linux-kernel - Re: [PATCH 1/2] firmware: arm_scmi: Fix frequency truncation by promoting multiplier to u64

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Date:   Fri, 1 Dec 2023 16:17:56 +0000
From:   Cristian Marussi <cristian.marussi@....com>
To:     Sudeep Holla <sudeep.holla@....com>
Cc:     linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        quic_mdtipton@...cinc.com, quic_asartor@...cinc.com,
        quic_lingutla@...cinc.com, Sibi Sankar <quic_sibis@...cinc.com>,
        linux-arm-msm@...r.kernel.org
Subject: Re: [PATCH 1/2] firmware: arm_scmi: Fix frequency truncation by
 promoting multiplier to u64

On Fri, Dec 01, 2023 at 02:39:35PM +0000, Sudeep Holla wrote:
> On Thu, Nov 30, 2023 at 08:43:42PM +0000, Sudeep Holla wrote:
> > Fix the frequency truncation for all values equal to or greater 4GHz by
> > updating the multiplier 'mult_factor' to u64 type. It is also possible
> > that the multiplier itself can be greater than or equal to 2^32. So we need
> > to also fix the equation computing the value of the multiplier.
> > 
> > Fixes: a9e3fbfaa0ff ("firmware: arm_scmi: add initial support for performance protocol")
> > Reported-by: Sibi Sankar <quic_sibis@...cinc.com>
> > Closes: https://lore.kernel.org/all/20231129065748.19871-3-quic_sibis@quicinc.com/
> > Cc: Cristian Marussi <cristian.marussi@....com>
> > Signed-off-by: Sudeep Holla <sudeep.holla@....com>
> > ---
> >  drivers/firmware/arm_scmi/perf.c | 6 +++---
> >  1 file changed, 3 insertions(+), 3 deletions(-)
> > 
> > diff --git a/drivers/firmware/arm_scmi/perf.c b/drivers/firmware/arm_scmi/perf.c
> > index 81dd5c5e5533..8ce449922e55 100644
> > --- a/drivers/firmware/arm_scmi/perf.c
> > +++ b/drivers/firmware/arm_scmi/perf.c
> > @@ -152,7 +152,7 @@ struct perf_dom_info {
> >  	u32 opp_count;
> >  	u32 sustained_freq_khz;
> >  	u32 sustained_perf_level;
> > -	u32 mult_factor;
> > +	u64 mult_factor;
> 
> I have now changed this to unsigned long instead of u64 to fix the 32-bit
> build failure[1].

Right, I was caught a few times too by this kind of failures on v7 :D

... but this 32bit issue makes me wonder what to do in such a case...

...I mean, on 32bit if the calculated freq oveflows, there is just
nothing we can do on v7 without overcomplicating the code...but I suppose
it is unplausible to have such high freq on a v7... as a palliative I can
only think of some sort of overflow check (only on v7) that could trigger
a warning ... but it is hardly worth the effort probably..

Thanks,
Cristian