Re: [PATCH v1] driver core: Add state_synced sysfs file for devices that support it

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

On Wed, May 13, 2020 at 11:49:57AM -0700, Saravana Kannan wrote:
> On Wed, May 13, 2020 at 1:34 AM Saravana Kannan <saravanak@google.com> wrote:
> >
> > On Wed, May 13, 2020 at 1:22 AM Greg Kroah-Hartman
> > <gregkh@linuxfoundation.org> wrote:
> > >
> > > On Tue, May 12, 2020 at 06:34:15PM -0700, Saravana Kannan wrote:
> > > > This can be used to check if a device supports sync_state() callbacks
> > > > and therefore keeps resources left on by the bootloader enabled till all
> > > > its consumers have probed.
> > > >
> > > > This can also be used to check if sync_state() has been called for a
> > > > device or whether it is still trying to keep resources enabled because
> > > > they were left enabled by the bootloader and all its consumers haven't
> > > > probed yet.
> > > >
> > > > Signed-off-by: Saravana Kannan <saravanak@google.com>
> > > > ---
> > > >  .../ABI/testing/sysfs-devices-state_synced    | 24 +++++++++++++++++++
> > > >  drivers/base/dd.c                             | 16 +++++++++++++
> > > >  2 files changed, 40 insertions(+)
> > > >  create mode 100644 Documentation/ABI/testing/sysfs-devices-state_synced
> > > >
> > > > diff --git a/Documentation/ABI/testing/sysfs-devices-state_synced b/Documentation/ABI/testing/sysfs-devices-state_synced
> > > > new file mode 100644
> > > > index 000000000000..0c922d7d02fc
> > > > --- /dev/null
> > > > +++ b/Documentation/ABI/testing/sysfs-devices-state_synced
> > > > @@ -0,0 +1,24 @@
> > > > +What:                /sys/devices/.../state_synced
> > > > +Date:                May 2020
> > > > +Contact:     Saravana Kannan <saravanak@google.com>
> > > > +Description:
> > > > +             The /sys/devices/.../state_synced attribute is only present for
> > > > +             devices whose bus types or driver provides the .sync_state()
> > > > +             callback. The number read from it (0 or 1) reflects the value
> > > > +             of the device's 'state_synced' field. A value of 0 means the
> > > > +             .sync_state() callback hasn't been called yet. A value of 1
> > > > +             means the .sync_state() callback has been called.
> > > > +
> > > > +             Generally, if a device has sync_state() support and has some of
> > > > +             the resources it provides enabled at the time the kernel starts
> > > > +             (Eg: enabled by hardware reset or bootloader or anything that
> > > > +             run before the kernel starts), then it'll keep those resources
> > > > +             enabled and in a state that's compatible with the state they
> > > > +             were in at the start of the kernel. The device will stop doing
> > > > +             this only when the sync_state() callback has been called --
> > > > +             which happens only when all its consumer devices are registered
> > > > +             and have probed successfully. Resources that were left disabled
> > > > +             at the time the kernel starts are not affected or limited in
> > > > +             any way by sync_state() callbacks.
> > > > +
> > > > +
> > > > diff --git a/drivers/base/dd.c b/drivers/base/dd.c
> > > > index 48ca81cb8ebc..72599436ae84 100644
> > > > --- a/drivers/base/dd.c
> > > > +++ b/drivers/base/dd.c
> > > > @@ -458,6 +458,13 @@ static void driver_deferred_probe_add_trigger(struct device *dev,
> > > >               driver_deferred_probe_trigger();
> > > >  }
> > > >
> > > > +static ssize_t state_synced_show(struct device *dev,
> > > > +                              struct device_attribute *attr, char *buf)
> > > > +{
> > > > +     return sprintf(buf, "%u\n", dev->state_synced);
> > > > +}
> > > > +static DEVICE_ATTR_RO(state_synced);
> > > > +
> > > >  static int really_probe(struct device *dev, struct device_driver *drv)
> > > >  {
> > > >       int ret = -EPROBE_DEFER;
> > > > @@ -531,9 +538,16 @@ static int really_probe(struct device *dev, struct device_driver *drv)
> > > >               goto dev_groups_failed;
> > > >       }
> > > >
> > > > +     if (dev_has_sync_state(dev) &&
> > > > +         device_create_file(dev, &dev_attr_state_synced)) {
> > > > +             dev_err(dev, "state_synced sysfs add failed\n");
> > > > +             goto dev_sysfs_state_synced_failed;
> > > > +     }
> > >
> > > Why not add this to the groups above this and only enable it if needed
> > > at runtime?
> >
> > Those groups above seem to be driver specific groups. Looking at the
> > code, some drivers seem to be setting them. Also, this attribute can
> > only be decided after a driver has successfully bound to the device
> > because dev_has_sync_state() has to check the bus and the driver for
> > sync_state() support.
> >
> > > The is_visible() callback should be what you need to use here.
> >
> > If this is an attribute specific property, it might work. I'll take a look.
> 
> I took a look at is_visible(). It only makes sense for a group of
> attributes that are exposed in a sub directory. But state_synced is a
> top level property IMO. What I'm doing is similar to the "online"
> attribute that's exposed for a device.

No, no need for a subdirectory for an attribute group.  If you don't
have a name for the group, the files will be placed in the device's
directory.

> And even if I create a group with one lone attribute, I'll still have
> to add it after the device has probed or have to call
> sysfs_update_groups() after the device is probed to update the
> visibility. Seems quite messy and unnecessarily complicated.

Ah, ok, that is messier.

> I'd like to keep this an attribute and not create a group for that
> reason. Does that sound okay to you? I'll obviously still do the clean
> up correctly.

Ok, that's fine, we can change it later if we figure it out.

thanks,

greg k-h