Re: [PATCH RESEND 0/2] leds: gpio: Add devlink between the leds-gpio device and the gpio used.

[Herve Codina <herve.codina@bootlin.com>]

Hi Saravana,

On Tue, 20 Feb 2024 19:28:17 -0800
Saravana Kannan <saravanak@google.com> wrote:

...
> > >
> > > Do I understand correctly that the devlink is created between "led-5"
> > > and "tca6424_dock_2" but actually should also be created between
> > > "leds-dock" and "tca6424_dock_2"?
> > >  
> >
> > Yes, that's my understanding too.  
> 
> I'm replying here instead of the RESEND because here's where the
> context and example are provided.
> 
> I quickly poked into the gpio-leds driver. Please correct me if I'm
> misunderstanding anything.
> 
> It looks like led-5 will be added as a class device. But the
> dev->fwnode is not set before it's added because it uses
> device_create_with_groups(). So, fw_devlink doesn't create a link
> between led-5 and tca6424_dock_2 unless tca6424_dock_2 is added after
> led-5. Which coincidentally seems to be the case here. Might want to
> explicitly create the device in gpio-leds driver.
> 
> The issue you are trying to fix is a generic issue that I'd like to
> fix in a generic fashion. It's one of my TODOs which I've mentioned
> before in conferences/emails to LKML: device links framework has a
> bunch of gaps when it comes to class devices. I've been thinking about
> it for a while, but it needs a lot more work and testing. I'll roll in
> this case when I deal with it in a generic fashion. But here's the
> general idea of things that need to be addressed:
> 
> 1. "Managed" device links allow having a class device as a supplier,
> but that'll mean the consumer will never probe.
> 2. What if a class device is a consumer and the supplier isn't ready.
> What does it mean for the class device to be added? Is it available
> for use? Probably not. Can we do something here that'll be useful for
> the class implementation?
> 3. What if the supplier and consumer are class devices, when does the
> consumer class device become "available" (do we check the suppliers of
> the supplier?)?
> 4. What happens if the supplier of a class device gets removed? Do we
> notify the class so it can do the right thing? Do we force unbind the
> first ancestor that's on a bus? (your case).
> 5. What if a supplier class device is removed, should we unbind the
> consumer (if it's a bus device)?
> 
> I'm currently working on a patch to break dependency cycles. Once
> that's in, the next TODO item I work on is going to be this or clock
> framework sync_state() support.
> 
> So, I'd recommend waiting this out if it's not urgent.
> 
> Heh, here's my commit on my local repo from a year ago when I touched
> on this and realised the scope of the work.
> 
> commit 7dcaad52e569209104408f3e472fde4ef8cd5585 (class-devlinks-v1)
> Author: Saravana Kannan <saravanak@google.com>
> Date:   Mon Feb 13 13:40:43 2023 -0800
> 
>     add class support to device links
> 
> 
> Thanks,
> Saravana

Did you move forward on this topic ?

Best regards,
Hervé