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Date:   Fri, 5 Mar 2021 16:01:36 +0100
From:   Greg KH <gregkh@linuxfoundation.org>
To:     Bartosz Golaszewski <brgl@bgdev.pl>
Cc:     Geert Uytterhoeven <geert@linux-m68k.org>,
        Joel Becker <jlbec@evilplan.org>,
        Christoph Hellwig <hch@lst.de>, Shuah Khan <shuah@kernel.org>,
        Linus Walleij <linus.walleij@linaro.org>,
        Andy Shevchenko <andriy.shevchenko@linux.intel.com>,
        Uwe =?iso-8859-1?Q?Kleine-K=F6nig?= 
        <u.kleine-koenig@pengutronix.de>,
        Kent Gibson <warthog618@gmail.com>,
        Jonathan Corbet <corbet@lwn.net>,
        "open list:GPIO SUBSYSTEM" <linux-gpio@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        "open list:DOCUMENTATION" <linux-doc@vger.kernel.org>,
        Bartosz Golaszewski <bgolaszewski@baylibre.com>
Subject: Re: [PATCH v2 08/12] drivers: export device_is_bound()
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On Fri, Mar 05, 2021 at 03:20:27PM +0100, Bartosz Golaszewski wrote:
> On Fri, Mar 5, 2021 at 12:27 PM Greg KH <gregkh@linuxfoundation.org> wrote:
> >
> > On Fri, Mar 05, 2021 at 11:58:18AM +0100, Bartosz Golaszewski wrote:
> > > On Fri, Mar 5, 2021 at 11:24 AM Greg KH <gregkh@linuxfoundation.org> wrote:
> > > >
> > > > On Fri, Mar 05, 2021 at 10:16:10AM +0100, Bartosz Golaszewski wrote:
> > > > > On Fri, Mar 5, 2021 at 9:55 AM Greg KH <gregkh@linuxfoundation.org> wrote:
> > > > > >
> > > > > > On Fri, Mar 05, 2021 at 09:45:41AM +0100, Bartosz Golaszewski wrote:
> > > > > > > On Fri, Mar 5, 2021 at 9:34 AM Greg KH <gregkh@linuxfoundation.org> wrote:
> > > > > > > >
> > > > > > > > On Fri, Mar 05, 2021 at 09:18:30AM +0100, Geert Uytterhoeven wrote:
> > > > > > > > > CC Greg
> > > > > > > > >
> > > > > > > > > On Thu, Mar 4, 2021 at 11:30 AM Bartosz Golaszewski <brgl@bgdev.pl> wrote:
> > > > > > > > > >
> > > > > > > > > > From: Bartosz Golaszewski <bgolaszewski@baylibre.com>
> > > > > > > > > >
> > > > > > > > > > Export the symbol for device_is_bound() so that we can use it in gpio-sim
> > > > > > > > > > to check if the simulated GPIO chip is bound before fetching its driver
> > > > > > > > > > data from configfs callbacks in order to retrieve the name of the GPIO
> > > > > > > > > > chip device.
> > > > > > > > > >
> > > > > > > > > > Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>
> > > > > > > > > > ---
> > > > > > > > > >  drivers/base/dd.c | 1 +
> > > > > > > > > >  1 file changed, 1 insertion(+)
> > > > > > > > > >
> > > > > > > > > > diff --git a/drivers/base/dd.c b/drivers/base/dd.c
> > > > > > > > > > index 9179825ff646..c62c02e3490a 100644
> > > > > > > > > > --- a/drivers/base/dd.c
> > > > > > > > > > +++ b/drivers/base/dd.c
> > > > > > > > > > @@ -353,6 +353,7 @@ bool device_is_bound(struct device *dev)
> > > > > > > > > >  {
> > > > > > > > > >         return dev->p && klist_node_attached(&dev->p->knode_driver);
> > > > > > > > > >  }
> > > > > > > > > > +EXPORT_SYMBOL_GPL(device_is_bound);
> > > > > > > >
> > > > > > > > No.  Please no.  Why is this needed?  Feels like someone is doing
> > > > > > > > something really wrong...
> > > > > > > >
> > > > > > > > NACK.
> > > > > > > >
> > > > > > >
> > > > > > > I should have Cc'ed you the entire series, my bad.
> > > > > > >
> > > > > > > This is the patch that uses this change - it's a new, improved testing
> > > > > > > module for GPIO using configfs & sysfs as you (I think) suggested a
> > > > > > > while ago:
> > > > > > >
> > > > > > > https://lkml.org/lkml/2021/3/4/355
> > > > > > >
> > > > > > > The story goes like this: committing the configfs item registers a
> > > > > > > platform device.
> > > > > >
> > > > > > Ick, no, stop there, that's not a "real" device, please do not abuse
> > > > > > platform devices like that, you all know I hate this :(
> > > > > >
> > > > > > Use the virtbus code instead perhaps?
> > > > > >
> > > > >
> > > > > I have no idea what virtbus is and grepping for it only returns three
> > > > > hits in: ./drivers/pci/iov.c and it's a function argument.
> > > > >
> > > > > If it stands for virtual bus then for sure it sounds like the right
> > > > > thing but I need to find more info on this.
> > > >
> > > > Sorry, wrong name, see Documentation/driver-api/auxiliary_bus.rst for
> > > > the details.  "virtbus" was what I think about it as that was my
> > > > original name for it, but it eventually got merged with a different
> > > > name.
> > > >
> 
> Unless I'm not seeing something - it completely doesn't look like the
> right solution. This auxiliary bus sounds like MFD with extra steps.
> Its aim seems to be to provide virtual devices for sub-modules of real
> devices.
> 
> What I have here really is a dummy device for which no HW exists.

