Re: [PATCH 0/3] Implement role-switch notifications from dwc3-drd to dwc3-qcom

[Bjorn Andersson <bjorn.andersson@linaro.org>]

On Wed 25 Aug 09:43 PDT 2021, Heikki Krogerus wrote:

> On Wed, Aug 25, 2021 at 08:53:29AM -0700, Bjorn Andersson wrote:
> > On Wed 25 Aug 01:18 PDT 2021, Bryan O'Donoghue wrote:
> > 
> > > On 25/08/2021 06:51, Felipe Balbi wrote:
> > > > > Hi Peter, I took a proper look at this again, hoping to find a way to
> > > > > pass a callback pointer from dwc3-qcom to the dwc3 core, that can be
> > > > > called from __dwc3_set_mode() to inform the Qualcomm glue about mode
> > > > > changes.
> > > > I would rather keep the strict separation between glue and core.
> > > 
> > > # 1 __dwc3_set_mode
> > > Felipe wants to keep a strict separation between core and glue
> > > 
> > > # notifier
> > > Requires the core probe() to complete before the glue probe to work
> > > reliably. This then would lead us to remaking the dwc3-qcom::probe() to
> > > facilitate probe deferral.
> > > 
> > > We can be sure bugs would be introduced in this process.
> > > 
> > > AFAIK Felipe is not opposed to this, Bjorn likes it
> 
> Notifiers were proposed for the USB role switches already some time
> ago [1], and I don't think anybody was against them, but in the end I
> don't think there were any users for those notifier, so they were
> never added.
> 
> If something needs to only react to the role changes like I think in
> this case, then I would just add those notifiers to the USB role
> switches.
> 
> [1] https://lore.kernel.org/linux-usb/20191002231617.3670-3-john.stultz@linaro.org/
> 

Afaict this would end up pretty much identical to the notification chain
that Bryan proposed earlier; the dwc3 drd code registers a
usb_role_switch and the glue code somehow needs to get hold of that
resource to register the notification.

But the glue code has no way to know when the core/drd code is done
registering, so it has no way to know when there is a notification chain
to register with.

Regards,
Bjorn