RE: [PATCH 3/5] drivers: clk-qoriq: Add clockgen support for lx2160a

[Andy Tang]

Hi Scott,

Please see my replay in line.

> -----Original Message-----
> From: Linuxppc-dev
> <linuxppc-dev-bounces+b29983=freescale.com@xxxxxxxxxxxxxxxx> On
> Behalf Of Scott Wood
> Sent: 2018å9æ1æ 4:29
> To: Andy Tang <andy.tang@xxxxxxx>; Vabhav Sharma
> <vabhav.sharma@xxxxxxx>; linux-kernel@xxxxxxxxxxxxxxx;
> devicetree@xxxxxxxxxxxxxxx; robh+dt@xxxxxxxxxx;
> mark.rutland@xxxxxxx; linuxppc-dev@xxxxxxxxxxxxxxxx;
> linux-arm-kernel@xxxxxxxxxxxxxxxxxxx; mturquette@xxxxxxxxxxxx;
> sboyd@xxxxxxxxxx; rjw@xxxxxxxxxxxxx; viresh.kumar@xxxxxxxxxx;
> linux-clk@xxxxxxxxxxxxxxx; linux-pm@xxxxxxxxxxxxxxx;
> linux-kernel-owner@xxxxxxxxxxxxxxx; catalin.marinas@xxxxxxx;
> will.deacon@xxxxxxx; gregkh@xxxxxxxxxxxxxxxxxxx; arnd@xxxxxxxx;
> kstewart@xxxxxxxxxxxxxxxxxxx; yamada.masahiro@xxxxxxxxxxxxx
> Cc: Yogesh Narayan Gaur <yogeshnarayan.gaur@xxxxxxx>;
> linux@xxxxxxxxxxxxxxx; Udit Kumar <udit.kumar@xxxxxxx>; Varun Sethi
> <V.Sethi@xxxxxxx>
> Subject: Re: [PATCH 3/5] drivers: clk-qoriq: Add clockgen support for
> lx2160a
> 
> On Fri, 2018-08-31 at 06:12 +0000, Andy Tang wrote:
> > Hi Scott,
> >
> > Please see my replay inline.
> >
> > > -----Original Message-----
> > > From: linux-arm-kernel
> > > <linux-arm-kernel-bounces@xxxxxxxxxxxxxxxxxxx>
> > > On Behalf Of Scott Wood
> > > Sent: 2018å8æ31æ 1:43
> > > To: Vabhav Sharma <vabhav.sharma@xxxxxxx>;
> > > linux-kernel@xxxxxxxxxxxxxxx; devicetree@xxxxxxxxxxxxxxx;
> > > robh+dt@xxxxxxxxxx; mark.rutland@xxxxxxx;
> > > linuxppc-dev@xxxxxxxxxxxxxxxx; linux-arm-kernel@xxxxxxxxxxxxxxxxxxx;
> > > mturquette@xxxxxxxxxxxx; sboyd@xxxxxxxxxx; rjw@xxxxxxxxxxxxx;
> > > viresh.kumar@xxxxxxxxxx; linux-clk@xxxxxxxxxxxxxxx;
> > > linux-pm@xxxxxxxxxxxxxxx; linux-kernel-owner@xxxxxxxxxxxxxxx;
> > > catalin.marinas@xxxxxxx; will.deacon@xxxxxxx;
> > > gregkh@xxxxxxxxxxxxxxxxxxx; arnd@xxxxxxxx;
> > > kstewart@xxxxxxxxxxxxxxxxxxx; yamada.masahiro@xxxxxxxxxxxxx
> > > Cc: Yogesh Narayan Gaur <yogeshnarayan.gaur@xxxxxxx>; Andy Tang
> > > <andy.tang@xxxxxxx>; linux@xxxxxxxxxxxxxxx; Varun Sethi
> > > <V.Sethi@xxxxxxx>; Udit Kumar <udit.kumar@xxxxxxx>
> > > Subject: Re: [PATCH 3/5] drivers: clk-qoriq: Add clockgen support
> > > for lx2160a
> > >
> > > On Thu, 2018-08-30 at 12:39 -0500, Scott Wood wrote:
> > > > On Thu, 2018-08-30 at 07:36 +0000, Vabhav Sharma wrote:
> > > > > >
> > > > > > Why are you increasing NUM_CMUX beyond 8 for a chip that
> only
> > >
> > > has
> > > > > > 8 entries in cmux_to_group?
> > > > >
> > > > > Configuration is 16 cores,8 cluster with 2 cores in each cluster
> > > >
> > > > So?  This is about cmuxes, not cores.  You're increasing the array
> > > > without ever using the new size.
> > >
> > > Oh, and you also broke p4080 which has 8 cmuxes but no -1
> > > terminator, because the array was of length 8.  Probably the array
> > > should be changed to NUM_CMUX+1 so every array can be -1
> terminated.
> > >
> >
> > [Andy] How about we add -1 terminator to p4080 and increase
> NUM_CMUX to 16?
> 
> Why 16?  What does such a change have to do with this chip, which
> according to the rest of the patch has 8 cmuxes?
[Andy] NUM_CMUX is a limitation number. We better give it an extra buffer, not exactly equal to the limitation.
16 is the limitation number with extra buffer.

> 
> > We don't want to increase NUM_CMUX each time new soc with more
> cmuxes added.
> 
> You don't want to have to make a trivial change each time you exceed a
> limit that has yet to be exceeded once since NUM_CMUX was added?
> This isn't ABI or in any other way hard to change.  It's right in the same file
> as the chip description you'd be adding.
> 
> And even if a chip did come along with 16 cmuxes, you'd then need to
> increase the array to 17 to hold the -1 if you don't want to leave a situation
> like the
> p4080 is in now, where a chip's cmux array could be broken by increasing
> NUM_CMUX further.
> 
[Andy] Adding buffer to a limitation number is always a good habit when coding. We often forget to increase this value when
a new chip with more cmuxes added. Like this patch, we didn't increase this value at first. We spent a lot of time finding out that NUM_CMUX needs to be increased too.
It is a personal preference how to set this value. I think it is better to increase it to 16, not NUM_CMUX+1 as long as we fix the P4080 issue
even though it is a trivial change. And I agree the description needs to be updated.

BR,
Andy

> -Scott