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

[Laurent Pinchart <laurent.pinchart@ideasonboard.com>]

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.

-- 
Regards,

Laurent Pinchart