Message-ID: <674470dc.5d0a0220.119e75.b012@mx.google.com>
Date: Mon, 25 Nov 2024 13:43:06 +0100
From: Christian Marangi <ansuelsmth@...il.com>
To: Viresh Kumar <viresh.kumar@...aro.org>
Cc: "Rafael J. Wysocki" <rafael@...nel.org>, linux-kernel@...r.kernel.org,
	linux-pm@...r.kernel.org, Lorenzo Bianconi <lorenzo@...nel.org>,
	upstream@...oha.com, ulf.hansson@...aro.org
Subject: Re: [PATCH v2] cpufreq: airoha: Add EN7581 Cpufreq SMC driver

On Tue, Nov 19, 2024 at 04:14:21PM +0530, Viresh Kumar wrote:
> On 19-11-24, 10:04, Christian Marangi wrote:
> > On Tue, Nov 19, 2024 at 12:50:54PM +0530, Viresh Kumar wrote:
> > > On 17-10-24, 21:07, Christian Marangi wrote:
> > > > Add simple Cpufreq driver for Airoha EN7581 SoC that control CPU
> > > > frequency scaling with SMC APIs.
> > > > 
> > > > All CPU share the same frequency and can't be controlled independently.
> > > > Current shared CPU frequency is returned by the related SMC command.
> > > > 
> > > > Add SoC compatible to cpufreq-dt-plat block list as a dedicated cpufreq
> > > > driver is needed with OPP v2 nodes declared in DTS.
> > > > 
> > > > Signed-off-by: Christian Marangi <ansuelsmth@...il.com>
> > > > ---
> > > > Changes v2:
> > > > - Fix kernel bot error with missing slab.h and bitfield.h header
> > > > - Limit COMPILE_TEST to ARM64 due to smcc 1.2
> > > 
> > > Hi,
> > > 
> > > Sorry for delay at my side to review this driver.
> > > 
> > > Now that I looked at it, I don't see a lot of special stuff happening in the
> > > driver. There are many other platforms with similar situation. What we have done
> > > for all them, which rely on OPPs coming from DT, is to add a clk for the CPUs
> > > and do all this magically smcc stuff from clk_get_rate() and clk_set_rate().
> > > Once that is done, you should be able to reuse the cpufreq-dt driver as is.
> > > 
> > > So a CPU clk is the only missing thing in your case I guess.
> > >
> > 
> > Hi Viresh,
> > 
> > thanks a lot for the follow-up. I will see what I can do, 2 main problem
> > I see is that, contrary to other driver, for this Airoha SoC, there are
> > no parents or no clock to enable... It's really just entirely handled by
> > ATF and smccc call.
> > 
> > And also the SMCCC requires an index and not the clock itself. This was
> > handy for a cpufreq driver as it passed the OPP index
> 
> Right, but the OPP table for the CPU must contain frequencies too, isn't it ? So
> you already have index to frequency conversion available ?
> 
> Can't you just add a clk driver for the CPU, which uses OPP core to parse the
> OPP table of the CPU and set a clk-index table in the clk driver ? So once the
> clk driver gets a request for a particular frequency, it just finds the
> respective index and sets it ?
> 
> > problematic for a
> > clock driver as set_rate pass the clock. So I guess I will have to
> > define the OPP phandle also in the clock node struct. (and map it?)
> 
> I am not suggesting a clk in DT, but just in code, added by a cpufreq driver for
> your platform, which at the end creates cpufreq-dt device. There are many which
> are creating the device on the fly, like tegra20-cpufreq.c.
> 
> > The main problem in doing that is the performance hit on having to cycle
> > every time the OPPs to find the correct index...
> 
> I think it would be traversing an array in the clk driver eventually and that
> won't be that bad ?
> 
> > (yes they really implemented this thing with the ATF specifically with
> > the cpufreq scenario in mind)
>

Hi Viresh,

sorry for the delay... I checked the example and other cpufreq driver
that register a simple cpufreq-dt. None of the current driver implements
a full clk provider internally and I have some fear it might be
problematic to have mixed stuff, eventually I feel I should implement a
small clk driver that implements determine_rate, set_rate, a compatible
and all sort. And still we would have the double reference of OPP
Index->Freq Clock->OPP index.

I wonder if a much easier and better solution for this is (similar to
how we do with suspend and resume) add entry in the struct
cpufreq_dt_platform_data, to permit to define simple .target_index and
.get and overwrite the default one cpufreq-dt.

This permits to both reduce greatly the airoha-cpufreq driver, register
a simple cpufreq-dt and prevent any kind of overhead. After all the
.target_index and .get doesn't do anything fancy, they just call the OPP
set and clk get rate.

What do you think? Changes are really trivial since this is already
implemented for suspend and resume.

-- 
	Ansuel

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