by Geert Uytterhoeven

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Hi Linus,

On Thu, Mar 12, 2020 at 3:57 PM Linus Walleij <[email protected]> wrote:
> thanks for your patience and again sorry for procrastination on my part :(
>
> Overall I start to like this driver a lot. It has come a long way.
>
> Some comments below are nitpicky, bear with me if they seem stupid.

Thanks a lot for your comments!

> On Tue, Feb 18, 2020 at 4:18 PM Geert Uytterhoeven
> <[email protected]> wrote:
> > +#define DRV_NAME "gpio-aggregator"
> > +#define pr_fmt(fmt) DRV_NAME ": " fmt
>
> I would just use dev_[info|err] for all messages to get rid of this.

See below.

> > +#include <linux/bitmap.h>
> > +#include <linux/bitops.h>
> > +#include <linux/ctype.h>
> > +#include <linux/gpio/consumer.h>
> > +#include <linux/gpio/driver.h>
> > +#include <linux/gpio/machine.h>
> > +#include <linux/idr.h>
> > +#include <linux/kernel.h>
> > +#include <linux/module.h>
> > +#include <linux/mutex.h>
> > +#include <linux/overflow.h>
> > +#include <linux/platform_device.h>
> > +#include <linux/spinlock.h>
> > +#include <linux/string.h>
> > +
> > +#include "gpiolib.h"
>
> When this file is includes I prefer if there is a comment next to
> this include saying why we have to touch internals and which
> ones.

I have just discovered gpiod_to_chip(), which removes the need for two
of the three users ;-)

> > +struct gpio_aggregator {
> > + struct gpiod_lookup_table *lookups;
> > + struct platform_device *pdev;
>
> What about just storing struct device *dev?
>
> Then callbacks can just
>
> dev_err(aggregator->dev, "myerror\n");

&aggr->pdev.dev or aggr->dev does't make much of a difference.

> > +static char *get_arg(char **args)
> > +{
> > + char *start = *args, *end;
> > +
> > + start = skip_spaces(start);
> > + if (!*start)
> > + return NULL;
> > +
> > + if (*start == '"') {
> > + /* Quoted arg */
> > + end = strchr(++start, '"');
> > + if (!end)
> > + return ERR_PTR(-EINVAL);
> > + } else {
> > + /* Unquoted arg */
> > + for (end = start; *end && !isspace(*end); end++) ;
> > + }
> > +
> > + if (*end)
> > + *end++ = '\0';
> > +
> > + *args = end;
> > + return start;
> > +}
>
> Isn't this function reimplementing strsep()?
> while ((s = strsep(&p, " \""))) {
> or something.
>
> I'm not the best with strings, just asking so I know you tried it
> already.

strsep(&p, " \"") would terminate the token if a space or double quote is
seen. I.e. it wouldn't handle spaces in quoted arguments.
There's also argv_split(), but that doesn't handle quoted args, and
duplicates all arguments.

Line names assigned by "gpio-lines-names" may contain spaces, so support
for quoted args is mandatory.

> > +static int aggr_parse(struct gpio_aggregator *aggr)
> > +{
> > + unsigned int first_index, last_index, i, n = 0;
> > + char *name, *offsets, *first, *last, *next;
> > + char *args = aggr->args;
> > + int error;
> > +
> > + for (name = get_arg(&args), offsets = get_arg(&args); name;
> > + offsets = get_arg(&args)) {
> > + if (IS_ERR(name)) {
> > + pr_err("Cannot get GPIO specifier: %pe\n", name);
>
> If gpio_aggregrator contained struct device *dev this would be
> dev_err(aggr->dev, "...\n");

aggr_parse() is called before the platform device is created, and before
aggr->pdev is populated. So there is no device to print yet.

> > +static void gpio_aggregator_free(struct gpio_aggregator *aggr)
> > +{
> > + platform_device_unregister(aggr->pdev);
>
> Aha maybe store both the pdev and the dev in the struct then?
>
> Or print using &aggr->pdev.dev.

Same for aggr->pdev.dev (or aggr->dev).

> > + /*
> > + * If any of the GPIO lines are sleeping, then the entire forwarder
> > + * will be sleeping.
> > + * If any of the chips support .set_config(), then the forwarder will
> > + * support setting configs.
> > + */
> > + for (i = 0; i < ngpios; i++) {
> > + dev_dbg(dev, "gpio %u => gpio-%d (%s)\n", i,
> > + desc_to_gpio(descs[i]), descs[i]->label ? : "?");
>
> If this desc->label business is why you need to include
> "gpiolib.h" then I'd prefer if you just add a

It was the third reason to include that file...

> const char *gpiod_get_producer_name(struct gpio_desc *desc);
>
> to gpiolib (add in <linux/gpio/consumer.h> so that gpiolib can
> try to give you something reasonable to print for the label here.
> I ran into that problem before (wanting to print something like this)
> and usually just printed the offset.
>
> But if it is a serious debug issue, let's fix a helper for this.
>
> gpiod_get_producer_name() could return the thing in
> desc->label if that is set or else something along
> "chipname-offset" or "unknown", I'm not very picky
> with that.

