Date:   Mon, 17 Aug 2020 20:21:58 +0200
From:   Bartosz Golaszewski <bgolaszewski@...libre.com>
To:     Kent Gibson <warthog618@...il.com>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        linux-gpio <linux-gpio@...r.kernel.org>,
        Linus Walleij <linus.walleij@...aro.org>
Subject: Re: [PATCH v4 12/20] gpiolib: cdev: support setting debounce

On Fri, Aug 14, 2020 at 5:05 AM Kent Gibson <warthog618@...il.com> wrote:
>
> Add support for setting debounce on a line via the GPIO uAPI.
> Where debounce is not supported by hardware, a software debounce is
> provided.
>
> Signed-off-by: Kent Gibson <warthog618@...il.com>
> ---
>
> The implementation of the software debouncer waits for the line to be
> stable for the debounce period before determining if a level change,
> and a corresponding edge event, has occurred.  This provides maximum
> protection against glitches, but also introduces a debounce_period
> latency to edge events.
>
> The software debouncer is integrated with the edge detection as it
> utilises the line interrupt, and integration is simpler than getting
> the two to interwork.  Where software debounce AND edge detection is
> required, the debouncer provides both.
>
> Due to the tight integration between the debouncer and edge detection,
> and to avoid particular corner cases, it is not allowed to alter the
> debounce value if edge detection is enabled.  Changing the debounce with
> edge detection enabled is a very unlikely use case, so it is preferable
> to disallow it rather than complicate the code to allow it.
> Should the user wish to alter the debounce value in such cases they will
> need to release and re-request the line.
>
> Changes for v4:
>  - fix handling of mask in line_get_values
>
> Changes for v3:
>  - only GPIO_V2 field renaming
>
> Changes for v2:
>  - improve documentation on fields shared by threads.
>  - use READ_ONCE/WRITE_ONCE for shared fields rather than atomic_t
>    which was overkill.
>
>  drivers/gpio/gpiolib-cdev.c | 265 +++++++++++++++++++++++++++++++++++-
>  drivers/gpio/gpiolib.h      |   4 +
>  2 files changed, 263 insertions(+), 6 deletions(-)
>
> diff --git a/drivers/gpio/gpiolib-cdev.c b/drivers/gpio/gpiolib-cdev.c
> index de88b7a5ba0f..77fabf815de8 100644
> --- a/drivers/gpio/gpiolib-cdev.c
> +++ b/drivers/gpio/gpiolib-cdev.c
> @@ -6,6 +6,7 @@
>  #include <linux/build_bug.h>
>  #include <linux/cdev.h>
>  #include <linux/compat.h>
> +#include <linux/compiler.h>
>  #include <linux/device.h>
>  #include <linux/err.h>
>  #include <linux/file.h>
> @@ -22,6 +23,7 @@
>  #include <linux/spinlock.h>
>  #include <linux/timekeeping.h>
>  #include <linux/uaccess.h>
> +#include <linux/workqueue.h>
>  #include <uapi/linux/gpio.h>
>
>  #include "gpiolib.h"
> @@ -395,6 +397,9 @@ static int linehandle_create(struct gpio_device *gdev, void __user *ip)
>   * for the corresponding line request. Ths is drawn from the @line.
>   * @line_seqno: the seqno for the current edge event in the sequence of
>   * events for this line.
> + * @work: the worker that implements software debouncing
> + * @sw_debounced: flag indicating if the software debouncer is active
> + * @level: the current debounced physical level of the line
>   */
>  struct edge_detector {
>         struct line *line;
> @@ -406,7 +411,27 @@ struct edge_detector {
>          */
>         u64 timestamp;
>         u32 seqno;
> +       /*
> +        * line_seqno is used by either edge_irq_thread() or
> +        * debounce_work_func() which are themselves mutually exclusive.
> +        */
>         u32 line_seqno;
> +       /*
> +        * -- debouncer specific fields --
> +        */
> +       struct delayed_work work;
> +       /*
> +        * sw_debounce is shared by line_set_config(), which is the only
> +        * setter, and line_ioctl(), which can live with a slightly stale
> +        * value.
> +        */
> +       unsigned int sw_debounced;
> +       /*
> +        * level is shared by debounce_work_func(), which is the only
> +        * setter, and line_ioctl() which can live with a slightly stale
> +        * value.
> +        */
> +       unsigned int level;
>  };
>
>  /**
> @@ -523,6 +548,10 @@ static int edge_detector_start(struct edge_detector *edet)
>         int ret, irq, irqflags = 0;
>         struct gpio_desc *desc;
>
> +       if (READ_ONCE(edet->sw_debounced))
> +               /* debouncer is setup and will provide edge detection */
> +               return 0;
> +
>         desc = edge_detector_desc(edet);
>         irq = gpiod_to_irq(desc);
>
