linux-kernel - Re: [PATCH] PM / OPP: fix debugfs files for 64-bit

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Date:	Mon, 19 Oct 2015 08:40:09 -0700
From:	Michael Turquette <mturquette@...libre.com>
To:	Viresh Kumar <viresh.kumar@...aro.org>,
	"Arnd Bergmann" <arnd@...db.de>
Cc:	"Greg Kroah-Hartman" <gregkh@...uxfoundation.org>,
	"Stephen Boyd" <sboyd@...eaurora.org>,
	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	"Len Brown" <len.brown@...el.com>, "Pavel Machek" <pavel@....cz>,
	"Bartlomiej Zolnierkiewicz" <b.zolnierkie@...sung.com>,
	"Nishanth Menon" <nm@...com>,
	"Dmitry Torokhov" <dtor@...omium.org>, linux-pm@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH] PM / OPP: fix debugfs files for 64-bit

Quoting Viresh Kumar (2015-10-08 00:48:28)
> On 07-10-15, 21:12, Arnd Bergmann wrote:
> > I think it clearly makes sense to have a fixed length for each of these
> > members:
> >
> > either 32 bit is enough to represent all possible values, then
> > there is no need to make them 'long' on 64-bit architectures, or 32 bit
> > is not enough and then the code is broken on 32-bit architectures today
> > and should be fixed.
> 
> I agree.
> 
> But I am not 100% sure why it was done this way to start with.
> Probably this is the logic behind that:
> - Max clock rate supported by a u32 is ~ 4.295 GHz
> - People expected that, we will not reach this rate for 32 bit systems
>   but for 64 bit ones.
> - If above is true, then making it u64 for all will generate not very
>   optimized code for 32bit systems, as we need to fetch two 32bit
>   values everytime then.
> - And making it u32 for 64 bit systems wouldn't be great as well, as
>   we need to mask out half of the read value.
> 
> Ofcourse, Mike and Stephen can correct me here :)

I chose unsigned long for the common clock framework _implemenation_,
because the long-standing clk.h _api_ returns this type for clk_get_rate
and passes this type in for clk_set_rate and clk_round_rate.

> 
> > In my patch, I assumed that if 32-bit architectures work fine today, then
> > we don't need more range on 64-bit architectures either.
> 
> The problem here is that we haven't fixed it properly.
> - clock framework expects it to be unsigned long
> - DT is sending a 64 bit value in Hz
> - But we are storing and exposing it in u32

Don't forget cpufreq is using unsigned int for KHz.

> 
> That's weird, isn't it?

Yes it is.

> 
> So, either we update clock API and other similar APIs to u64 or u32
> (which may not be the right thing to do), Or we keep it unsigned long
> here as well and add debugfs_create_ulong().

This comes up every now and then. I'm still trying to figure out if
sub-Hertz quantities should be considered (e.g. representing freqs in
milliHertz).

Regards,
Mike

> 
> -- 
> viresh
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