[PATCH 1/1] usb: dwc3: meson-g12a: fix shared reset control use

[Philipp Zabel]

On Tue, 2020-08-25 at 16:20 +0200, Jerome Brunet wrote:
> On Tue 25 Aug 2020 at 12:20, Philipp Zabel <p.zabel at pengutronix.de> wrote:
> 
> > On Mon, 2020-08-24 at 16:26 +0200, Jerome Brunet wrote:
> > [...]
> > > In practice, I think your proposition would work since the drivers
> > > sharing this USB reset line are likely to be probed/suspended/resumed at
> > > the same time but ...
> > > 
> > > If we imagine a situation where 2 device share a reset line, 1 go in
> > > suspend, the other does not - if the first device as control of the
> > > reset, it could trigger it and break the 2nd device. Same goes for
> > > probe/remove()
> > > 
> > > I agree it could be seen as unlikely but leaving such race condition
> > > open looks dangerous to me.
> > 
> > You are right, this is not good enough.
> > 
> > > > Is this something that would be feasible for your combination of
> > > > drivers? Otherwise it is be unclear to me under which condition a driver
> > > > should be allowed to call the proposed reset_control_clear().
> > > 
> > > I was thinking of reset_control_clear() as the counter part of
> > > reset_control_reset().
> > 
> > I'm not particularly fond of reset_control_clear as a name, because it
> > is very close to "clearing a reset bit" which usually would deassert a
> > reset line (or the inverse).
> 
> It was merely a suggestion :) any other name you prefer is fine by me

Naming is hard. All metaphors I can think of are either a obscure or
clash with other current usage. My instinct would be to call this
"resetting the (reset) control", but _reset() is already taken, with the
opposite meaning. How about _rewind() or _rearm()? Not sure if those are
good metaphors either, but at least there is no obvious but incorrect
interpretation. I kind of wish reset_control_reset() would be called
reset_control_trigger() instead.

> > > When a reset_control_reset() has been called once, "triggered_count" is
> > > incremented which signals that the ressource under the reset is
> > > "in_use" and the reset should not be done again.
> > 
> > "triggered_count" would then have to be renamed to something like
> > "trigger_requested_count", or "use_count". I wonder it might be possible
> > to merge this with "deassert_count" as they'd share the same semantics
> > (while the count is > 0, the reset line must stay deasserted).
> 
> Sure. Could investigate this as a 2nd step ?

Yes.

> I'd like to bring a solution for our meson-usb use case quickly - even
> with the revert suggested, we are having an ugly warning around suspend

I understand. Let's still do this carefully :)

> > > reset_control_clear()
> > > would be the way to state that the ressource is no longer used and, that
> > > from the caller perspective, the reset can fired again if necessary.
> > > 
> > > If we take the probe / suspend / resume example:
> > > * 1st device using the shared will actually trigger it (as it is now)
> > > * following device just increase triggered_count
> > > 
> > > If all devices go to suspend (calling reset_control_clear()) then
> > > triggered_count will reach zero, allowing the first device resuming to
> > > trigger the reset again ... this is important since it might not be the
> > > one which would have got the exclusive control
> > > 
> > > If any device don't go to suspend, meaning the ressource under reset
> > > keep on being used, no reset will performed. With exlusive control,
> > > there is a risk that the resuming device resets something already in use.
> > > 
> > > Regarding the condition, on shared resets, call reset_control_reset()
> > > should be balanced reset_control_clear() - no clear before reset.
> > 
> > Martin, is this something that would be useful for the current users of
> > the shared reset trigger functionality (phy-meson-gxl-usb2 and phy-
> > meson8b-usb2 with reset-meson)?
> 
> I'm not Martin but these devices are the origin of the request/suggestion.

So these two phy drivers are used together with dwc3-meson-g12a?
Will you change them to use the new API as well?

regards
Philipp