[PATCH v3 00/10] clk: implement clock rate protection mechanism

[boris.brezillon@free-electrons.com (Boris Brezillon)]

+Quentin

On Tue, 20 Jun 2017 14:32:30 +0200
Jerome Brunet <jbrunet@baylibre.com> wrote:

> On Tue, 2017-06-20 at 13:54 +0200, Linus Walleij wrote:
> > On Tue, Jun 20, 2017 at 12:50 PM, Jerome Brunet <jbrunet@baylibre.com> wrote:  
> > > On Tue, 2017-06-20 at 11:07 +0200, Linus Walleij wrote:
> > > When the rate is critical to perform a particular task. My example is the
> > > audio
> > > and i2s output. You can't tolerate glitches during the playback, the end
> > > user
> > > would complain (longer description in the original RFC)  
> > 
> > I see. Thanks for your detailed explanation!
> >   
> > > It also fixes the behavior of CLK_SET_RATE_GATE flag, which is why I put you
> > > in
> > > CC.
> > > 
> > > ux500 uses this flag several time, I'd like make sure people are not relying
> > > on
> > > its broken implementation.  
> > 
> > Ux500 audio is broken, but I'm fixing it a little at a time...  
> 
> No problem with Ux500 audio, don't worry :)
> Audio is just one application among others.
> 
> The concern regarding ux500 is that the clock controller declares clocks using
> the CLK_SET_RATE_GATE flag (like qcom, at91 and several other)
> 
> here is the definition:
> #define CLK_SET_RATE_GATE	BIT(0) /* must be gated across rate change */
> 
> My interpretation it that, as long as clock is enabled, rate can't change.
> 
> The implementation of this flag is currently broken:
> * If you call set_rate on directly on the clock while it is enabled, you will
> get -EBUSY, as expected
> * If you call set_rate on its parent, rate will change, changing the rate of the
> child clock as well ...
> 
> With this patchset applied, calling set_rate on the parent would also return
> -EBUSY, enforcing the "rate can't change while enabled" property.
> 
> To build confidence that this won't be causing regression, I'd like to check
> that platform using this flag are no relying on the broken behavior.
> 
> I've included the clock maintainers of at91, qcom, and ux500 (you) in this
> thread because they are the heaviest users of this flag, so the more likely to
> report a problem.

This flag in at91 clk drivers really means that rate cannot be changed
while the clk is enabled. We have other clock where this is not a
problem (at least, nothing in mentioned in the datasheet).

> 
> If you could apply this series and just do things as usual, It'd be awesome !

Actually, this is a feature I was pushing for a while back [1],
because I had the same problem (one user messing up with a PLL while
others were relying on its rate). I'm glad someone finally had time to
provide a solution for this problem.

Quentin tested the series, and I guess he'll soon add his Tested-by.
Note that this series does not address all problems. For example, when
several drivers are setting a rate on different clks that take the
same parent, the first one to set the rate and enable the clk wins.

He has an hack-ish solution for our audio-pll case preventing non audio
users to mess up with the audio PLL. It'd be great to have a generic
solution, though I don't know how this can be solved without advanced
description of the clk-rate setting policy.

[1]https://patchwork.kernel.org/patch/6204221/