linux-kernel - Re: [PATCH 1/5] clk: berlin: add support for berlin plls

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Message-ID: <20140321160147.GH6443@piout.net>
Date:	Fri, 21 Mar 2014 17:01:47 +0100
From:	Alexandre Belloni <alexandre.belloni@...e-electrons.com>
To:	Sebastian Hesselbarth <sebastian.hesselbarth@...il.com>
Cc:	Mike Turquette <mturquette@...aro.org>, linux-doc@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	Antoine Ténart 
	<antoine.tenart@...e-electrons.com>
Subject: Re: [PATCH 1/5] clk: berlin: add support for berlin plls

On 21/03/2014 at 17:03:52 +0100, Sebastian Hesselbarth wrote :
> On 03/21/2014 04:45 PM, Alexandre Belloni wrote:
> >[all commentis I agree on are snipped]
> 
> :)
> 
> >On 21/03/2014 at 13:49:32 +0100, Sebastian Hesselbarth wrote :
> >>On 03/21/2014 12:43 PM, Alexandre Belloni wrote:
> >>obj-y += pll.o
> >>obj-$(CONFIG_MACH_BERLIN_BG2)	+= pll-berlin2.o
> >>obj-$(CONFIG_MACH_BERLIN_BG2CD)	+= pll-berlin2.o
> >>obj-$(CONFIG_MACH_BERLIN_BG2Q)	+= pll-berlin2q.o
> >>
> >>Which reminds me, that we forgot to add MACH_BERLIN_BG2Q to
> >>arch/arm/mach-berlin/Kconfig. Can you spin a patch?
> >>
> >
> >I will do that.
> >
> >>>+static const u8 vcodiv_berlin2[] = {10, 15, 20, 25, 30, 40, 50, 60, 80};
> >>>+
> >>>+static struct berlin_pllmap berlin_pll_map = {
> >>>+	.vcodiv = vcodiv_berlin2,
> >>>+	.fbdiv_mask = 0x1FF,
> >>>+	.rfdiv_mask = 0x1F,
> >>>+	.divsel_mask = 0xF,
> >>
> >>divsel_mask allows 16 possible values, vcodiv_berlin2[] only provides
> >>9, and common pll driver below does not know how many valid vcodiv
> >>values are passed. That can become dangerous..
> >>
> >>I'd rather extend vcodiv_berlin2 to full divsel range and provide
> >>safe (=1) divisiors. This way wrong/new register values will only
> >>break clock frequency derived.
> >>
> >
> >Good catch ! Then, what about simply shrinking the mask so that we don't
> >overflow the table. We'll put it back to its supposed real value whant
> >we know what are the remaining divisors (my guess is that they are already
> >all listed here). I would say that we are getting the divisor wrong if
> >divsel > 8 anyway.
> 
> Hmm, maybe I should look up valid vcodiv myself, but your vcodiv_berlin2
> has 9 values and I guess they are all valid, aren't they?
> 
> The next possible, larger mask where 0-8 fits in, is 0xf. You used that
> above and that reveals 16 possible indices.
> 
> The only option for shrinking the table that I see, would be min/max
> allowed indices, but that is as useful as having a slightly larger
> table.
> 

You are absolutely right :) I definitely need to take a break, right now !

> >>>+	.fbdiv_shift = 6,
> >>>+	.rfdiv_shift = 1,
> >>>+	.divsel_shift = 7,
> >>
> >>Have .foo_mask and .foo_shift together?
> >>
> >
> >This will make the struct larger but I don't really have an opinion.
> 
> Maybe, I wasn't clear enough. Just assign .foo_mask and .foo_shift in
> subsequent lines of code, i.e.
> 
> static struct berlin_pllmap berlin_pll_map = {
> 	.vcodiv = vcodiv_berlin2,
> 	.fbdiv_mask = 0x1FF,
> 	.fbdiv_shift = 6,
> 	.rfdiv_mask = 0x1F,
> 	.rfdiv_shift = 1,
> 	.divsel_mask = 0xF,
> 	.divsel_shift = 7,
> };
> 

yeah, I figured that out a few minutes ago...

Thanks again for your reviews and patience !

-- 
Alexandre Belloni, Free Electrons
Embedded Linux, Kernel and Android engineering
http://free-electrons.com
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