Re: [PATCH v5 2/2] drm/bridge: ti-sn65dsi86: Implement the pwm_chip

From: Bjorn Andersson
Date: Fri Sep 24 2021 - 18:04:47 EST

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In reply to: Uwe Kleine-König: "Re: [PATCH v5 2/2] drm/bridge: ti-sn65dsi86: Implement the pwm_chip"
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On Fri 24 Sep 02:54 CDT 2021, Uwe Kleine-K?nig wrote:

> Hello,
>
> On Thu, Sep 23, 2021 at 09:12:25PM -0500, Bjorn Andersson wrote:
> > The SN65DSI86 provides the ability to supply a PWM signal on GPIO 4,
> > with the primary purpose of controlling the backlight of the attached
> > panel. Add an implementation that exposes this using the standard PWM
> > framework, to allow e.g. pwm-backlight to expose this to the user.
> >
> > Signed-off-by: Bjorn Andersson <bjorn.andersson@xxxxxxxxxx>
> > ---
> >
> > Changes since v4:
> > - Rebased the patch
> > - Turned ti_sn65dsi86_write_u16 into using regmap bulk write
> > - Moved pwm_refclk_freq out of the #if CONFIG_PWM, to allow removing the guard
> > from ti_sn_bridge_set_refclk_freq()
> > - Updates "Limitations" text
> > - pm_runtime_put_sync() on pm_runtime_get_sync() failure
> > - Added parenthesis around (scale + 1) in the PWM_FREQ formula and thereby
> > redid all the math
> > - Rewrote the comments related to the math
> > - Rewrote the math for calculating the backlight (duty cycle) register value
> > - Dropped some !! on already boolean state->enabled
> > - Dropped a spurious newline
> > - Rewrote comment in ti_sn65dsi86_probe() talking about future PWM work
> >
> > drivers/gpu/drm/bridge/ti-sn65dsi86.c | 369 +++++++++++++++++++++++++-
> > 1 file changed, 361 insertions(+), 8 deletions(-)
> >
> > diff --git a/drivers/gpu/drm/bridge/ti-sn65dsi86.c b/drivers/gpu/drm/bridge/ti-sn65dsi86.c
> > index 41d48a393e7f..857a53dc6c75 100644
> > --- a/drivers/gpu/drm/bridge/ti-sn65dsi86.c
> > +++ b/drivers/gpu/drm/bridge/ti-sn65dsi86.c
> > @@ -4,7 +4,9 @@
> > * datasheet: https://www.ti.com/lit/ds/symlink/sn65dsi86.pdf
> > */
> >
> > +#include <linux/atomic.h>
> > #include <linux/auxiliary_bus.h>
> > +#include <linux/bitfield.h>
> > #include <linux/bits.h>
> > #include <linux/clk.h>
> > #include <linux/debugfs.h>
> > @@ -15,6 +17,7 @@
> > #include <linux/module.h>
> > #include <linux/of_graph.h>
> > #include <linux/pm_runtime.h>
> > +#include <linux/pwm.h>
> > #include <linux/regmap.h>
> > #include <linux/regulator/consumer.h>
> >
> > @@ -91,6 +94,13 @@
> > #define SN_ML_TX_MODE_REG 0x96
> > #define ML_TX_MAIN_LINK_OFF 0
> > #define ML_TX_NORMAL_MODE BIT(0)
> > +#define SN_PWM_PRE_DIV_REG 0xA0
> > +#define SN_BACKLIGHT_SCALE_REG 0xA1
> > +#define BACKLIGHT_SCALE_MAX 0xFFFF
> > +#define SN_BACKLIGHT_REG 0xA3
> > +#define SN_PWM_EN_INV_REG 0xA5
> > +#define SN_PWM_INV_MASK BIT(0)
> > +#define SN_PWM_EN_MASK BIT(1)
