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

From: Uwe Kleine-König
Date: Sat Aug 14 2021 - 16:47:38 EST


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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