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[82.243.161.21]) by smtp.gmail.com with ESMTPSA id fx15sm13847072ejb.1.2020.08.25.07.21.00 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 25 Aug 2020 07:21:00 -0700 (PDT) References: <20200713160522.19345-1-dan@dlrobertson.com> <20200713160522.19345-2-dan@dlrobertson.com> <1jy2maekzf.fsf@starbuckisacylon.baylibre.com> <1j8se43yrw.fsf@starbuckisacylon.baylibre.com> <18105405541cbc32cecaff181e1427f5434fafc3.camel@pengutronix.de> User-agent: mu4e 1.3.3; emacs 26.3 From: Jerome Brunet To: Philipp Zabel , Dan Robertson , Martin Blumenstingl , Neil Armstrong , Kevin Hilman Cc: linux-amlogic@lists.infradead.org, linux-usb@vger.kernel.org, linux-arm-kernel@lists.infradead.org, aouledameur@baylibre.com Subject: Re: [PATCH 1/1] usb: dwc3: meson-g12a: fix shared reset control use In-reply-to: <18105405541cbc32cecaff181e1427f5434fafc3.camel@pengutronix.de> Date: Tue, 25 Aug 2020 16:20:59 +0200 Message-ID: <1j5z964xis.fsf@starbuckisacylon.baylibre.com> MIME-Version: 1.0 Content-Type: text/plain Sender: linux-usb-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-usb@vger.kernel.org On Tue 25 Aug 2020 at 12:20, Philipp Zabel 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 > >> 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 ? 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 > >> 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. > > regards > Philipp