linux-kernel - Re: [RFC] clk: add flags to distinguish xtal clocks

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Message-ID: <20131023092427.11662.41825@quantum>
Date:	Wed, 23 Oct 2013 02:24:27 -0700
From:	Mike Turquette <mturquette@...aro.org>
To:	Luca Coelho <luca@...lho.fi>, balbi@...com
Cc:	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	James Hogan <james.hogan@...tec.com>
Subject: Re: [RFC] clk: add flags to distinguish xtal clocks

Quoting Luca Coelho (2013-10-16 03:24:27)
> Hi,
> 
> Sorry for the delayed response.
> 
> On Tue, 2013-10-08 at 10:27 -0500, Felipe Balbi wrote:
> > Fixing Luca's address since he left TI
> 
> Thanks, Felipe! I wouldn't have seen this otherwise.
> 
> 
> > On Mon, Oct 07, 2013 at 12:44:24AM -0700, Mike Turquette wrote:
> > > Quoting Luciano Coelho (2013-07-29 06:50:42)
> > > > Hi Mike,
> > > > 
> > > > On Fri, 2013-07-05 at 10:54 +0300, Luciano Coelho wrote:
> > > > > On Thu, 2013-07-04 at 16:19 -0700, Mike Turquette wrote:
> > > > > > Quoting Luciano Coelho (2013-07-04 15:37:45)
> > > > > > > On Thu, 2013-07-04 at 15:25 -0700, Mike Turquette wrote:
> > > > > > > > Or is it the same clock input and basically the problem is that you need
> > > > > > > > to know what kind of waveform to expect (e.g. square versus sine)?
> > > > > > > 
> > > > > > > It's the same clock input in the chip's perspective.  One clock input
> > > > > > > that can be any of the combinations I mentioned above.  Again, I'm not
> > > > > > > familiar with clocks, so I guess the square vs. sine explanation is
> > > > > > > plausible.  What I could see in the firmware is that it handles the
> > > > > > > clocks differently if they're xtal or not.
> > > > > > 
> > > > > > OMAP has a similar thing where sys_clkin (the fast reference clock for
> > > > > > the chip) can be 19.2, 26, 38.4, etc. This is easy to handle since only
> > > > > > the rates matter.
> > > > > 
> > > > > Right, this part is easy and I already have the code for that.  What I'm
> > > > > missing is a way to pass this XTAL flag to the chip.
> > > > > 
> > > > > 
> > > > > > In your case you need some extra metadata to know what to do. I'm really
> > > > > > not sure if CLK_IS_TYPE_XTAL is the most useful form this metadata can
> > > > > > take. It would be best to know if the waveform is what you really need
> > > > > > to know, or perhaps something else. For instance you might be affected
> > > > > > by some clock signal stabilization time. Can you talk to your hardware
> > > > > > guys and figure it out? I'd rather model the actual needs instead of
> > > > > > just tossing a flag in there.
> > > > > 
> > > > > I get your point.  I have tried to investigate how this flag is used by
> > > > > the firmware and I could see that it is used to set different "buffer
> > > > > gains" and "delays" when waking up (I guess this means when the clock is
> > > > > starting, so probably related to stabilization time).  They specify two
> > > > > "modes", "boost" and "normal" and use different delay values for each.
> > > > 
> > > > I tried but I couldn't find any more information on how exactly this
> > > > works.  But since this change is really simple and there seems to be
> > > > other people who need the same information, couldn't we add it as is and
> > > > try to figure out more specific information about the clocks later on?
> > > > 
> > > > Even if XTAL is not that useful if we know the other details, at least
> > > > it wouldn't hurt to have the flag there anyway.
> > > 
> > > Luca,
> > > 
> > > By any chance did you come to a different solution for this problem? I
> > > can take the patch, but I do not feel like we're solving the right
> > > problem the right way.
> 
> Unfortunately I didn't find any better solution for this.  From the
> WiLink firmware's point of view, there was not much information about
> the details of stabilization time requirements and such.
> 
>  
> > > If not I will take it for 3.13.
> 
> That would be great! As I mentioned above, the XTAL/non-XTAL flag
> wouldn't hurt, even if in most cases it may not provide the necessary
> details for the clock code.  But at least for the WiLink hardware it is
> very useful. ;)

Why is the CLK_IS_TYPE_DEFINED necessary?

Also we're adding a property to the fixed-rate binding so
Documentation/devicetree/bindings/clock/fixed-clock.txt needs to be
updated.

Regards,
Mike

> 
> --
> Cheers,
> Luca.
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