Re: [PATCH v5 3/3] pwm: Add support for Xilinx AXI Timer

[Uwe Kleine-König]

Hello Sean,

sorry for having you let waiting so long. Now here some more feedback:

On Mon, Jul 19, 2021 at 06:13:22PM -0400, Sean Anderson wrote:
> +static int xilinx_pwm_apply(struct pwm_chip *chip, struct pwm_device *unused,
> +			    const struct pwm_state *state)
> +{
> +	bool enabled;
> +	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);
> +	period_cycles = mul_u64_u32_div(state->period, rate, NSEC_PER_SEC);

cool, I didn't know mul_u64_u32_div.

Hmm, we still have a problem here if

	state->period * rate > 1000000000 * U64_MAX. 

So to be entirely save, we either need:

	/*
	 * To ensure that period * rate / NSEC_PER_SEC fits into an u64
	 * we need:
	 *            U64_MAX * NSEC_PER_SEC
	 *   period < ----------------------
	 *                    rate
         *
	 * . If rate is not bigger than NSEC_PER_SEC this is true for
	 * sure as the RHS is bigger than U64_MAX. Otherwise we can
	 * calculate the RHS using mul_u64_u32_div.
	 */
	if (rate > NSEC_PER_SEC)
		period = min(state->period, mul_u64_u32_div(U64_MAX, NSEC_PER_SEC, rate);
	else
		period = state->period;

or we go a step further and check the priv->max limit in the same step:

	period = min(state->period, ((u64)priv->max + 2) * NSEC_PER_SEC / rate)

. The latter is simpler and it's safe as priv->max is an u32 and so
there is no overflow.

> +	if (period_cycles - 2 > priv->max || period_cycles < 2)

I'd check for period_cycles < 2 first, because otherwise period_cycles -
2 might underflow. Nothing bad happens in this case, but reading from
left to right my first thought was I found a bug. Also please decrease
period_cycles if it's bigger than priv->max + 2. (With the suggestion
above you don't need to check for period_cycles - 2 > priv->max any more
however.)

> +		return -ERANGE;
> +	duty_cycles = mul_u64_u32_div(state->duty_cycle, rate, NSEC_PER_SEC);
> +
> +	/*
> +	 * 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);
> +
> +	enabled = xilinx_timer_pwm_enabled(tcsr0, tcsr1);
> +	if (state->enabled) {
> +		/*
> +		 * If the PWM is already running, then the counters will be
> +		 * reloaded at the end of the current cycle.
> +		 */

If state->enabled is false, $enabled isn't used, so you can move the
assignment into the if body and also limit the scope of $enabled.

> +		if (!enabled) {
> +			/* 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);

xilinx_timer_get_period rounds down, this is however wrong for
.get_state().

> +	state->duty_cycle = xilinx_timer_get_period(priv, tlr1, tcsr1);

ditto for duty_cycle.

> +	state->enabled = xilinx_timer_pwm_enabled(tcsr0, tcsr1);
> +	state->polarity = PWM_POLARITY_NORMAL;
> [...]
> +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;
> +	u32 pwm_cells, one_timer;
> +	void __iomem *regs;
> +
> +	ret = of_property_read_u32(np, "#pwm-cells", &pwm_cells);
> +	if (ret == -EINVAL)
> +		return -ENODEV;
> +	else if (ret)
> +		return dev_err_probe(dev, ret, "could not read #pwm-cells\n");

Please capitalize error messages.

> [...]
> +	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");

s/pwm/PWM/

> +	}
> +
> +	return 0;
> +}

Best regards
Uwe

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

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