> @@ -554,17 +583,215 @@ static int edge_detector_start(struct edge_detector *edet)
>         return 0;
>  }
>
> +/*
> + * returns the current debounced logical value.
> + */
> +static int debounced_value(struct edge_detector *edet)
> +{
> +       int value;
> +
> +       /*
> +        * minor race - debouncer may be stopped here, so edge_detector_stop
> +        * must leave the value unchanged so the following will read the level
> +        * from when the debouncer was last running.
> +        */
> +       value = READ_ONCE(edet->level);
> +
> +       if (test_bit(FLAG_ACTIVE_LOW, &edge_detector_desc(edet)->flags))
> +               value = !value;
> +
> +       return value;
> +}
> +
> +static irqreturn_t debounce_irq_handler(int irq, void *p)
> +{
> +       struct edge_detector *edet = p;
> +       struct gpio_desc *desc = edge_detector_desc(edet);
> +
> +       mod_delayed_work(system_wq,
> +                        &edet->work,
> +                        usecs_to_jiffies(READ_ONCE(desc->debounce_period)));
> +
> +       return IRQ_HANDLED;
> +}
> +
> +static void debounce_work_func(struct work_struct *work)
> +{
> +       struct gpio_v2_line_event le;
> +       int ret, level;
> +       struct edge_detector *edet =
> +               container_of(work, struct edge_detector, work.work);
> +       struct gpio_desc *desc = edge_detector_desc(edet);
> +       struct line *line;
> +
> +       level = gpiod_get_raw_value_cansleep(desc);
> +       if (level < 0) {
> +               pr_debug_ratelimited("debouncer failed to read line value\n");
> +               return;
> +       }
> +
> +       if (READ_ONCE(edet->level) == level)
> +               return;
> +
> +       WRITE_ONCE(edet->level, level);
> +
> +       /* -- edge detection -- */
> +       if (!edet->flags)
> +               return;
> +
> +       /* switch from physical level to logical - if they differ */
> +       if (test_bit(FLAG_ACTIVE_LOW, &desc->flags))
> +               level = !level;
> +
> +       /* ignore edges that are not being monitored */
> +       if (((edet->flags == GPIO_V2_LINE_FLAG_EDGE_RISING) && !level) ||
> +           ((edet->flags == GPIO_V2_LINE_FLAG_EDGE_FALLING) && level))
> +               return;
> +
> +       /* Do not leak kernel stack to userspace */
> +       memset(&le, 0, sizeof(le));
> +
> +       line = edet->line;
> +       le.timestamp = ktime_get_ns();
> +       le.offset = gpio_chip_hwgpio(desc);
> +       edet->line_seqno++;
> +       le.line_seqno = edet->line_seqno;
> +       le.seqno = (line->num_descs == 1) ?
> +               le.line_seqno : atomic_inc_return(&line->seqno);
> +
> +       if (level)
> +               /* Emit low-to-high event */
> +               le.id = GPIO_V2_LINE_EVENT_RISING_EDGE;
> +       else
> +               /* Emit high-to-low event */
> +               le.id = GPIO_V2_LINE_EVENT_FALLING_EDGE;
> +
> +       ret = kfifo_in_spinlocked_noirqsave(&line->events, &le,
> +                                           1, &line->wait.lock);
> +       if (ret)
> +               wake_up_poll(&line->wait, EPOLLIN);
> +       else
> +               pr_debug_ratelimited("event FIFO is full - event dropped\n");
> +}
> +
> +static int debounce_setup(struct edge_detector *edet,
> +                         unsigned int debounce_period)
> +{
> +       int ret, level, irq, irqflags;
> +       struct gpio_desc *desc = edge_detector_desc(edet);
> +
> +       /* try hardware */
> +       ret = gpiod_set_debounce(desc, debounce_period);
> +       if (!ret) {
> +               WRITE_ONCE(desc->debounce_period, debounce_period);
> +               return ret;
> +       }
> +       if (ret != -ENOTSUPP)
> +               return ret;
> +
> +       if (debounce_period) {
> +               /* setup software debounce */
> +               level = gpiod_get_raw_value_cansleep(desc);
> +               if (level < 0)
> +                       return level;
> +
> +               irq = gpiod_to_irq(desc);
> +               if (irq <= 0)
> +                       return -ENODEV;
> +
> +               WRITE_ONCE(edet->level, level);
> +               edet->line_seqno = 0;
> +               irqflags = IRQF_TRIGGER_FALLING | IRQF_TRIGGER_RISING;
> +               ret = request_irq(irq,
> +                                 debounce_irq_handler,
> +                                 irqflags,
> +                                 edet->line->label,
> +                                 edet);

Nit: Don't break lines too eagerly: this call could fit in two lines.
I think I saw some other instances in other patches.

