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

From: Stephen Warren
Date: Thu Jul 31 2014 - 15:53:57 EST

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

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


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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