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

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Date:	Wed, 18 Jun 2014 15:19:27 -0700
From:	Stéphane Marchesin <stephane.marchesin@...il.com>
To:	Thierry Reding <thierry.reding@...il.com>
Cc:	Tomeu Vizoso <tomeu.vizoso@...labora.com>,
	devicetree@...r.kernel.org, Mike Turquette <mturquette@...aro.org>,
	Stephen Warren <swarren@...dotorg.org>,
	linux-pm@...r.kernel.org, "Rafael J. Wysocki" <rjw@...ysocki.net>,
	Linux Kernel list <linux-kernel@...r.kernel.org>,
	"dri-devel@...ts.freedesktop.org" <dri-devel@...ts.freedesktop.org>,
	Kyungmin Park <kyungmin.park@...sung.com>,
	myungjoo.ham@...sung.com,
	"linux-tegra@...r.kernel.org" <linux-tegra@...r.kernel.org>,
	linux-arm-kernel@...ts.infradead.org
Subject: Re: [RFC PATCH 1/4] memory: tegra124-emc: Add EMC driver

On Wed, Jun 18, 2014 at 3:00 PM, Thierry Reding
<thierry.reding@...il.com> wrote:
> 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).

Is there any motivation for keeping MC and EMC separate? In my mind,
the solution was always to handle those together.

Stéphane
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