linux-kernel - Re: [PATCH v4 3/8] clk: add support for clocks provided by SCP(System Control Processor)

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Message-ID: <55A7D79B.3080305@arm.com>
Date:	Thu, 16 Jul 2015 17:11:07 +0100
From:	Sudeep Holla <sudeep.holla@....com>
To:	Stephen Boyd <sboyd@...eaurora.org>
CC:	Sudeep Holla <sudeep.holla@....com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>,
	"linux-clk@...r.kernel.org" <linux-clk@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	Liviu Dudau <Liviu.Dudau@....com>,
	Lorenzo Pieralisi <Lorenzo.Pieralisi@....com>,
	"Jon Medhurst (Tixy)" <tixy@...aro.org>,
	Arnd Bergmann <arnd@...db.de>,
	Kevin Hilman <khilman@...nel.org>,
	Olof Johansson <olof@...om.net>,
	Mike Turquette <mturquette@...libre.com>
Subject: Re: [PATCH v4 3/8] clk: add support for clocks provided by SCP(System
 Control Processor)

Hi Stephen,

On 08/07/15 02:46, Stephen Boyd wrote:
> On 07/07, Sudeep Holla wrote:
>>
>>
>> On 06/07/15 20:52, Stephen Boyd wrote:
>>>>
>>>
>>> If I have time I may try to start doing the clk_register() conversion,
>>> but it will take a while so I doubt it will be in v4.3. I'm asking if
>>> you can add a clk_hw based API that does something like
>>> clk_set_rate_range() without requiring a struct clk pointer. i.e.
>>> clk_hw_set_rate_range(struct clk_hw *hw, min, max) that constraints the
>>> min/max rate of the clock. This way, the driver is only using clk
>>> provider APIs and not clk consumer APIs.
>>>
>>
>> I understand the intention of separating clk provider helpers/APIs
>> and clk consumer APIs. Since {min,max}_rate are part of struct clk
>> itself, I was thinking that you would have moved it to struct clk_core
>> as part of the rework you mentioned and hence asked about the patches.
>>
>> IIUC, if {min,max}_rate remain part of struct clk, then how are we
>> restricting that operation to just the clk providers ? clk consumer
>> can still directly modify or use clk_set_rate_range.
>>
>> Do we continue to provide that feature for both provider and consumer ?
>> If so I assume {min,max}_rate range requested by consumer should be
>> within the limits set by provider and do we maintain both the limits ?
>>
>> Sorry if I am missing something fundamental since I don't have much
>> knowledge of clk layer internals.
>>
>
> Yes struct clk would have min/max, and struct clk_core would have
> min/max. Then some sort of provider API (or possibly even
> clk_init_data) would take the min/max fields and copy them over
> to struct clk_core. Then during set_rate operations we would
> aggregate the constraints from struct clk like we already do and
> add in the constrains in struct clk_core.
>
> One downside to adding new fields to clk_init_data is that there
> are drivers out there that aren't initializing that structure to
> 0, and they're putting it on the stack, so stack junk can come
> through. Furthermore, min/max would mean that every driver needs
> to specify some large number for max or we have to special case
> min == max == 0 and ignore it. Somehow it needs to be opt-in. If
> we want to go down the clk_init_data route then perhaps we need
> some sort of rate_constraint struct pointer in there that drivers
> can optionally setup.
>
> 	struct clk_rate_constraint {
> 		unsigned long min;
> 		unsigned long max;
> 	};
>
> 	struct clk_init_data {
> 		...
> 		struct clk_rate_constraint *rate_constraint;
> 	};
>
> I haven't thought it through completely, but I can probably write
> up some patch tomorrow after I sleep on it.
>

I am hoping to get this series for v4.3. In order to avoid using
consumer API, I can revert back to the min,max check I had in the
round_rate earlier if that's fine with you ? Let me know so that I can
post the next version based on that. All the other comments are already
addressed.

Also since this series depends on SCPI, I was thinking to get it merged
via ARM-SoC, but that might conflict with the round_rate prototype
change. Do do plan to share a stable base with arm-soc guys or you
expect all the changes to be contained in clk tree ?

Regards,
Sudeep
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