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

[Stephen Warren <swarren-3lzwWm7+Weoh9ZMKESR00Q@public.gmane.org>]

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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.



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