> > #define SN_AUX_CMD_STATUS_REG 0xF4
> > #define AUX_IRQ_STATUS_AUX_RPLY_TOUT BIT(3)
> > #define AUX_IRQ_STATUS_AUX_SHORT BIT(5)
> > @@ -113,11 +123,14 @@
> >
> > #define SN_LINK_TRAINING_TRIES 10
> >
> > +#define SN_PWM_GPIO_IDX 3 /* 4th GPIO */
> > +
> > /**
> > * struct ti_sn65dsi86 - Platform data for ti-sn65dsi86 driver.
> > * @bridge_aux: AUX-bus sub device for MIPI-to-eDP bridge functionality.
> > * @gpio_aux: AUX-bus sub device for GPIO controller functionality.
> > * @aux_aux: AUX-bus sub device for eDP AUX channel functionality.
> > + * @pwm_aux: AUX-bus sub device for PWM controller functionality.
> > *
> > * @dev: Pointer to the top level (i2c) device.
> > * @regmap: Regmap for accessing i2c.
> > @@ -145,11 +158,17 @@
> > * bitmap so we can do atomic ops on it without an extra
> > * lock so concurrent users of our 4 GPIOs don't stomp on
> > * each other's read-modify-write.
> > + *
> > + * @pchip: pwm_chip if the PWM is exposed.
> > + * @pwm_enabled: Used to track if the PWM signal is currently enabled.
> > + * @pwm_pin_busy: Track if GPIO4 is currently requested for GPIO or PWM.
> > + * @pwm_refclk_freq: Cache for the reference clock input to the PWM.
> > */
> > struct ti_sn65dsi86 {
> > struct auxiliary_device bridge_aux;
> > struct auxiliary_device gpio_aux;
> > struct auxiliary_device aux_aux;
> > + struct auxiliary_device pwm_aux;
> >
> > struct device *dev;
> > struct regmap *regmap;
> > @@ -172,6 +191,12 @@ struct ti_sn65dsi86 {
> > struct gpio_chip gchip;
> > DECLARE_BITMAP(gchip_output, SN_NUM_GPIOS);
> > #endif
> > +#if defined(CONFIG_PWM)
> > + struct pwm_chip pchip;
> > + bool pwm_enabled;
> > + atomic_t pwm_pin_busy;
> > +#endif
> > + unsigned int pwm_refclk_freq;
> > };
> >
> > static const struct regmap_range ti_sn65dsi86_volatile_ranges[] = {
> > @@ -190,11 +215,31 @@ static const struct regmap_config ti_sn65dsi86_regmap_config = {
> > .cache_type = REGCACHE_NONE,
> > };
> >
> > +static int ti_sn65dsi86_read_u16(struct ti_sn65dsi86 *pdata,
> > + unsigned int reg, u16 *val)
> > +{
> > + unsigned int tmp;
> > + int ret;
> > +
> > + ret = regmap_read(pdata->regmap, reg, &tmp);
> > + if (ret)
> > + return ret;
> > + *val = tmp;
> > +
> > + ret = regmap_read(pdata->regmap, reg + 1, &tmp);
> > + if (ret)
> > + return ret;
> > + *val |= tmp << 8;
> > +
> > + return 0;
> > +}
> > +
> > static void ti_sn65dsi86_write_u16(struct ti_sn65dsi86 *pdata,
> > unsigned int reg, u16 val)
> > {
> > - regmap_write(pdata->regmap, reg, val & 0xFF);
> > - regmap_write(pdata->regmap, reg + 1, val >> 8);
> > + u8 buf[2] = { val & 0xff, val >> 8 };
> > +
> > + regmap_bulk_write(pdata->regmap, reg, &buf, ARRAY_SIZE(buf));
>
> Given that ti_sn65dsi86_write_u16 uses regmap_bulk_write I wonder why
> ti_sn65dsi86_read_u16 doesn't use regmap_bulk_read.
>