> +               if (ret)
> +                       return ret;
> +
> +               WRITE_ONCE(edet->sw_debounced, 1);
> +               edet->irq = irq;
> +       }
> +       WRITE_ONCE(desc->debounce_period, debounce_period);
> +       return 0;
> +}
> +
>  static void edge_detector_stop(struct edge_detector *edet)
>  {
> +       struct gpio_desc *desc = edge_detector_desc(edet);
> +
>         if (edet->irq) {
>                 free_irq(edet->irq, edet);
>                 edet->irq = 0;
>         }
> +
> +       cancel_delayed_work_sync(&edet->work);
> +       WRITE_ONCE(edet->sw_debounced, 0);
> +       /* do not change edet->level - see comment in debounced_value */
> +
> +       if (desc)
> +               WRITE_ONCE(desc->debounce_period, 0);
> +}
> +
> +static int debounce_update(struct edge_detector *edet,
> +                          unsigned int debounce_period)
> +{
> +       struct gpio_desc *desc = edge_detector_desc(edet);
> +
> +       if (READ_ONCE(desc->debounce_period) == debounce_period)
> +               return 0;
> +
> +       if (!READ_ONCE(edet->sw_debounced))
> +               return debounce_setup(edet, debounce_period);
> +
> +       if (!debounce_period)
> +               edge_detector_stop(edet);
> +       else
> +               WRITE_ONCE(desc->debounce_period, debounce_period);
> +       return 0;
> +}
> +
> +static int gpio_v2_line_config_debounced(struct gpio_v2_line_config *lc,
> +                                        int line_idx)
> +{
> +       int i;
> +       unsigned long long mask = BIT_ULL(line_idx);
> +
> +       for (i = 0; i < lc->num_attrs; i++) {
> +               if ((lc->attrs[i].attr.id == GPIO_V2_LINE_ATTR_ID_DEBOUNCE) &&
> +                   (lc->attrs[i].mask & mask))
> +                       return 1;
> +       }
> +       return 0;
> +}
> +
> +static u32 gpio_v2_line_config_debounce_period(struct gpio_v2_line_config *lc,
> +                                              int line_idx)
> +{
> +       int i;
> +       unsigned long long mask = BIT_ULL(line_idx);
> +
> +       for (i = 0; i < lc->num_attrs; i++) {
> +               if ((lc->attrs[i].attr.id == GPIO_V2_LINE_ATTR_ID_DEBOUNCE) &&
> +                   (lc->attrs[i].mask & mask))
> +                       return lc->attrs[i].attr.debounce_period;
> +       }
> +       return 0;
>  }
>
>  static int edge_detector_setup(struct edge_detector *edet,
> -                              struct gpio_v2_line_config *lc)
> +                              struct gpio_v2_line_config *lc,
> +                              int line_idx)
>  {
> +       int ret;
> +
> +       if (gpio_v2_line_config_debounced(lc, line_idx)) {
> +               ret = debounce_setup(edet,
> +                       gpio_v2_line_config_debounce_period(lc, line_idx));
> +               if (ret)
> +                       return ret;
> +       }
>         if (edet->flags)
>                 return edge_detector_start(edet);
>         return 0;
> @@ -703,6 +930,11 @@ static int gpio_v2_line_config_validate(struct gpio_v2_line_config *lc,
>                 ret = gpio_v2_line_flags_validate(flags);
>                 if (ret)
>                         return ret;
> +
> +               /* debounce requires explicit input */
> +               if (gpio_v2_line_config_debounced(lc, i) &&
> +                   !(flags & GPIO_V2_LINE_FLAG_INPUT))
> +                       return -EINVAL;
>         }
>         return 0;
>  }
> @@ -761,7 +993,7 @@ static long line_get_values(struct line *line, void __user *ip)
>         struct gpio_v2_line_values lv;
>         DECLARE_BITMAP(vals, GPIO_V2_LINES_MAX);
>         struct gpio_desc **descs;
> -       int ret, i, didx, num_get = 0;
> +       int ret, i, val, didx, num_get = 0;
>
>         /* NOTE: It's ok to read values of output lines. */
>         if (copy_from_user(&lv, ip, sizeof(lv)))
> @@ -799,7 +1031,11 @@ static long line_get_values(struct line *line, void __user *ip)
>         lv.bits = 0;
>         for (didx = 0, i = 0; i < line->num_descs; i++) {
>                 if (lv.mask & BIT_ULL(i)) {
> -                       if (test_bit(didx, vals))
