Re: [PATCH V2 3/8] soc: qcom-geni-se: Add interconnect support to fix earlycon crash

From: Bjorn Andersson <bjorn.andersson@linaro.org>
To: Evan Green <evgreen@chromium.org>
Cc: Akash Asthana <akashast@codeaurora.org>,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	Andy Gross <agross@kernel.org>,
	wsa@the-dreams.de, Mark Brown <broonie@kernel.org>,
	Mark Rutland <mark.rutland@arm.com>,
	Rob Herring <robh+dt@kernel.org>,
	linux-i2c@vger.kernel.org, linux-spi@vger.kernel.org,
	"open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" 
	<devicetree@vger.kernel.org>, Stephen Boyd <swboyd@chromium.org>,
	Manu Gautam <mgautam@codeaurora.org>,
	linux-arm-msm <linux-arm-msm@vger.kernel.org>,
	linux-serial@vger.kernel.org,
	Doug Anderson <dianders@chromium.org>
Subject: Re: [PATCH V2 3/8] soc: qcom-geni-se: Add interconnect support to fix earlycon crash
Date: Fri, 27 Mar 2020 16:23:38 -0700	[thread overview]
Message-ID: <20200327232338.GL5063@builder> (raw)
In-Reply-To: <CAE=gft6AGkcdUAkoyevZgmtBgaiEkoQzzJcg7sYjbpy5Kh2fyA@mail.gmail.com>

On Fri 20 Mar 09:30 PDT 2020, Evan Green wrote:

