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

From: Philipp Zabel <p.zabel@pengutronix.de>
To: Jerome Brunet <jbrunet@baylibre.com>,
	Dan Robertson <dan@dlrobertson.com>,
	 Martin Blumenstingl <martin.blumenstingl@googlemail.com>,
	Neil Armstrong <narmstrong@baylibre.com>,
	Kevin Hilman <khilman@baylibre.com>
Cc: linux-amlogic@lists.infradead.org, linux-usb@vger.kernel.org,
	aouledameur@baylibre.com, linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH 1/1] usb: dwc3: meson-g12a: fix shared reset control use
Date: Tue, 25 Aug 2020 12:20:16 +0200	[thread overview]
Message-ID: <18105405541cbc32cecaff181e1427f5434fafc3.camel@pengutronix.de> (raw)
In-Reply-To: <1j8se43yrw.fsf@starbuckisacylon.baylibre.com>

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).

> 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).

> 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)?

regards
Philipp

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