linux-kernel - Re: [PATCH v2 05/18] gpiolib: cdev: support GPIO_GET_LINE_IOCTL and GPIOLINE_GET_VALUES

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Date:   Sun, 2 Aug 2020 11:31:58 +0800
From:   Kent Gibson <warthog618@...il.com>
To:     Bartosz Golaszewski <bgolaszewski@...libre.com>
Cc:     Andy Shevchenko <andy.shevchenko@...il.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        "open list:GPIO SUBSYSTEM" <linux-gpio@...r.kernel.org>,
        Linus Walleij <linus.walleij@...aro.org>
Subject: Re: [PATCH v2 05/18] gpiolib: cdev: support GPIO_GET_LINE_IOCTL and
 GPIOLINE_GET_VALUES_IOCTL

On Fri, Jul 31, 2020 at 06:05:10PM +0200, Bartosz Golaszewski wrote:
> On Sun, Jul 26, 2020 at 3:12 AM Kent Gibson <warthog618@...il.com> wrote:
> >
> 
> [snip]
> 
> > >
> > > > +static bool padding_not_zeroed(__u32 *padding, int pad_size)
> > > > +{
> > > > +       int i, sum = 0;
> > > > +
> > > > +       for (i = 0; i < pad_size; i++)
> > > > +               sum |= padding[i];
> > > > +
> > > > +       return sum;
> > > > +}
> > >
> > > Reimplementation of memchr_inv() ?
> > >
> >
> > I was hoping to find an existing function, surely checking a region is
> > zeroed is a common thing, right?, so this was a place holder as much
> > as anything.  Not sure memchr_inv fits the bill, but I'll give it a
> > try...
> >
> 
> If you don't find an appropriate function: please put your new
> implementation in lib/ so that others may reuse it.
> 

Changed to memchr_inv.

> > > ...
> > >
> > > > +static u64 gpioline_config_flags(struct gpioline_config *lc, int line_idx)
> > > > +{
> > > > +       int i;
> > > > +
> > > > +       for (i = lc->num_attrs - 1; i >= 0; i--) {
> > >
> > > Much better to read is
> > >
> > > unsigned int i = lc->num_attrs;
> > >
> > > while (i--) {
> > >  ...
> > > }
> > >
> >
> > Really? I find that the post-decrement in the while makes determining the
> > bounds of the loop more confusing.
> >
> 
> Agreed, Andy: this is too much nit-picking. :)
> 

I was actually hoping for some feedback on the direction of that loop,
as it relates to the handling of multiple instances of the same
attribute associated with a given line.

The reverse loop here implements a last in wins policy, but I'm now
thinking the kernel should be encouraging userspace to only associate a
given attribute with a line once, and that a first in wins would help do
that - as additional associations would be ignored.

Alternatively, the kernel should enforce that an attribute can only be
associated once, but that would require adding more request validation.

> [snip]
> 
> > > ...
> > >
> > > > +               struct gpio_desc *desc = gpiochip_get_desc(gdev->chip, offset);
> > >
> > > I prefer to see this split, but it's minor.
> > >
> > > > +               if (IS_ERR(desc)) {
> > > > +                       ret = PTR_ERR(desc);
> > > > +                       goto out_free_line;
> > > > +               }
> > >
> > > ...
> > >
> > > > +               dev_dbg(&gdev->dev, "registered chardev handle for line %d\n",
> > > > +                       offset);
> > >
> > > Perhaps tracepoint / event?
> > >
> >
> > Again, a cut-and-paste from V1, and I have no experience with
> > tracepoints or events, so I have no opinion on that.
> >
> > So, yeah - perhaps?
> >
> 
> I think it's a good idea to add some proper instrumentation this time
> other than much less reliable logs. Can you take a look at
> include/trace/events/gpio.h? Adding new GPIO trace events should be
> pretty straightforward by copy-pasti... drawing inspiration from
> existing ones.
> 

You only want tracepoints to replace those dev_dbg()s, so when a line
is requested? What about the release?  Any other points?

Cheers,
Kent.