Seems reasonable to make that use the bulk interface as well and this
would look better in its own patch.

> > }
> >
> > static u32 ti_sn_bridge_get_dsi_freq(struct ti_sn65dsi86 *pdata)
> > @@ -253,6 +298,12 @@ static void ti_sn_bridge_set_refclk_freq(struct ti_sn65dsi86 *pdata)
> >
> > regmap_update_bits(pdata->regmap, SN_DPPLL_SRC_REG, REFCLK_FREQ_MASK,
> > REFCLK_FREQ(i));
> > +
> > + /*
> > + * The PWM refclk is based on the value written to SN_DPPLL_SRC_REG,
> > + * regardless of its actual sourcing.
> > + */
> > + pdata->pwm_refclk_freq = ti_sn_bridge_refclk_lut[i];
> > }
> >
> > static void ti_sn65dsi86_enable_comms(struct ti_sn65dsi86 *pdata)
> > @@ -1259,9 +1310,283 @@ static struct auxiliary_driver ti_sn_bridge_driver = {
> > };
> >
> > /* -----------------------------------------------------------------------------
> > - * GPIO Controller
> > + * PWM Controller
> > + */
> > +#if defined(CONFIG_PWM)
> > +static int ti_sn_pwm_pin_request(struct ti_sn65dsi86 *pdata)
> > +{
> > + return atomic_xchg(&pdata->pwm_pin_busy, 1) ? -EBUSY : 0;
> > +}
> > +
> > +static void ti_sn_pwm_pin_release(struct ti_sn65dsi86 *pdata)
> > +{
> > + atomic_set(&pdata->pwm_pin_busy, 0);
> > +}
> > +
> > +static struct ti_sn65dsi86 *pwm_chip_to_ti_sn_bridge(struct pwm_chip *chip)
> > +{
> > + return container_of(chip, struct ti_sn65dsi86, pchip);
> > +}
> > +
> > +static int ti_sn_pwm_request(struct pwm_chip *chip, struct pwm_device *pwm)
> > +{
> > + struct ti_sn65dsi86 *pdata = pwm_chip_to_ti_sn_bridge(chip);
> > +
> > + return ti_sn_pwm_pin_request(pdata);
> > +}
> > +
> > +static void ti_sn_pwm_free(struct pwm_chip *chip, struct pwm_device *pwm)
> > +{
> > + struct ti_sn65dsi86 *pdata = pwm_chip_to_ti_sn_bridge(chip);
> > +
> > + ti_sn_pwm_pin_release(pdata);
> > +}
> > +
> > +/*
> > + * Limitations:
> > + * - The PWM signal is not driven when the chip is powered down, or in its
> > + * reset state and the driver does not implement the "suspend state"
> > + * described in the documentation. In order to save power, state->enabled is
> > + * interpreted as denoting if the signal is expected to be valid, and is used
> > + * to determine if the chip needs to be kept powered.
> > + * - Changing both period and duty_cycle is not done atomically, neither is the
> > + * multi-byte register updates, so the output might briefly be undefined
> > + * during update.
> > */
> > +static int ti_sn_pwm_apply(struct pwm_chip *chip, struct pwm_device *pwm,
> > + const struct pwm_state *state)
> > +{
> > + struct ti_sn65dsi86 *pdata = pwm_chip_to_ti_sn_bridge(chip);
> > + unsigned int pwm_en_inv;
> > + unsigned int backlight;
> > + unsigned int pre_div;
> > + unsigned int scale;
> > + u64 period_max;
> > + u64 period;
> > + int ret;
> > +
> > + if (!pdata->pwm_enabled) {
> > + ret = pm_runtime_get_sync(pdata->dev);
> > + if (ret < 0) {
> > + pm_runtime_put_sync(pdata->dev);
> > + return ret;
> > + }
> > + }
> > +
> > + if (state->enabled) {
> > + if (!pdata->pwm_enabled) {
> > + /*
> > + * The chip might have been powered down while we
> > + * didn't hold a PM runtime reference, so mux in the
> > + * PWM function on the GPIO pin again.
> > + */
> > + ret = regmap_update_bits(pdata->regmap, SN_GPIO_CTRL_REG,
> > + SN_GPIO_MUX_MASK << (2 * SN_PWM_GPIO_IDX),
> > + SN_GPIO_MUX_SPECIAL << (2 * SN_PWM_GPIO_IDX));
> > + if (ret) {
> > + dev_err(pdata->dev, "failed to mux in PWM function\n");
> > + goto out;
> > + }
> > + }
> > +
> > + /*
> > + * Per the datasheet the PWM frequency is given by:
> > + *
> > + * REFCLK_FREQ
> > + * PWM_FREQ = -----------------------------------
> > + * PWM_PRE_DIV * BACKLIGHT_SCALE + 1
> > + *
> > + * Unfortunately the math becomes overly complex unless we
> > + * assume that this formula is missing parenthesis around
> > + * BACKLIGHT_SCALE + 1.
>
> We shouldn't assume a different formula than the reference manual
> describes because it's complex. The reasoning should be that the
> reference manual is wrong and that someone has convinced themselves the
> assumed formula is the right one instead.
>

Given the effort put in to conclude that there must be some parenthesis
there, I agree that "assume" might be to weak of a word...

