linux-kernel - Re: [PATCH 8/8] clk: tegra: Add EMC clock driver

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Message-ID: <53DA9ED2.5000003@wwwdotorg.org>
Date:	Thu, 31 Jul 2014 13:53:54 -0600
From:	Stephen Warren <swarren@...dotorg.org>
To:	Mike Turquette <mturquette@...aro.org>,
	Thierry Reding <thierry.reding@...il.com>
CC:	Mikko Perttunen <mperttunen@...dia.com>,
	Peter De Schrijver <pdeschrijver@...dia.com>,
	Prashant Gaikwad <pgaikwad@...dia.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"linux-tegra@...r.kernel.org" <linux-tegra@...r.kernel.org>
Subject: Re: [PATCH 8/8] clk: tegra: Add EMC clock driver

On 07/31/2014 01:06 PM, Mike Turquette wrote:
> Quoting Thierry Reding (2014-07-30 02:34:57)
>> On Tue, Jul 29, 2014 at 04:14:44PM -0600, Stephen Warren wrote:
>>> On 07/29/2014 02:19 PM, Mike Turquette wrote:
>>>> Quoting Mikko Perttunen (2014-07-29 01:47:35)
>>>>> On 22/07/14 19:57, Stephen Warren wrote:
>>>>>> On 07/11/2014 08:18 AM, Mikko Perttunen wrote:
>>>>>>> +static int emc_debug_rate_set(void *data, u64 rate)
>>>>>>> +{
>>>>>>> +    struct tegra_emc *tegra = data;
>>>>>>> +
>>>>>>> +    return clk_set_rate(tegra->hw.clk, rate);
>>>>>>> +}
>>>>>>> +
>>>>>>> +DEFINE_SIMPLE_ATTRIBUTE(emc_debug_rate_fops, emc_debug_rate_get,
>>>>>>> +                    emc_debug_rate_set, "%lld\n");
>>>>>>
>>>>>> I think the rate can already be obtained through
>>>>>> ...debug/clock/clock_summary. I'm not sure about changing the rate, but
>>>>>> shouldn't that be a feature of the common clock core, not individual
>>>>>> drivers?
>>>>>
>>>>> The core doesn't allow writing to the rate debugfs files, so this is the
>>>>> only way to trigger an EMC clock change for now. I agree that the core
>>>>> might be a better place. I don't know if there are any philosophical
>>>>> objections to that. I'd like to keep this in until a possible core
>>>>> feature addition. Mike, any comments?
>>>>
>>>> Yes, there is a philosophical rejection to exposing rate-change knobs to
>>>> userspace through debugfs. These can and will ship in real products
>>>> (typically Android) with lots of nasty userspace hacks, and also
>>>> represent pretty dangerous things to expose to userspace. I have always
>>>> maintained that such knobs should remain out of tree or, with the advent
>>>> of the custom debugfs entries, should be burden of the clock drivers.
>>>
>>> That argument seems a bit inconsistent.
>>>
>>> I can see the argument to disallow code that lets user-space fiddle with
>>> clocks. However, if that argument holds, then surely it must apply to either
>>> the clock core *or* a clock driver; the end effect of allowing the code in
>
> Stephen,
>
> You meant to say, "it must apply to both the clock core and a clock
> driver"? I agree.

Sure; s/either/both/ in what I said.

>>> either place is that people will be able to implement the user-space hacks
>>> you want to avoid. Yet, if we allow the code because it's a useful debug
>>> tool, then surely it should be in the clock core so we don't implement it
>>> redundantly in each clock driver.
>
> I don't want it anywhere to be honest. Read-only debugfs stuff is great
> and I'm happy to merge it. I have repeatedly NAK'd any attempt to get
> the userspace rate-change stuff merged into the core.
>
> Recently we have the ability to assign custom debugfs entries that are
> specific to the clock driver. I'm trying to find the right balance
> between giving the clock driver authors the right amount of autonomy to
> implement what they need while trying to keep the crazy out of the
> kernel. Maybe in this case I should stick to my guns and NAK this patch.

If someone implements the same thing in some downstream/product kernel, 
and it can't be upstreamed, I'd argue they should still do that in the 
clock core rather than individual clock drivers, since they'll probably 
want the feature for multiple clocks. I don't think  the per-clock 
debugfs hook is useful in this case, although I can certainly imagine 
other read-only per-clock debug files could be useful.
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