Re: [PATCH v3 12/13] gpiolib: add new ioctl() for monitoring changes in line info

[Andy Shevchenko <andy.shevchenko@xxxxxxxxx>]

On Thu, Dec 19, 2019 at 7:17 PM Bartosz Golaszewski <brgl@xxxxxxxx> wrote:
>
> From: Bartosz Golaszewski <bgolaszewski@xxxxxxxxxxxx>
>
> Currently there is no way for user-space to be informed about changes
> in status of GPIO lines e.g. when someone else requests the line or its
> config changes. We can only periodically re-read the line-info. This
> is fine for simple one-off user-space tools, but any daemon that provides
> a centralized access to GPIO chips would benefit hugely from an event
> driven line info synchronization.
>
> This patch adds a new ioctl() that allows user-space processes to reuse
> the file descriptor associated with the character device for watching
> any changes in line properties. Every such event contains the updated
> line information.
>
> Currently the events are generated on three types of status changes: when
> a line is requested, when it's released and when its config is changed.
> The first two are self-explanatory. For the third one: this will only
> happen when another user-space process calls the new SET_CONFIG ioctl()
> as any changes that can happen from within the kernel (i.e.
> set_transitory() or set_debounce()) are of no interest to user-space.

> -       } else if (cmd == GPIO_GET_LINEINFO_IOCTL) {
> +       } else if (cmd == GPIO_GET_LINEINFO_IOCTL ||
> +                  cmd == GPIO_GET_LINEINFO_WATCH_IOCTL) {

Wouldn't be better for maintenance to have them separated from the day 1?

...

> +       if (test_bit(desc_to_gpio(desc), priv->watched_lines)) {

if (!test_bit(...))
  return NOTIFY_DONE;

?

> +                       pr_debug_ratelimited(
> +                               "%s: lineinfo event FIFO is full - event dropped\n",

> +                               __func__);

This is in 99.9% cases redundant in *_dbg() calls.

> +               ret = NOTIFY_OK;
> +       }
> +
> +       return ret;

return NOTIFY_OK;
?

> +}

...

> @@ -3111,6 +3285,7 @@ static int gpio_set_bias(struct gpio_chip *chip, struct gpio_desc *desc)
>                 if (ret != -ENOTSUPP)
>                         return ret;
>         }
> +
>         return 0;
>  }
>

This hunk doesn't belong to this patch.

...

> +/**
> + * struct gpioline_info_changed - Information about a change in status
> + * of a GPIO line
> + * @info: updated line information
> + * @timestamp: estimate of time of status change occurrence, in nanoseconds
> + * and GPIOLINE_CHANGED_CONFIG
> + * @event_type: one of GPIOLINE_CHANGED_REQUESTED, GPIOLINE_CHANGED_RELEASED
> + */
> +struct gpioline_info_changed {

> +       struct gpioline_info info;

Is this guaranteed to be always 8 byte aligned?
I'm expecting to see some comments there and / or here about it.

> +       __u64 timestamp;
> +       __u32 event_type;
> +       __u32 padding[5]; /* for future use */
> +};

-- 
With Best Regards,
Andy Shevchenko