Quoting Thierry Reding (2014-09-29 06:54:00)

On Mon, Sep 29, 2014 at 01:34:36PM +0200, Maxime Ripard wrote:

quoted

On Mon, Sep 29, 2014 at 12:44:57PM +0200, Thierry Reding wrote:

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Plus, speaking more specifically about the clocks, that won't prevent
your clock to be shut down as a side effect of a later clk_disable
call from another driver.

quoted

Furthermore isn't it a bug for a driver to call clk_disable() before a
preceding clk_enable()? There are patches being worked on that will
enable per-user clocks and as I understand it they will specifically
disallow drivers to disable the hardware clock if other drivers are
still keeping them on via their own referenc.

Calling clk_disable() preceding clk_enable() is a bug.

Calling clk_disable() after clk_enable() will disable the clock (and
its parents)
if the clock subsystem thinks there are no other users, which is what will
happen here.

Right. I'm not sure this is really applicable to this situation, though.

It's actually very easy to do. Have a driver that probes, enables its
clock, fails to probe for any reason, call clk_disable in its exit
path. If there's no other user at that time of this particular clock
tree, it will be shut down. Bam. You just lost your framebuffer.

Really, it's just that simple, and relying on the fact that some other
user of the same clock tree will always be their is beyond fragile.

Perhaps the meaning clk_ignore_unused should be revised, then. What you
describe isn't at all what I'd expect from such an option. And it does
not match the description in Documentation/kernel-parameters.txt either.

From e156ee56cbe26c9e8df6619dac1a993245afc1d5 Mon Sep 17 00:00:00 2001

From: Mike Turquette <redacted>
Date: Tue, 30 Sep 2014 14:24:38 -0700
Subject: [PATCH] doc/kernel-parameters.txt: clarify clk_ignore_unused

Refine the definition around clk_ignore_unused, which caused some
confusion recently on the linux-fbdev and linux-arm-kernel mailing
lists[0].

[0] http://lkml.kernel.org/r/<20140929135358.GC30998@ulmo>

Signed-off-by: Mike Turquette <redacted>
---
Thierry,

Please let me know if this wording makes the feature more clear.

 Documentation/kernel-parameters.txt | 14 +++++++++-----
 1 file changed, 9 insertions(+), 5 deletions(-)

diff --git a/Documentation/kernel-parameters.txt b/Documentation/kernel-parameters.txt
index 10d51c2..0ce01fb 100644
--- a/Documentation/kernel-parameters.txt
+++ b/Documentation/kernel-parameters.txt

@@ -605,11 +605,15 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
 			See Documentation/s390/CommonIO for details.
 	clk_ignore_unused
 			[CLK]
-			Keep all clocks already enabled by bootloader on,
-			even if no driver has claimed them. This is useful
-			for debug and development, but should not be
-			needed on a platform with proper driver support.
-			For more information, see Documentation/clk.txt.
+			Prevents the clock framework from automatically gating
+			clocks that have not been explicitly enabled by a Linux
+			device driver but are enabled in hardware at reset or
+			by the bootloader/firmware. Note that this does not
+			force such clocks to be always-on nor does it reserve
+			those clocks in any way. This parameter is useful for
+			debug and development, but should not be needed on a
+			platform with proper driver support.  For more
+			information, see Documentation/clk.txt.
 
 	clock=		[BUGS=X86-32, HW] gettimeofday clocksource override.
 			[Deprecated]

-- 
1.8.3.2