> With the assumed formula: What happens if PWM_PRE_DIV is 0?
>

Looking at the specification again and I don't see anything prohibiting
from writing 0 in the PRE_DIV register, and writing a 0 to the register
causes no visual on my backlight from writing 1.

Per our new understanding this should probably have resulted in a period
of 0. That said, if the formula from the datasheet was correct then we'd
have a period T_pwm (given T_ref as refclk pulse length) of:

T_pwm = T_ref * (div * scale + 1)

And with div = 0 we'd have:

T_pwm = T_ref * 1

Which wouldn't give any room for adjusting the duty cycle, and I can
still do that. So that's not correct either.

Unfortunately I still don't want to dismantle my working laptop, so I
don't know if I can do any better than this. Perhaps they do the typical
trick of encoding prediv off-by-1 and the PWM duty is off by a factor
of 2. Perhaps the comparator for the prediv counter trips at 1 even
though the register is configured at 0...

> > + * With that the formula can be written:
> > + *
> > + * T_pwm * REFCLK_FREQ = PWM_PRE_DIV * (BACKLIGHT_SCALE + 1)
> > + *
> > + * In order to keep BACKLIGHT_SCALE within its 16 bits,
> > + * PWM_PRE_DIV must be:
> > + *
> > + * T_pwm * REFCLK_FREQ
> > + * PWM_PRE_DIV >= -------------------------
> > + * BACKLIGHT_SCALE_MAX + 1
> > + *
> > + * To simplify the search and optimize the resolution of the
> > + * PWM, the lowest possible PWM_PRE_DIV is used. Finally the
> > + * scale is calculated as:
> > + *
> > + * T_pwm * REFCLK_FREQ
> > + * BACKLIGHT_SCALE = ---------------------- - 1
> > + * PWM_PRE_DIV
> > + *
> > + * Here T_pwm is represented in seconds, so appropriate scaling
> > + * to nanoseconds is necessary.
> > + */
> > +
> > + /* Minimum T_pwm is (0 * 1) / REFCLK_FREQ */
>
> So the minimal T_pwm (which corresponds to .period, right?) is 0? That
> sounds wrong.
>

Yes, that seems more likely.

> > + if (state->period <= NSEC_PER_SEC / pdata->pwm_refclk_freq) {
>
> The implemented check assumes 1 / REFCLK_FREQ, which looks more reasonable.
>

I agree.

> > + ret = -EINVAL;
> > + goto out;
> > + }
> > +
> > + /*
> > + * Maximum T_pwm is 255 * (65535 + 1) / REFCLK_FREQ
> > + * Limit period to this to avoid overflows
> > + */
> > + period_max = div_u64((u64)NSEC_PER_SEC * 255 * (65535 + 1), pdata->pwm_refclk_freq);
> > + if (period > period_max)
> > + period = period_max;
> > + else
> > + period = state->period;
> > +
> > + pre_div = DIV64_U64_ROUND_UP(period * pdata->pwm_refclk_freq,
> > + (u64)NSEC_PER_SEC * (BACKLIGHT_SCALE_MAX + 1));
> > + scale = div64_u64(period * pdata->pwm_refclk_freq, (u64)NSEC_PER_SEC * pre_div) - 1;
>
> You're loosing precision here as you divide by the result of a division.
>

As described above, I'm trying to find suitable values for PRE_DIV and
BACKLIGHT_SCALE in:

T_pwm * refclk_freq
--------------------- = PRE_DIV * (BACKLIGHT_SCALE + 1)
NSEC_PER_SEC

BACKLIGHT_SCALE must fit in 16 bits and in order to be useful for
driving the backlight control be large enough that the resolution
(BACKLIGTH = [0..BACKLIGHT_SCALE]) covers whatever resolution
pwm-backlight might expose.

