Re: [PATCH v4 1/2] dt-bindings: usb: Add binding for discrete onboard USB hubs

[Matthias Kaehlcke]

On Sat, Oct 03, 2020 at 08:41:42AM -0400, Alan Stern wrote:
> On Fri, Oct 02, 2020 at 05:58:22PM -0500, Rob Herring wrote:
> > On Fri, Oct 2, 2020 at 1:36 PM Alan Stern <stern@xxxxxxxxxxxxxxxxxxx> wrote:
> > > Regardless of how the situation is represented in DT, there remains the
> > > issue of where (i.e., in which driver module) the appropriate code
> > > belongs.  This goes far beyond USB.  In general, what happens when one
> > > sort of device normally isn't hooked up through a power regulator, so
> > > its driver doesn't have any code to enable a regulator, but then some
> > > system does exactly that?
> > >
> > > Even worse, what if the device is on a discoverable bus, so the driver
> > > doesn't get invoked at all until the device is discovered, but on the
> > > new system it can't be discovered until the regulator is enabled?
> > 
> > Yep, it's the same issue here with USB, MDIO which just came up a few
> > weeks ago, MMC/SD which hacked around it with 'mmc-pwrseq' binding
> > (not something I want to duplicate) and every other discoverable bus.
> > What do they all have in common? The kernel's driver model being
> > unable to cope with this situation. We really need a common solution
> > here and not bus or device specific hack-arounds.
> 
> To me this doesn't seem quite so much to be a weakness of the kernel's 
> driver model.
> 
> It's a platform-specific property, one that is not discoverable and 
> therefore needs to be represented somehow in DT or ACPI or something 
> similar.  Something that says "Device A cannot operate or be discovered 
> until power regulator B is enabled", for example.
> 
> The decision to enable the power regulator at system startup would be 
> kernel policy, not a part of the DT description.  But there ought to be 
> a standard way of recognizing which resource requirements of this sort 
> should be handled at startup.  Then there could be a special module (in 
> the driver model core? -- that doesn't really seem appropriate) which 
> would search through the whole DT database for resources of this kind 
> and enable them.

This might work for some cases that only have a single resource or multiple
resources but no timing/sequencing requirements. For the more complex cases
it would probably end up in something similar to the pwrseq series
(https://lore.kernel.org/patchwork/project/lkml/list/?series=314989&state=%2A&archive=both),
which was nack-ed by Rafael, Rob also expressed he didn't want to go
down that road.

It seems to me that initialization of the resources needs to be done by
the/a driver for the device, which knows about the sequencing requirements.
Potentially this could be done in a pre-probe function that you brought up
earlier.