> +                       if (line->edets[i].sw_debounced)
> +                               val = debounced_value(&line->edets[i]);
> +                       else
> +                               val = test_bit(didx, vals);
> +                       if (val)
>                                 lv.bits |= BIT_ULL(i);
>                         didx++;
>                 }
> @@ -907,6 +1143,12 @@ static long line_set_config_locked(struct line *line,
>                         ret = gpiod_direction_input(desc);
>                         if (ret)
>                                 return ret;
> +                       if (gpio_v2_line_config_debounced(lc, i)) {
> +                               ret = debounce_update(&line->edets[i],
> +                                       gpio_v2_line_config_debounce_period(lc, i));
> +                               if (ret)
> +                                       return ret;
> +                       }
>                 }
>
>                 blocking_notifier_call_chain(&desc->gdev->notifier,
> @@ -1109,8 +1351,11 @@ static int line_create(struct gpio_device *gdev, void __user *ip)
>                 goto out_free_line;
>         }
>
> -       for (i = 0; i < lr.num_lines; i++)
> +       for (i = 0; i < lr.num_lines; i++) {
>                 line->edets[i].line = line;
> +               WRITE_ONCE(line->edets[i].sw_debounced, 0);
> +               INIT_DELAYED_WORK(&line->edets[i].work, debounce_work_func);
> +       }
>
>         /* Make sure this is terminated */
>         lr.consumer[sizeof(lr.consumer)-1] = '\0';
> @@ -1177,7 +1422,7 @@ static int line_create(struct gpio_device *gdev, void __user *ip)
>                         if (ret)
>                                 goto out_free_line;
>                         line->edets[i].flags = flags & GPIO_V2_LINE_EDGE_FLAGS;
> -                       ret = edge_detector_setup(&line->edets[i], lc);
> +                       ret = edge_detector_setup(&line->edets[i], lc, i);
>                         if (ret)
>                                 goto out_free_line;
>                 }
> @@ -1649,6 +1894,8 @@ static void gpio_desc_to_lineinfo(struct gpio_desc *desc,
>         struct gpio_chip *gc = desc->gdev->chip;
>         bool ok_for_pinctrl;
>         unsigned long flags;
> +       u32 debounce_period;
> +       int num_attrs = 0;
>
>         memset(info, 0, sizeof(*info));
>         info->offset = gpio_chip_hwgpio(desc);
> @@ -1708,7 +1955,13 @@ static void gpio_desc_to_lineinfo(struct gpio_desc *desc,
>         if (test_bit(FLAG_EDGE_FALLING, &desc->flags))
>                 info->flags |= GPIO_V2_LINE_FLAG_EDGE_FALLING;
>
> -       info->num_attrs = 0;
> +       debounce_period = READ_ONCE(desc->debounce_period);
> +       if (debounce_period) {
> +               info->attrs[num_attrs].id = GPIO_V2_LINE_ATTR_ID_DEBOUNCE;
> +               info->attrs[num_attrs].debounce_period = debounce_period;
> +               num_attrs++;
> +       }
> +       info->num_attrs = num_attrs;

AFAICT this (reading it in gpio_desc_to_lineinfo) is the only reason
to store the debounce period in struct gpio_desc. I'm wondering if we
can avoid extending this struct only for such uncommon case and store
it elsewhere. In all other cases where you read or write to it - you
have access to the underlying edge detector. Would the single-line
struct line I suggested elsewhere be a good place? On the other hand
I'm not sure how to get it having only the desc. I need to think about
it more.

Bart

>
>         spin_unlock_irqrestore(&gpio_lock, flags);
>  }
> diff --git a/drivers/gpio/gpiolib.h b/drivers/gpio/gpiolib.h
> index 39b356160937..671805a79a15 100644
> --- a/drivers/gpio/gpiolib.h
> +++ b/drivers/gpio/gpiolib.h
> @@ -124,6 +124,10 @@ struct gpio_desc {
>  #ifdef CONFIG_OF_DYNAMIC
>         struct device_node      *hog;
>  #endif
> +#ifdef CONFIG_GPIO_CDEV
> +       /* debounce period in microseconds */
> +       unsigned int            debounce_period;
> +#endif
>  };
>
>  int gpiod_request(struct gpio_desc *desc, const char *label);
> --
> 2.28.0
>

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