Then just use a "normal" virtual device.  We have loads of them.  But if
you want to bind a "driver" to it, then use the aux bus please.  Do NOT
abuse a platform device for this.

> Also: while the preferred way is to use configfs to instantiate these
> simulated devices, then can still be registered from device-tree (this
> is a feature that was requested and eventually implemented in
> gpio-mockup which we want to phase out so we can't just drop it).
> AFAIK only platform devices can be populated from DT.

If you really are using DT, then ok, a platform device can be used, but
you didn't say that :)

> I guess we could create something like a "virtual bus" that would be
> there for devices that don't exist on any physical bus but this would
> end up in big part being the same thing as platform devices.

That's what the aux bus code is there for.  So maybe you do need to use
it.

> > > > > > > As far as I understand - there's no guarantee that
> > > > > > > the device will be bound to a driver before the commit callback (or
> > > > > > > more specifically platform_device_register_full() in this case)
> > > > > > > returns so the user may try to retrieve the name of the device
> > > > > > > immediately (normally user-space should wait for the associated uevent
> > > > > > > but nobody can force that) by doing:
> > > > > > >
> > > > > > > mv /sys/kernel/config/gpio-sim/pending/foo /sys/kernel/config/gpio-sim/live/
> > > > > > > cat /sys/kernel/config/gpio-sim/live/foo/dev_name
> > > > > > >
> > > > > > > If the device is not bound at this point, we'll have a crash in the
> > > > > > > kernel as opposed to just returning -ENODEV.
> > > > > >
> > > > > > How will the kernel crash?  What has created the dev_name sysfs file
> > > > > > before it is possible to be read from?  That feels like the root
> > > > > > problem.
> > > > > >
> > > > >
> > > > > It's not sysfs - it's in configfs. Each chip has a read-only configfs
> > > > > attribute that returns the name of the device - I don't really have a
> > > > > better idea to map the configfs items to devices that committing
> > > > > creates.
> > > >
> > > > Same question, why are you exporting a configfs attribute that can not
> > > > be read from?  Only export it when your driver is bound to the device.
> > > >
> > >
> > > The device doesn't know anything about configfs. Why would it? The
> > > configuration of a GPIO chip can't be changed after it's instantiated,
> > > this is why we have committable items.
> > >
> > > We export a directory in configfs: gpio-sim -> user creates a new
> > > directory (item) in gpio-sim/pending/foo and it's not tied to any
> > > device yet but exports attributes which we use to configure the device
> > > (label, number of lines, line names etc.), then we mv
> > > gpio-sim/pending/foo gpio-sim/live and this is when the device gets
> > > created and registered with the subsystem. We take all the configured
> > > attributes and put them into device properties for both the driver and
> > > gpiolib core (for standard properties) to read - just like we would
> > > with a regular GPIO driver because this is the goal: test the core
> > > code.
> >
> > Ok, but they why are you trying to have dev_name be an exported thing?
> > I don't understand an attribute here that is visable but can not be read
> > from.
> >
> 
> Because once the associated configfs item is committed and the device
> created, it will become readable. The list of attributes is fixed in
> configfs. I'm not sure what the better approach would be - return
> "none" if the device handle is NULL?

Sounds reasonable, I don't know how configfs works, it's been a decade
since I last touched it.

> > And why not just use the default device name function: dev_name(), which
> > will always return a string that will work no matter if the device is
> > bound to a driver or not.
> >
> 
> I can do this but then it's possible that user-space gets the name of
> the device which doesn't exist in sysfs. I guess we can mention that
> in the documentation.

Device names can change over time, nothing new there.

thanks,

greg k-h