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From: Kent Gibson <warthog618@gmail.com>
To: Bartosz Golaszewski <bgolaszewski@baylibre.com>
Cc: Andy Shevchenko <andy.shevchenko@gmail.com>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	"open list:GPIO SUBSYSTEM" <linux-gpio@vger.kernel.org>,
	Linus Walleij <linus.walleij@linaro.org>
Subject: Re: [PATCH v2 05/18] gpiolib: cdev: support GPIO_GET_LINE_IOCTL and GPIOLINE_GET_VALUES_IOCTL
Date: Tue, 4 Aug 2020 07:01:18 +0800
Message-ID: <20200803230118.GA3650@sol> (raw)
In-Reply-To: <CAMpxmJWZpMFbrMBkLiR9q7chdamVnjw0geDf-pgKrz=AWD8mNg@mail.gmail.com>

On Mon, Aug 03, 2020 at 10:02:50PM +0200, Bartosz Golaszewski wrote:
> On Sun, Aug 2, 2020 at 5:32 AM Kent Gibson <warthog618@gmail.com> wrote:
> >
> > 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@gmail.com> wrote:
> 
> [snip!]
> 
> > > > >
> > > > > > +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.
> >
> 
> I guess this would result in a lot of churn to do validation which is
> largely unnecessary? To me the first in wins sounds more consistent.
> 

Fully validating the attrs would involve a lot of tedious looping, which
would be pointless 99.99% of the time, so I was hoping to avoid it.
OTOH we're interacting with hardware so I don't want to be doing
anything that userspace hasn't explicitly requested.

But I would be satisfied with clearly documenting the behaviour - and
in most cases libgpiod will be taking care of it anyway...

> Also: I just started going through the patches - nice idea with the
> GPIO attributes, I really like it. Although I need to give it a longer
> thought tomorrow - I'm wondering if we can maybe unify them and the
> flags.
> 

I had an earlier draft that did just that - and that is partially why
the loop is last in wins - I was using slot 0 as the default flags.
But the default flags cover a lot of use cases, including all of v1, and
it was simple and cheap to provide a default - and it simplified the
initial port of libgpiod to v2...

Cheers,
Kent.


  reply index

Thread overview: 46+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-07-25  4:19 [PATCH v2 00/18] gpio: cdev: add uAPI V2 Kent Gibson
2020-07-25  4:19 ` [PATCH v2 01/18] gpio: uapi: define GPIO_MAX_NAME_SIZE for array sizes Kent Gibson
2020-07-25  4:19 ` [PATCH v2 02/18] gpio: uapi: define uAPI v2 Kent Gibson
2020-08-04 17:42   ` Bartosz Golaszewski
2020-08-05  5:18     ` Kent Gibson
2020-08-05 17:47       ` Bartosz Golaszewski
2020-08-06  1:03         ` Kent Gibson
2020-08-06  1:15       ` Kent Gibson
2020-08-06 15:53         ` Bartosz Golaszewski
2020-07-25  4:19 ` [PATCH v2 03/18] gpiolib: make cdev a build option Kent Gibson
2020-07-25 20:29   ` Andy Shevchenko
2020-07-26 22:25   ` Linus Walleij
2020-07-27  1:46     ` Kent Gibson
2020-07-27  5:57       ` Kent Gibson
2020-07-27  8:15         ` Linus Walleij
2020-07-25  4:19 ` [PATCH v2 04/18] gpiolib: add build option for CDEV v1 ABI Kent Gibson
2020-07-25  4:19 ` [PATCH v2 05/18] gpiolib: cdev: support GPIO_GET_LINE_IOCTL and GPIOLINE_GET_VALUES_IOCTL Kent Gibson
2020-07-25 20:51   ` Andy Shevchenko
2020-07-26  1:12     ` Kent Gibson
2020-07-26  3:24       ` Kent Gibson
2020-07-29  2:28       ` Kent Gibson
2020-07-29  8:05         ` Andy Shevchenko
2020-07-29 10:01           ` Kent Gibson
2020-07-31 16:05       ` Bartosz Golaszewski
2020-08-02  3:31         ` Kent Gibson
2020-08-02  9:32           ` Kent Gibson
2020-08-03 19:59             ` Bartosz Golaszewski
2020-08-03 20:02           ` Bartosz Golaszewski
2020-08-03 23:01             ` Kent Gibson [this message]
2020-08-04 17:47               ` Bartosz Golaszewski
2020-08-05  3:06                 ` Kent Gibson
2020-07-25  4:19 ` [PATCH v2 06/18] gpiolib: cdev: support GPIO_GET_LINEINFO_V2_IOCTL and GPIO_GET_LINEINFO_WATCH_V2_IOCTL Kent Gibson
2020-07-25  4:19 ` [PATCH v2 07/18] gpiolib: cdev: support edge detection for uAPI v2 Kent Gibson
2020-08-04 19:28   ` Bartosz Golaszewski
2020-07-25  4:19 ` [PATCH v2 08/18] gpiolib: cdev: support GPIOLINE_SET_CONFIG_IOCTL Kent Gibson
2020-07-25  4:19 ` [PATCH v2 09/18] gpiolib: cdev: support GPIOLINE_SET_VALUES_IOCTL Kent Gibson
2020-07-25  4:19 ` [PATCH v2 10/18] gpiolib: cdev: support setting debounce Kent Gibson
2020-07-25  4:19 ` [PATCH v2 11/18] gpio: uapi: document uAPI v1 as deprecated Kent Gibson
2020-08-04 19:43   ` Bartosz Golaszewski
2020-07-25  4:19 ` [PATCH v2 12/18] tools: gpio: port lsgpio to v2 uAPI Kent Gibson
2020-07-25  4:19 ` [PATCH v2 13/18] tools: gpio: port gpio-watch " Kent Gibson
2020-07-25  4:19 ` [PATCH v2 14/18] tools: gpio: rename nlines to num_lines Kent Gibson
2020-07-25  4:19 ` [PATCH v2 15/18] tools: gpio: port gpio-hammer to v2 uAPI Kent Gibson
2020-07-25  4:19 ` [PATCH v2 16/18] tools: gpio: port gpio-event-mon " Kent Gibson
2020-07-25  4:19 ` [PATCH v2 17/18] tools: gpio: add debounce support to gpio-event-mon Kent Gibson
2020-07-25  4:19 ` [PATCH v2 18/18] tools: gpio: add multi-line monitoring " Kent Gibson

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