For the intended use case this seems pretty good, but I presume you
could pick better values to get closer to the requested period.

Do you have a better suggestion for how to pick PRE_DIV and
BACKLIGHT_SCALE?

> > +
> > + /*
> > + * The documentation has the duty ratio given as:
> > + *
> > + * duty BACKLIGHT
> > + * ------- = ---------------------
> > + * period BACKLIGHT_SCALE + 1
> > + *
> > + * Solve for BACKLIGHT, substituting BACKLIGHT_SCALE according
> > + * to definition above and adjusting for nanosecond
> > + * representation of duty cycle gives us:
> > + */
> > + backlight = div64_u64(state->duty_cycle * pdata->pwm_refclk_freq, (u64)NSEC_PER_SEC * pre_div);
> > + if (backlight > scale)
> > + backlight = scale;
> > +
> > + ret = regmap_write(pdata->regmap, SN_PWM_PRE_DIV_REG, pre_div);
> > + if (ret) {
> > + dev_err(pdata->dev, "failed to update PWM_PRE_DIV\n");
> > + goto out;
> > + }
> >
> > + ti_sn65dsi86_write_u16(pdata, SN_BACKLIGHT_SCALE_REG, scale);
> > + ti_sn65dsi86_write_u16(pdata, SN_BACKLIGHT_REG, backlight);
> > + }
> > +
> > + pwm_en_inv = FIELD_PREP(SN_PWM_EN_MASK, state->enabled) |
> > + FIELD_PREP(SN_PWM_INV_MASK, state->polarity == PWM_POLARITY_INVERSED);
> > + ret = regmap_write(pdata->regmap, SN_PWM_EN_INV_REG, pwm_en_inv);
> > + if (ret) {
> > + dev_err(pdata->dev, "failed to update PWM_EN/PWM_INV\n");
> > + goto out;
> > + }
> > +
> > + pdata->pwm_enabled = state->enabled;
> > +out:
> > +
> > + if (!pdata->pwm_enabled)
> > + pm_runtime_put_sync(pdata->dev);
> > +
> > + return ret;
> > +}
> > +
> > +static void ti_sn_pwm_get_state(struct pwm_chip *chip, struct pwm_device *pwm,
> > + struct pwm_state *state)
> > +{
> > + struct ti_sn65dsi86 *pdata = pwm_chip_to_ti_sn_bridge(chip);
> > + unsigned int pwm_en_inv;
> > + unsigned int pre_div;
> > + u16 backlight;
> > + u16 scale;
> > + int ret;
> > +
> > + ret = regmap_read(pdata->regmap, SN_PWM_EN_INV_REG, &pwm_en_inv);
> > + if (ret)
> > + return;
> > +
> > + ret = ti_sn65dsi86_read_u16(pdata, SN_BACKLIGHT_SCALE_REG, &scale);
> > + if (ret)
> > + return;
> > +
> > + ret = ti_sn65dsi86_read_u16(pdata, SN_BACKLIGHT_REG, &backlight);
> > + if (ret)
> > + return;
> > +
> > + ret = regmap_read(pdata->regmap, SN_PWM_PRE_DIV_REG, &pre_div);
> > + if (ret)
> > + return;
> > +
> > + state->enabled = FIELD_GET(SN_PWM_EN_MASK, pwm_en_inv);
> > + if (FIELD_GET(SN_PWM_INV_MASK, pwm_en_inv))
> > + state->polarity = PWM_POLARITY_INVERSED;
> > + else
> > + state->polarity = PWM_POLARITY_NORMAL;
> > +
> > + state->period = DIV_ROUND_UP_ULL((u64)NSEC_PER_SEC * pre_div * (scale + 1),
> > + pdata->pwm_refclk_freq);
> > + state->duty_cycle = DIV_ROUND_UP_ULL((u64)NSEC_PER_SEC * pre_div * backlight,
> > + pdata->pwm_refclk_freq);
>
> What happens if scale + 1 < backlight?
>

When we write the backlight register above, we cap it at scale, so for
the typical case that shouldn't happen.

So I guess the one case when this could happen is if we get_state()
before pwm_apply(), when someone else has written broken values
(reset values are scale = 0xff and backlight = 0)

Thanks,
Bjorn

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