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

From: Stephen Warren
Date: Tue Jul 29 2014 - 18:14:50 EST

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

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 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.

We could always taint the kernel if the feature is used. Admittedly that wouldn't stop people using the feature as a hack in Android/product kernels, but at least nobody would have to unknowingly debug problems due to such manipulation, in the context of an upstream kernel.
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