer-only\n");
> +
> + if (one_timer)
> + return dev_err_probe(dev, -EINVAL,
> + "Two timers required for PWM mode\n");
> +
> +
> + ret = of_property_read_u32(np, "xlnx,count-width", &width);
> + if (ret == -EINVAL)
> + width = 32;
> + else if (ret)
> + return dev_err_probe(dev, ret,
> + "Could not read xlnx,count-width\n");
> +
> + if (width != 8 && width != 16 && width != 32)
> + return dev_err_probe(dev, -EINVAL,
> + "Invalid counter width %d\n", width);
> + priv->max = BIT_ULL(width) - 1;
> +
> + /*
> + * The polarity of the generate outputs must be active high for PWM

s/generate/generated/

> + * mode to work. We could determine this from the device tree, but
> + * alas, such properties are not allowed to be used.
> + */
> +
> + priv->clk = devm_clk_get(dev, "s_axi_aclk");
> + if (IS_ERR(priv->clk))
> + return dev_err_probe(dev, PTR_ERR(priv->clk),
> + "Could not get clock\n");
> +
> + ret = clk_prepare_enable(priv->clk);
> + if (ret)
> + return dev_err_probe(dev, ret, "Clock enable failed\n");
> + clk_rate_exclusive_get(priv->clk);
> +
> + pwm->chip.dev = dev;
> + pwm->chip.ops = &xilinx_pwm_ops;
> + pwm->chip.npwm = 1;
> + ret = pwmchip_add(&pwm->chip);
> + if (ret) {
> + clk_rate_exclusive_put(priv->clk);
> + clk_disable_unprepare(priv->clk);
> + return dev_err_probe(dev, ret, "Could not register PWM chip\n");
> + }
> +
> + return 0;
> +}
> +
> +static int xilinx_timer_remove(struct platform_device *pdev)
> +{
> + struct xilinx_pwm_device *pwm = platform_get_drvdata(pdev);
> +
> + pwmchip_remove(&pwm->chip);
> + clk_rate_exclusive_put(pwm->priv.clk);
> + clk_disable_unprepare(pwm->priv.clk);
> + return 0;
> +}
> +
> +static const struct of_device_id xilinx_timer_of_match[] = {
> + { .compatible = "xlnx,xps-timer-1.00.a", },
> + {},
> +};
> +MODULE_DEVICE_TABLE(of, xilinx_timer_of_match);
> +
> +static struct platform_driver xilinx_timer_driver = {
> + .probe = xilinx_timer_probe,
> + .remove = xilinx_timer_remove,
> + .driver = {
> + .name = "xilinx-timer",

Doesn't this give a wrong impression as this is a PWM driver, not a
timer driver?

> + .of_match_table = of_match_ptr(xilinx_timer_of_match),
> + },
> +};
> +module_platform_driver(xilinx_timer_driver);
> +
> +MODULE_ALIAS("platform:xilinx-timer");
> +MODULE_DESCRIPTION("Xilinx LogiCORE IP AXI Timer driver");
> +MODULE_LICENSE("GPL v2");
> diff --git a/include/clocksource/timer-xilinx.h b/include/clocksource/timer-xilinx.h
> new file mode 100644
> index 000000000000..1f7757b84a5e
> --- /dev/null
> +++ b/include/clocksource/timer-xilinx.h
> @@ -0,0 +1,91 @@
> +/* SPDX-License-Identifier: GPL-2.0+ */
> +/*
> + * Copyright (C) 2021 Sean Anderson <sean.anderson@xxxxxxxx>
> + */
> +
> +#ifndef XILINX_TIMER_H
> +#define XILINX_TIMER_H
> +
> +#include <linux/compiler.h>
> +
> +#define TCSR0 0x00
> +#define TLR0 0x04
> +#define TCR0 0x08
> +#define TCSR1 0x10
> +#define TLR1 0x14
> +#define TCR1 0x18
> +
> +#define TCSR_MDT BIT(0)
> +#define TCSR_UDT BIT(1)
> +#define TCSR_GENT BIT(2)
> +#define TCSR_CAPT BIT(3)
> +#define TCSR_ARHT BIT(4)
> +#define TCSR_LOAD BIT(5)
> +#define TCSR_ENIT BIT(6)
> +#define TCSR_ENT BIT(7)
> +#define TCSR_TINT BIT(8)
> +#define TCSR_PWMA BIT(9)
> +#define TCSR_ENALL BIT(10)
> +#define TCSR_CASC BIT(11)
> +
> +struct clk;
> +struct device_node;
> +struct regmap;
> +
> +/**
> + * struct xilinx_timer_priv - Private data for Xilinx AXI timer drivers
> + * @map: Regmap of the device, possibly with an offset
> + * @clk: Parent clock
> + * @max: Maximum value of the counters
> + */
> +struct xilinx_timer_priv {
> + struct regmap *map;
> + struct clk *clk;
> + u32 max;
> +};
> +
> +/**
> + * xilinx_timer_tlr_cycles() - Calculate the TLR for a period specified
> + * in clock cycles
> + * @priv: The timer's private data
> + * @tcsr: The value of the TCSR register for this counter
> + * @cycles: The number of cycles in this period
> + *
> + * Callers of this function MUST ensure that @cycles is representable as
> + * a TLR.
> + *
> + * Return: The calculated value for TLR
> + */
> +u32 xilinx_timer_tlr_cycles(struct xilinx_timer_priv *priv, u32 tcsr,
> + u64 cycles);
> +
> +/**
> + * xilinx_timer_get_period() - Get the current period of a counter
> + * @priv: The timer's private data
> + * @tlr: The value of TLR for this counter
> + * @tcsr: The value of TCSR for this counter
> + *
> + * Return: The period, in ns
> + */
> +unsigned int xilinx_timer_get_period(struct xilinx_timer_priv *priv,
> + u32 tlr, u32 tcsr);
> +
> +/**
> + * xilinx_timer_common_init() - Perform common initialization for Xilinx
> + * AXI timer drivers.
> + * @priv: The timer's private data
> + * @np: The devicetree node for the timer
> + * @one_timer: Set to %1 if there is only one timer
> + *
> + * This performs common initialization, such as detecting endianness,
> + * and parsing devicetree properties. @priv->regs must be initialized
> + * before calling this function. This function initializes @priv->read,
> + * @priv->write, and @priv->width.
> + *
> + * Return: 0, or negative errno
> + */
> +int xilinx_timer_common_init(struct device_node *np,
> + struct xilinx_timer_priv *priv,
> + u32 *one_timer);
> +
> +#endif /* XILINX_TIMER_H */
> --
> 2.25.1
>
>

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

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