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

[Jerome Brunet]

On Wed 26 Aug 2020 at 10:14, Philipp Zabel <p.zabel at pengutronix.de> wrote:

> 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.

We'll pick something for the v1 ... maybe the inspiration will come
later on and we'll make a v2 ;)

>
>> > > 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 :)

will do

>
>> > > 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?

Yes, reset is shared by the different usb devices of the SoCs

> Will you change them to use the new API as well?

That's the plan, yes.

>
> regards
> Philipp