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

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Date:	Wed, 18 Jun 2014 19:23:47 +0200
From:	Tomeu Vizoso <tomeu.vizoso@...labora.com>
To:	Stephen Warren <swarren@...dotorg.org>,
	Thierry Reding <thierry.reding@...il.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 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.

How does it sound?

Regards,

Tomeu
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