linux-kernel - Re: [RFC PATCH 1/4] memory: tegra124-emc: Add EMC driver

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Date:	Thu, 19 Jun 2014 01:14:11 +0200
From:	Thierry Reding <thierry.reding@...il.com>
To:	Stephen Warren <swarren@...dotorg.org>
Cc:	Tomeu Vizoso <tomeu.vizoso@...labora.com>,
	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	David Airlie <airlied@...ux.ie>,
	Mike Turquette <mturquette@...aro.org>,
	myungjoo.ham@...sung.com, kyungmin.park@...sung.com,
	devicetree@...r.kernel.org, linux-tegra@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	linux-pm@...r.kernel.org, dri-devel@...ts.freedesktop.org
Subject: Re: [RFC PATCH 1/4] memory: tegra124-emc: Add EMC driver

On Wed, Jun 18, 2014 at 04:09:06PM -0600, Stephen Warren wrote:
> On 06/18/2014 04:03 PM, Thierry Reding wrote:
> > On Wed, Jun 18, 2014 at 11:46:49AM -0600, Stephen Warren wrote:
> >> On 06/18/2014 11:23 AM, Tomeu Vizoso wrote:
> >>> On 06/17/2014 06:15 PM, Stephen Warren wrote:
> >>>> On 06/17/2014 06:16 AM, Tomeu Vizoso wrote:
> >>>>> On 06/16/2014 10:02 PM, Stephen Warren wrote:
> >>>>>> On 06/16/2014 07:35 AM, Tomeu Vizoso wrote:
> >>>>>>> +#ifdef CONFIG_TEGRA124_EMC
> >>>>>>> +int tegra124_emc_reserve_bandwidth(unsigned int consumer, unsigned
> >>>>>>> long rate);
> >>>>>>> +void tegra124_emc_set_floor(unsigned long freq);
> >>>>>>> +void tegra124_emc_set_ceiling(unsigned long freq);
> >>>>>>> +#else
> >>>>>>> +int tegra124_emc_reserve_bandwidth(unsigned int consumer, unsigned
> >>>>>>> long rate)
> >>>>>>> +{ return -ENODEV; }
> >>>>>>> +void tegra124_emc_set_floor(unsigned long freq)
> >>>>>>> +{ return; }
> >>>>>>> +void tegra124_emc_set_ceiling(unsigned long freq)
> >>>>>>> +{ return; }
> >>>>>>> +#endif
> >>>>>>
> >>>>>> I'll repeat what I said off-list so that we can have the whole
> >>>>>> conversation on the list:
> >>>>>>
> >>>>>> That looks like a custom Tegra-specific API. I think it'd be much
> >>>>>> better
> >>>>>> to integrate this into the common clock framework as a standard clock
> >>>>>> constraints API. There are other use-cases for clock constraints
> >>>>>> besides
> >>>>>> EMC scaling (e.g. some in audio on Tegra, and I'm sure many on other
> >>>>>> SoCs too).
> >>>>>
> >>>>> Yes, I wrote a bit in the cover letter about our requirements and how
> >>>>> they map to the CCF. Could you please comment on that?
> >>>>
> >>>> My comments remain the same. I believe this is something that belongs in
> >>>> the clock driver, or at the least, some API that takes a struct clock as
> >>>> its parameter, so that drivers can use the existing DT clock lookup
> >>>> mechanism.
> >>>
> >>> Ok, let me put this strawman here to see if I have gotten close to what
> >>> you have in mind:
> >>>
> >>> * add per-client accounting (Rabin's patches referenced before)
> >>>
> >>> * add clk_set_floor, to be used by cpufreq, load stats, etc.
> >>>
> >>> * add clk_set_ceiling, to be used by battery drivers, thermal, etc.
> >>
> >> Yes. I'd expect those to be maintained per-client, and so the clock core
> >> (or whatever higher level code implements clk_set_floor/ceiling)
> >> performs the logic that "blends" together all the different requests
> >> from different clients.
> >>
> >> As an aside, for audio usage, I would expect clk_set_rate to be a
> >> per-client (rather than per HW clock) operation too, and to error out if
> >> one client says it wants to set pll_a to the rate needed for
> >> 44.1KHz-based audio and a different client wants the rate for
> >> 48KHz-based audio.
> > 
> > From what I remember, Mike was fairly strongly opposing the idea of
> > virtual clocks, but what you're proposing here sounds like it would
> > assume the existence of virtual clocks. clk_set_rate() per client
> > doesn't work with the current API as I understand it.
> > 
> > Or perhaps what you're proposing isn't about the individual clocks at
> > all but rather about a mechanism to express constraints for a set of
> > clocks?
> 
> This doesn't have anything to do with virtual clocks. As you mention,
> it's just about constraints.
> 
> One user of clock "cpu" wants min rate 216MHz. Another wants max rate
> 1GHz. cpufreq will request some rate between the 2, or be capped to
> those limits. These set of imposed constraints would need to be stored
> per client of the clock, not per HW clock, since many clients could set
> different max rates (e.g. thermal throttle 1.5GHz due to temperature,
> CPU policy 1GHz due to the user selecting low CPU power, etc.)
> 
> Similarly for audio, of there are N clients of 1 clock/PLL, and they
> each want the PLL to run at a different rate, something needs to detect
> that and deny it.

I'm wondering how this should work with the current API. Could the clock
core be modified to return a per-client struct clk * that references the
hardware clock internally? Or do we need to add a new API?

Thierry

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