I will just remove the printing of the label, as it is no longer useful.
Since I started using gpiod_lookup, the descriptor has already been
requested at this point, which means its label will usually be
"gpio-aggregator.N", i.e. it doesn't provide any help.
The only exception is for a GPIO line which has an associated line name
through "gpio-line-names" in DT. But just seeing the global GPIO number
should be good enough for debugging.

BTW, one day you may want to have your our printk() format specifier for
GPIOs? Oh, no "%pg" and "%pG" are already taken; "%pp" is still
available.

> > error = aggr_add_gpio(aggr, name, U16_MAX, &n);
>
> Is the reason why you use e.g. "gpiochip0" as name here that this
> is a simple ABI for userspace?

"name" is not the "gpiochipN" name here, but the line name, cfr. the
U16_MAX value for chip index, and the comment just above:

+ /* Named GPIO line */

That one is supposed to be stable, right?
Note that this is the most use-centric way to refer to a GPIO.

In the other caller:

+ error = aggr_add_gpio(aggr, name, i, &n);

"name" is a reference to the gpiochip, i.e. either its label, or the
"gpiochipN" name.

> Such like obtained from /sys/bus/gpio/devices/<chipname>?
>
> I would actually prefer to just add a sysfs attribute
> such as "name" and set it to the value of gpiochip->label.

Makes sense, but that would be a separate, unrelated patch, right?

> These labels are compulsory and supposed to be unique.
>
> Then whatever creates an aggregator can just use
> cat /sys/bus/gpio/devices/gpiochipN/name to send in
> through the sysfs interface to this kernel driver.
>
> This will protect you in the following way:
>
> When a system is booted and populated the N in
> gpiochipN is not stable and this aggregator will be used
> by scripts that assume it is. We already had this dilemma
> with things like network interfaces like eth0/1.
>
> This can be because of things like probe order which
> can be random, or because someone compiled a
> kernel with a new driver for a gpiochip that wasn't
> detected before. This recently happened to Raspberry Pi,
> that added gpio driver for "firmware GPIOs" (IIRC).
>
> The label on the chip is going to be more stable
> I think, so it is better to use that.

OK, so support for "gpiochipN" matching can be dropped, obsoleting
"[PATCH v5 1/5] gpiolib: Add support for gpiochipN-based table lookup".

Note that I added support for that in response to Bartosz' first try
https://lore.kernel.org/linux-gpio/CAMpxmJUF1s1zyXVtoUGfbV7Yk+heua4rNjY=DrX=jr-v8UfNxA@mail.gmail.com/

> This should also rid the need to include "gpiolib.h"
> which makes me nervous.

Consider it done!
Thanks!

Gr{oetje,eeting}s,

Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- [email protected]

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
-- Linus Torvalds

2020-03-17 11:33:17

by Geert Uytterhoeven

[permalink] [raw]

Subject: Re: [PATCH v5 3/5] gpio: Add GPIO Aggregator

On Tue, Feb 18, 2020 at 4:18 PM Geert Uytterhoeven
<[email protected]> wrote:
> GPIO controllers are exported to userspace using /dev/gpiochip*
> character devices. Access control to these devices is provided by
> standard UNIX file system permissions, on an all-or-nothing basis:
> either a GPIO controller is accessible for a user, or it is not.
> Currently no mechanism exists to control access to individual GPIOs.
>
> Hence add a GPIO driver to aggregate existing GPIOs, and expose them as
> a new gpiochip.
>
> This supports the following use cases:
> - Aggregating GPIOs using Sysfs
> This is useful for implementing access control, and assigning a set
> of GPIOs to a specific user or virtual machine.
> - Generic GPIO Driver
> This is useful for industrial control, where it can provide
> userspace access to a simple GPIO-operated device described in DT,
> cfr. e.g. spidev for SPI-operated devices.
>
> Signed-off-by: Geert Uytterhoeven <[email protected]>

> --- /dev/null
> +++ b/drivers/gpio/gpio-aggregator.c

> +static int gpio_fwd_set_config(struct gpio_chip *chip, unsigned int offset,
> + unsigned long config)
> +{
> + struct gpiochip_fwd *fwd = gpiochip_get_data(chip);
> +
> + chip = fwd->descs[offset]->gdev->chip;
> + if (chip->set_config)

- chip = fwd->descs[offset]->gdev->chip;
- if (chip->set_config)
+ chip = gpiod_to_chip(fwd->descs[offset]);
+ if (chip && chip->set_config)

> + return chip->set_config(chip, offset, config);

This is not correct: offset should be translated, too, i.e.

offset = gpio_chip_hwgpio(fwd->descs[offset]);

Which adds a new dependency on "gpiolib.h"...

Is there a better alternative, than providing a public gpiod_set_config()
helper?
Thanks!

Gr{oetje,eeting}s,

Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- [email protected]

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
-- Linus Torvalds