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

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Date:	Thu, 19 Jun 2014 00:00:09 +0200
From:	Thierry Reding <thierry.reding@...il.com>
To:	Tomeu Vizoso <tomeu.vizoso@...labora.com>
Cc:	Stephen Warren <swarren@...dotorg.org>,
	"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 07:23:47PM +0200, 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.
> 
> * an EMC driver would collect bandwidth and latency requests from consumers
> and call clk_set_floor on the EMC clock.
> 
> * the EMC driver would also register for rate change notifications in the
> EMC clock and would update the latency allowance registers at that point.

Latency allowance registers are part of the MC rather than the EMC. So I
think we have two options: a) have a unified driver for MC and EMC or b)
provide two parts of the API in two drivers.

Or perhaps c), create a generic framework that both MC and EMC can
register with (bandwidth for EMC, latency for MC).

Thierry

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