On Wed, Oct 1, 2014 at 12:30 AM, Thierry Reding <thierry.reding@xxxxxxxxx> wrote: > On Tue, Sep 30, 2014 at 02:37:53PM -0700, Mike Turquette wrote: >> Quoting Thierry Reding (2014-09-29 06:54:00) >> > On Mon, Sep 29, 2014 at 01:34:36PM +0200, Maxime Ripard wrote: >> > > On Mon, Sep 29, 2014 at 12:44:57PM +0200, Thierry Reding wrote: >> > > > > >> 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. >> > > > > >> > > > > > 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 <mturquette@xxxxxxxxxx> >> 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 <mturquette@xxxxxxxxxx> >> --- >> Thierry, >> >> Please let me know if this wording makes the feature more clear. > > I think that's better than before, but I don't think it's accurate yet. > As pointed out by Maxime unused clock may still be disabled if it's part > of a tree and that tree is being disabled because there are no users > left. It is entirely accurate. This feature does in fact "prevent the clock framework from *automatically* gating clock ...". And it was merged by Olof so that he could use simplefb with the Chromebook! > > What I had argued is that it's unexpected behavior, because the clock > is still unused (or becomes unused again), therefore shouldn't be > disabled at that point either. Leaving clocks enabled because nobody claimed them is not an option. > > So if you want to keep the current behaviour where an unused clock can > still be disabled depending on what other users do, then I think it'd be > good to mention that as a potential caveat. Do you have a suggestion on the wording? Thanks, Mike > > Thierry -- To unsubscribe from this list: send the line "unsubscribe linux-fbdev" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
