Re: [PATCH 1/4] gpiolib: cdev: relocate debounce_period_us from struct gpio_desc

[Bartosz Golaszewski <brgl@xxxxxxxx>]

On Wed, Dec 13, 2023 at 3:27 PM Kent Gibson <warthog618@xxxxxxxxx> wrote:
>
> On Wed, Dec 13, 2023 at 03:54:53PM +0200, Andy Shevchenko wrote:
> > On Tue, Dec 12, 2023 at 01:42:50PM +0800, Kent Gibson wrote:
> > > Store the debounce period for a requested line locally, rather than in
> > > the debounce_period_us field in the gpiolib struct gpio_desc.
> > >
> > > Add a global tree of lines containing supplemental line information
> > > to make the debounce period available to be reported by the
> > > GPIO_V2_GET_LINEINFO_IOCTL and the line change notifier.
> >
> > ...
> >
> > >  struct line {
> > >     struct gpio_desc *desc;
> > > +   struct rb_node node;
> >
> > If you swap them, would it benefit in a code generation (bloat-o-meter)?
> >
>
> Didn't consider that placement within the scruct could impact code
> generation.
> Having the rb_nodes at the beginning of struct is preferable?
>

I suppose it has something to do with 0 offset when using
container_of(). Not sure if that really matters though.

> > >  };
> >
> > ...
> >
> > > +struct supinfo {
> > > +   spinlock_t lock;
> > > +   struct rb_root tree;
> > > +};
> >
> > Same Q.
> >
>
> Same - I tend to put locks before the field(s) they cover.
> But if the node being first results in nicer code then happy to swap.
>
> > ...
> >
> > > +static struct supinfo supinfo;
> >
> > Why supinfo should be a struct to begin with? Seems to me as an unneeded
> > complication.
> >

I think we should keep it as a struct but defined the following way:

struct {
    spinlock_t lock;
    struct rb_root tree;
} supinfo;

>
> Yeah, that is a hangover from an earlier iteration where supinfo was
> contained in other object rather than being a global.
> Could merge the struct definition into the variable now.
>
> > ...
> >
> > > +                   pr_warn("%s: duplicate line inserted\n", __func__);
> >
> > I hope at bare minimum we have pr_fmt(), but even though this is poor message
> > that might require some information about exact duplication (GPIO chip label /
> > name, line number, etc). Generally speaking the __func__ in non-debug messages
> > _usually_ is a symptom of poorly written message.
> >
> > ...
>
> Yeah, I wasn't sure about the best way to log here.
>
> The details of chip or line etc don't add anything - seeing this error
> means there is a logic error in the code - we have inserted a line
> without erasing it.  Knowing which chip or line it happened to occur on
> wont help debug it.  It should never happen, but you can't just leave it
> unhandled, so I went with a basic log.
>

We should yell loudly in that case - use one of the WARN() variants
that'll print a stack trace too and point you to the relevant line in
the code.

> >
> > > +out_unlock:
> > > +   spin_unlock(&supinfo.lock);
> >
> > No use of cleanup.h?
> >
>
> Again, that is new to me, so no not yet.
>

Yep, please use a guard, they're awesome. :)

> > ...
> >
> > > +static inline bool line_is_supplemental(struct line *line)
> > > +{
> > > +   return READ_ONCE(line->debounce_period_us) != 0;
> >
> > " != 0" is redundant.
> >
>
> I prefer conversion from int to bool to be explicit, but if you
> insist...
>
> > > +}
> >
> > ...
> >
> > >     for (i = 0; i < lr->num_lines; i++) {
> > > -           if (lr->lines[i].desc) {
> > > -                   edge_detector_stop(&lr->lines[i]);
> > > -                   gpiod_free(lr->lines[i].desc);
> > > +           line = &lr->lines[i];
> > > +           if (line->desc) {
> >
> > Perhaps
> >
> >               if (!line->desc)
> >                       continue;
> >
> > ?
>
> Seems reasonable - I was just going with what was already there.
>
> >
> > > +                   edge_detector_stop(line);
> > > +                   if (line_is_supplemental(line))
> > > +                           supinfo_erase(line);
> > > +                   gpiod_free(line->desc);
> > >             }
> > >     }
> >
> > ...
> >
> > > +static int __init gpiolib_cdev_init(void)
> > > +{
> > > +   supinfo_init();
> > > +   return 0;
> > > +}
> >
> > It's a good practice to explain initcalls (different to the default ones),
> > can you add a comment on top to explain the choice of this initcall, please?
> >
>
> Not sure what you mean.  This section used gpiolib-sysfs as a template,
> and that has no documentation.
>
> > > +postcore_initcall(gpiolib_cdev_init);
> >
>
> Thanks for the review - always instructive.
>
> Cheers,
> Kent.

Bart