linux-kernel - Re: [PATCH v3 3/4] thermal/core: Build ascending ordered indexes for the trip points

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Message-ID: <e3bfb483aa6ce09756b38b3a568fac819b154beb.camel@intel.com>
Date:   Tue, 19 Jul 2022 22:17:20 +0800
From:   Zhang Rui <rui.zhang@...el.com>
To:     Daniel Lezcano <daniel.lezcano@...aro.org>, rafael@...nel.org
Cc:     quic_manafm@...cinc.com, amitk@...nel.org, lukasz.luba@....com,
        linux-pm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 3/4] thermal/core: Build ascending ordered indexes
 for the trip points

On Tue, 2022-07-19 at 09:22 +0200, Daniel Lezcano wrote:
> On 19/07/2022 03:14, Zhang Rui wrote:
> > On Mon, 2022-07-18 at 15:21 +0200, Daniel Lezcano wrote:
> > > 
> > > Hi Zhang,
> > > 
> > > thanks for the review
> > > 
> > > On 18/07/2022 07:28, Zhang Rui wrote:
> > > > On Fri, 2022-07-15 at 23:09 +0200, Daniel Lezcano wrote:
> > > 
> > > [ ... ]
> > > 
> > > > > Instead of taking the risk of breaking the existing
> > > > > platforms,
> > > > > use an
> > > > > array of temperature ordered trip identifiers and make it
> > > > > available
> > > > > for the code needing to browse the trip points in an ordered
> > > > > way.
> > > > > 
> > > > > Signed-off-by: Daniel Lezcano <daniel.lezcano@...aro.org>
> > > > > ---
> > > 
> > > [ ... ]
> > > 
> > > > > +static void sort_trips_indexes(struct thermal_zone_device
> > > > > *tz)
> > > > > +{
> > > > > +       int i, j;
> > > > > +
> > > > > +       for (i = 0; i < tz->trips; i++)
> > > > > +               tz->trips_indexes[i] = i;
> > > > > +
> > > > > +       for (i = 0; i < tz->trips; i++) {
> > > > > +               for (j = i + 1; j < tz->trips; j++) {
> > > > > +                       int t1, t2;
> > > > > +
> > > > > +                       tz->ops->get_trip_temp(tz, tz-
> > > > > > trips_indexes[i], &t1);
> > > > 
> > > > This line can be moved to the upper loop.
> > > 
> > > Right, thanks!
> > > 
> > > > > +                       tz->ops->get_trip_temp(tz, tz-
> > > > > > trips_indexes[j], &t2);
> > > > > +
> > > > 
> > > > what about the disabled trip points?
> > > > 
> > > > we should ignore those trip points and check the return value
> > > > to
> > > > make
> > > > sure we're comparing the valid trip_temp values.
> > > 
> > > We don't have to care about, whatever the position, the
> > > corresponding
> > > trip id will be disabled by the trip init function before calling
> > > this
> > > one and ignored in the handle_thermal_trip() function
> > 
> > hah, I missed this one and replied to your latest reply directly.
> > 
> > The thing I'm concerning is that if we don't check the return
> > value,
> > for a disabled trip point, the trip_temp (t1/t2) returned is some
> > random value, it all depends on the previous value set by last
> > successful .get_trip_temp(), and this may screw up the sorting.
> 
> The indexes array is the same size as the trip array, that makes the 
> code much less prone to errors.
> 
> To have the same number of trip points, the index of the disabled
> trip 
> must be inserted also in the array. We don't care about its position
> in 
> the indexes array because it is discarded in the handle_trip_point() 
> function anyway. For this reason, the random temperature of the
> disabled 
> trip point and the resulting position in the sorting is harmless.
> 
> It is made on purpose to ignore the return value, so we have a
> simpler code.
> 
Let's take below case for example,
say, we have three trip points 0, 1, 2, and trip point 1 is broken and
disabled.

trip temp for trip point 0 is 10 and for trip point 2 is 20.
.get_trip_temp(tz, 1, &t) fails, and t is an uninitialized random value


Initial:
   trip_indexes[0]=0,trip_indexes[1]=1,trip_indexes[2]=2
step1:
   i=0,j=1
   get trip temp for trip point trip_indexes[0]=0 and trip_indexes[1]=1
   trip point 1 returns trip temp 5, and it swaps with trip point 0
   so
   trip_indexes[0]=1,trip_indexes[1]=0,trip_indexes[2]=2
step2:
   i=0,j=2
   get trip temp for trip point trip_indexes[0]=1 and trip_indexes[2]=2
   trip point 1 returns trip temp 25, and it swaps with trip point 2
   so
   trip_indexes[0]=2,trip_indexes[1]=0,trip_indexes[2]=1

And the sorting is broken now.

please correct me if I'm missing anything.

thanks,
rui