Re: [PATCH] gpio: sim: don't fiddle with GPIOLIB private members

[Saravana Kannan <saravanak@xxxxxxxxxx>]

On Wed, Feb 21, 2024 at 4:59 AM Andy Shevchenko
<andriy.shevchenko@xxxxxxxxxxxxxxx> wrote:
>
> On Tue, Feb 20, 2024 at 05:46:27PM -0800, Saravana Kannan wrote:
> > On Mon, Sep 4, 2023 at 3:29 AM Andy Shevchenko
> > <andriy.shevchenko@xxxxxxxxxxxxxxx> wrote:
> > > On Mon, Sep 04, 2023 at 12:12:44PM +0200, Bartosz Golaszewski wrote:
> > > > On Mon, Sep 4, 2023 at 12:05 PM Andy Shevchenko
> > > > <andriy.shevchenko@xxxxxxxxxxxxxxx> wrote:
> > > > > On Mon, Sep 04, 2023 at 11:47:54AM +0200, Bartosz Golaszewski wrote:
> > > > > > On Mon, Sep 4, 2023 at 11:40 AM Andy Shevchenko
> > > > > > <andriy.shevchenko@xxxxxxxxxxxxxxx> wrote:
> > > > > > > On Mon, Sep 04, 2023 at 11:22:32AM +0200, Bartosz Golaszewski wrote:
> > > > > > > > On Mon, Sep 4, 2023 at 10:59 AM Andy Shevchenko
> > > > > > > > <andriy.shevchenko@xxxxxxxxxxxxxxx> wrote:
> > > > > > > > > On Sat, Sep 02, 2023 at 04:40:05PM +0200, Bartosz Golaszewski wrote:
> > > > > > > > > > On Fri, Sep 1, 2023 at 11:10 PM Andy Shevchenko
> > > > > > > > > > <andriy.shevchenko@xxxxxxxxxxxxxxx> wrote:
> > > > > > > > > > > On Fri, Sep 01, 2023 at 08:32:40PM +0200, Bartosz Golaszewski wrote:
>
> ...
>
> > > > > > > > > > > > -     /* Used by sysfs and configfs callbacks. */
> > > > > > > > > > > > -     dev_set_drvdata(&gc->gpiodev->dev, chip);
> > > > > > > > > > > > +     /* Used by sysfs callbacks. */
> > > > > > > > > > > > +     dev_set_drvdata(swnode->dev, chip);
> > > > > > > > > > >
> > > > > > > > > > > dev pointer of firmware node is solely for dev links. Is it the case here?
> > > > > > > > > > > Seems to me you luckily abuse it.
> > > > > > > > > >
> > > > > > > > > > I don't think so. If anything we have a helper in the form of
> > > > > > > > > > get_dev_from_fwnode() but it takes reference to the device while we
> > > > > > > > > > don't need it - we know it'll be there because we created it.
> > > > > > > > > >
> > > > > > > > > > This information (struct device of the GPIO device) can also be
> > > > > > > > > > retrieved by iterating over the device children of the top platform
> > > > > > > > > > device and comparing their fwnodes against the one we got passed down
> > > > > > > > > > from probe() but it's just so many extra steps.
> > > > > > > > > >
> > > > > > > > > > Or we can have a getter in gpio/driver.h for that but I don't want to
> > > > > > > > > > expose another interface is we can simply use the fwnode.
> > > > > > > > >
> >
> > Sorry for being late to the party.
>
> You decided to make a blast from the past due to the last patches from me? :-)

Yeah :) I meant to reply to this when you sent it, but was swamped and
forgot about it.

>
> > > > > > > > > dev pointer in the fwnode strictly speaking is optional. No-one, except
> > > > > > > > > its solely user, should rely on it (its presence and lifetime).
> > > > > > > >
> > > > > > > > Where is this documented? Because just by a quick glance into
> > > > > > > > drivers/base/core.c I can tell that if a device has an fwnode then
> > > > > > > > fwnode->dev gets assigned when the device is created and cleared when
> > > > > > > > it's removed (note: note even attached to driver, just
> > > > > > > > created/removed). Seems like pretty reliable behavior to me.
> > > > > > >
> > > > > > > Yes, and even that member in fwnode is a hack in my opinion. We should not mix
> > > > > > > layers and the idea in the future to get rid of the fwnode_handle to be
> > > > > > > _embedded_ into struct device. It should be separate entity, and device
> > > > > > > instance may use it as a linked list. Currently we have a few problems because
> > > > > > > of the this design mistake.
> > > > > >
> > > > > > I don't see how this would work if fwnodes can exist before struct
> > > > > > device is even created.
> > > > >
> > > > > That's whole idea behind swnodes. They (ideally) should be created _before_
> > > > > any other object they are being used with. This is how it works today.
> > > >
> > > > Yes, this is what I meant: if fwnodes can be created before struct
> > > > device (as it is now) and their life-time is separated then how could
> > > > you possibly make the fwnode part of struct device?
> > > >
> > > > > And doing swnode->dev = ... contradicts a lot: layering, lifetime objects, etc.
> >
> > I understand what you are trying to say about layering, but there are
> > no lifetime violations here.
>
> There is. Software node is not firmware node, their lifetime is the same or
> wider than the respective device (often, they are statically defined without
> any device in mind).
>
> > > > No it doesn't. We have the software node - the template for the
> > > > device. It can only be populated with a single device entry.
> > >
> > > Which is wrong assumption. Software nodes (and firmware nodes) in general
> > > can be shared. Which device pointer you want to add there?
> >
> > I don't think this is any harder to handle than how a device's
> > secondary fwnode is handled in set_primary_fwnode(). For secondary
> > fwnodes, you just WARN and overwrite it and move on.
>
> The whole concept of a single linked list with limitation to up to two
> nodes and being the part of the struct fwnode_handle itself appears to
> be problematic. We have a lot of tricks here and there instead of properly
> having a list head in the struct device without any limitations in number
> of nodes with a priority based on the appearance in the list.
>
> For the details you may ask USB DWC3 developers and related to that.
>
> > > Which one should be next when one of the devices is gone?
> >
> > Similar to how set_primary_fwnode() handles deletion (NULL), you can
> > handle the same for when a device is removed. You can check the parent
> > or the bus for another device with the same fwnode and set it.
>
> > > No, simply no. Do not use it!
> >
> > Using fwnode_handle->dev is no different than searching a bus for a
> > device which has dev->fwnode match the fwnode you are looking for.
> >
> > In both cases, you are just going to get the first device that was
> > added. It's completely pointless to force searching a bus to find the
> > device with a specific fwnode.
> >
> > In the special cases where one fwnode has multiple devices, no generic
> > code is going to always handle the device search correctly. The
> > framework adding those devices probably knows what's the right thing
> > to do based on which of the N devices with the same fwnode they are
> > trying to find.
> >
> > I understand it's not great, but blindly saying "search the bus" isn't
> > really improving anything here and just makes things unnecessarily
> > inefficient.
>
> Is there any _good_ documentation for devlinks and all that fields in the
> struct fwnode? Why should we use that without any understanding of the
> purposes of that field. We, as device property developers, hadn't introduced
> that field and never required it. It's an alien to device properties APIs.

If I add some inline documentation for these fields, will you be more
open to letting people use this as a way to look up devices? I'm happy
to do that for you.

Thanks,
Saravana