[PATCH] drm/bridge: ti-sn65dsi86: Fix multiple instances

[Geert Uytterhoeven]

Hi Greg,

On Mon, Oct 21, 2024 at 8:39 AM Greg KH <gregkh at linuxfoundation.org> wrote:
> On Sun, Oct 20, 2024 at 05:36:29PM +0300, Laurent Pinchart wrote:
> > On Fri, Oct 18, 2024 at 04:31:21PM +0200, Greg KH wrote:
> > > On Fri, Oct 18, 2024 at 05:25:22PM +0300, Laurent Pinchart wrote:
> > > > On Fri, Oct 18, 2024 at 04:09:26PM +0200, Greg KH wrote:
> > > > > On Fri, Oct 18, 2024 at 03:36:48PM +0200, Geert Uytterhoeven wrote:
> > > > > > On Fri, Oct 18, 2024 at 3:10 PM Laurent Pinchart wrote:
> > > > > > > On Fri, Oct 18, 2024 at 09:45:52AM +0200, Geert Uytterhoeven wrote:
> > > > > > > > Each bridge instance creates up to four auxiliary devices with different
> > > > > > > > names.  However, their IDs are always zero, causing duplicate filename
> > > > > > > > errors when a system has multiple bridges:
> > > > > > > >
> > > > > > > >     sysfs: cannot create duplicate filename '/bus/auxiliary/devices/ti_sn65dsi86.gpio.0'
> > > > > > > >
> > > > > > > > Fix this by using a unique instance ID per bridge instance.
> > > > > > >
> > > > > > > Isn't this something that should be handled by the AUX core ? The code
> > > > > > > below would otherwise need to be duplicated by all drivers, which seems
> > > > > > > a burden we should avoid.
> > > > > >
> > > > > > According to the documentation, this is the responsibility of the caller
> > > > > > https://elixir.bootlin.com/linux/v6.11.4/source/include/linux/auxiliary_bus.h#L81
> > > > > > I believe this is the same for platform devices.
> > > > > > See also the example at
> > > > > > https://elixir.bootlin.com/linux/v6.11.4/source/include/linux/auxiliary_bus.h#L116
> > > > > >
> > > > > > Note: the platform bus supports PLATFORM_DEVID_AUTO, but the auxiliary
> > > > > > bus does not.
> > > > >
> > > > > Yes, it does not as it's up to the caller to create a unique name, like
> > > > > your patch here does.  I'd argue that platform should also not do
> > > > > automatic device ids, but that's a different argument :)
> > > >
> > > > __auxiliary_device_add() creates the device name with
> > > >
> > > >   dev_set_name(dev, "%s.%s.%d", modname, auxdev->name, auxdev->id);
> > > >
> > > > I'm not calling for a PLATFORM_DEVID_AUTO-like feature here, but
> > > > shouldn't the first component of the device name use the parent's name
> > > > instead of the module name ?
> > >
> > > Why would the parent's name not be the module name?  That name is
> > > guaranteed unique in the system.  If you want "uniqueness" within the
> > > driver/module, use the name and id field please.
> > >
> > > That's worked well so far, but to be fair, aux devices are pretty new.
> > > What problem is this naming scheme causing?
> >
> > Auxiliary devices are created as children of a parent device. When
> > multiple instances of the same parent type exist, this will be reflected
> > in the /sys/devices/ devices tree hierarchy without any issue. The
> > problem comes from the fact the the auxiliary devices need a unique name
> > for /sys/bus/auxialiary/devices/, where we somehow have to differenciate
> > devices of identical types.
> >
> > Essentially, we're trying to summarize a whole hierarchy (path in
> > /sys/devices/) into a single string. There are different ways to solve
> > this. For platform devices, we use a device ID. For I2C devices, we use
> > the parent's bus number. Other buses use different schemes.
> >
> > Geert's patch implements a mechanism in the ti-sn65dsi86 driver to
> > handle this, and assign an id managed by the parent. In a sense we could
> > consider this to be similar to what is done for I2C, where the bus
> > number is also a property of the parent. However, the big difference is
> > that the I2C bus number is managed by the I2C subsystem, while here the
> > id is managed by the ti-sn65dsi86 driver, not by the auxiliary device
> > core. This would require duplicating the same mechanism in every single
> > driver creating auxiliary devices. This strikes me as a fairly bad idea.
> > The problem should be solved by the core, not by individual drivers.
>
> The "id" is just a unique number, it is "managed" by the thing that is
> creating the devices themselves, not the aux core code.  I don't see why
> the i2c bus number has to match the same number that the ti driver
> creates, it could be anything, as long as it doesn't match anything else
> currently created by that driver.

Laurent does not say it has to match the i2c bus number.
He does think the auxilliary bus should provide a mechanism to
allocate these IDs (e.g. usin g AUX_DEVID_AUTO?).

However, using i2c_client->adapter->nr instead of ida_alloc()
in the TI driver does sound like a good idea to me...

> If we had the aux core code create the id, it would just use a unique
> counter, and that would not reflect any mapping to anything, so I don't
> see how that is any different here.

And then we would get something like:

/sys/bus/auxiliary/devices
├── ti_sn65dsi86.gpio.0
├── ti_sn65dsi86.pwm.1
├── ti_sn65dsi86.aux.2
├── ti_sn65dsi86.bridge.3
├── ti_sn65dsi86.gpio.4
├── ti_sn65dsi86.pwm.5
├── ti_sn65dsi86.aux.6
└── ti_sn65dsi86.bridge.7

Which is similar to the first approach I tried (calling ida_alloc() in
ti_sn65dsi86_add_aux_device() instead of ti_sn65dsi86_probe()).

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert at linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds