Re: [PATCH v2 08/12] drivers: export device_is_bound()

[Greg KH <gregkh@linuxfoundation.org>]

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.
> >
> > > > > 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.

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.

thanks,

greg k-h