> On Fri, Mar 20, 2020 at 3:22 AM Akash Asthana <akashast@codeaurora.org> wrote:
> >
> > Hi Evan, Matthias,
> >
> > On 3/20/2020 1:13 AM, Matthias Kaehlcke wrote:
> > > On Wed, Mar 18, 2020 at 02:24:35PM +0530, Akash Asthana wrote:
> > >> Hi Matthias,
> > >>
> > >> On 3/17/2020 11:59 PM, Matthias Kaehlcke wrote:
> > >>> Hi Akash,
> > >>>
> > >>> On Tue, Mar 17, 2020 at 04:27:47PM +0530, Akash Asthana wrote:
> > >>>> Hi Matthias,
> > >>>>
> > >>>> On 3/14/2020 2:14 AM, Matthias Kaehlcke wrote:
> > >>>>> Hi Akash,
> > >>>>>
> > >>>>> On Fri, Mar 13, 2020 at 06:42:09PM +0530, Akash Asthana wrote:
> > >>>>>> V1 patch@https://patchwork.kernel.org/patch/11386469/ caused SC7180 system
> > >>>>>> to reset at boot time.
> > >>>>> The v1 patch isn't relevant in the commit message, please just describe the
> > >>>>> problem. Also the crash only occurs when earlycon is used.
> > >>>> ok
> > >>>>>> As QUP core clock is shared among all the SE drivers present on particular
> > >>>>>> QUP wrapper, the reset seen is due to earlycon usage after QUP core clock
> > >>>>>> is put to 0 from other SE drivers before real console comes up.
> > >>>>>>
> > >>>>>> As earlycon can't vote for it's QUP core need, to fix this add ICC
> > >>>>>> support to common/QUP wrapper driver and put vote for QUP core from
> > >>>>>> probe on behalf of earlycon and remove vote during sys suspend.
> > >>>>> Only removing the vote on suspend isn't ideal, the system might never get
> > >>>>> suspended. That said I don't have a really good alternative suggestion.
> > >>>>>
> > >>>>> One thing you could possibly do is to launch a delayed work, check
> > >>>>> console_device() every second or so and remove the vote when it returns
> > >>>>> non-NULL. Not claiming this would be a great solution ...
> > >>>>>
> > >>>>> The cleanest solution might be a notifier when the early console is
> > >>>>> unregistered, it seems somewhat over-engineered though ... Then again
> > >>>>> other (future) uart drivers with interconnect support might run into
> > >>>>> the same problem.
> > >>>> We are hitting this problem because QUP core clocks are shared among all the
> > >>>> SE driver present in particular QUP wrapper, if other HW controllers has
> > >>>> similar architecture we will hit this issue.
> > >>>>
> > >>>> How about if we expose an API from common driver(geni-se) for putting QUP
> > >>>> core BW vote to 0.
> > >>>>
> > >>>> We call this from console probe just after uart_add_one_port call (console
> > >>>> resources are enabled as part of this call) to put core quota to 0 on behalf
> > >>>> of earlyconsole?
> > >>>   From my notes from earlier debugging I have doubts this would work:
> > >>>
> > >>>     There is a short window where the early console and the 'real' console coexist:
> > >>>
> > >>>     [    3.858122] printk: console [ttyMSM0] enabled
> > >>>     [    3.875692] printk: bootconsole [qcom_geni0] disabled
> > >>>
> > >>>     The reset probably occurs when the early console tries to write, but the ICC
> > >>>     is effectively disabled because ttyMSM0 and the other geni ports are runtime
> > >>>     suspended.
> > >> Code flow from console driver probe(qcom_geni_serial.c)
> > >>
> > >> uart_add_one_port--->uart_configure_port--->{ 1) uart_change_pm(enable
> > >> console resources)  2)register_console(boot to real console switch happens
> > >> here)}
> > >>
> > >> Console resources are not disabled from anywhere before the switch happens
> > >> completely. I meant to say until we saw below logs.
> > >>
> > >> [    3.875692] printk: bootconsole [qcom_geni0] disabled
> > >>
> > >> I think the board reset issue cannot occur during the window where early
> > >> console and 'real' console coexist.
> > > Thanks for the clarification! Indeed my notes were only a hypothesis, I
> > > don't see evidence that there is an actual downvote shortly after console
> > > registration.
> > >
> > >> I have validated proposed solution by me, it is working fine.
> > >>
> > >> Currently voting is done for every QUP and not only to which earlycon is
> > >> connect, with the above approach we can't remove vote from other QUPs.
> > >>
> > >> However we can limit voting only to earlycon QUP by removing interconnect
> > >> from DT node of other QUPs.
> > >>
> > >> I am not sure how clean is this solution.
> > > I'm more inclined towards a solution along the lines of what Evan
> > > proposed, i.e. delaying the votes (either in geni or ICC) until we
> > > are ready.
> >
> > Based on discussion I think the delayed solution is most suited if
> > implemented in ICC core because other ICC client might face the similar
> > problem.
> >
> > However for geni case I am more inclined towards below proposed solution.
> >
> > -----------------------------------------------------------------------------------------------------
> >
> > How about if we expose an API from common driver(geni-se) for putting QUP
> > core BW vote to 0.
> >
> > We call this from console probe just after uart_add_one_port call (console
> > resources are enabled as part of this call) to put core quota to 0 on behalf
> > of earlyconsole?
> 
> This seems ok to me. Earlycon sets up a vote, and then real probe
> tears it down. As long as in the shuffle of all of these things into
> SE library helpers you still have a way of differentiating the
> earlycon vote from the real vote.

Note though that the boot console will outlive the real console when the
kernel is booted with "keep_bootcon" on the command line (something I do
from time to time).

So rather than relying on "real probe" to signal when we can release the
earlycon's icc vote I think we should specify dev->con->exit in
qcom_geni_serial_earlycon_setup(), so that it will signal when the
earlycon actually goes away - and until that point the clocks should
just be on.

> In other words, don't reuse this
> early icc_path for the real UART vote. You should probably also
> destroy the path once you've voted zero on it.

Maintaining the early and real votes completely separate sounds like a
sure way to keep this sane; and there's no drawback on having multiple
votes for the same thing and rely on the framework to keep track of the
various users.

Regards,
Bjorn