Message-ID: <50F63452.1060004@ti.com>
Date:	Wed, 16 Jan 2013 10:32:10 +0530
From:	Sekhar Nori <nsekhar@...com>
To:	Mike Turquette <mturquette@...aro.org>
CC:	Afzal Mohammed <afzal@...com>,
	Florian Tobias Schandinat <FlorianSchandinat@....de>,
	Tomi Valkeinen <tomi.valkeinen@...com>,
	Grant Likely <grant.likely@...retlab.ca>,
	Rob Herring <rob.herring@...xeda.com>,
	Rob Landley <rob@...dley.net>, <linux-omap@...r.kernel.org>,
	<linux-fbdev@...r.kernel.org>,
	<devicetree-discuss@...ts.ozlabs.org>, <linux-doc@...r.kernel.org>,
	<linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 12/12] video: da8xx-fb: set upstream clock rate (if
 reqd)

On 1/15/2013 9:02 PM, Mike Turquette wrote:
> Quoting Afzal Mohammed (2013-01-15 05:44:36)
>> LCDC IP has a clock divider to adjust pixel clock, this limits pixel
>> clock range to fck/255 - fck/2(fck - rate of input clock to LCDC IP).
>> In the case of AM335x, where this IP is present, default fck is not
>> sufficient to provide normal pixel clock rates, hence rendering this
>> driver unusable on AM335x.
>>
>> If input clock too is configurable, allowable range of pixel clock
>> would increase. Here initially it is checked whether with present fck,
>> divider in IP could be configured to obtain required rate, if not,
>> fck is adjusted. This makes it usable on AM335x.
>>
>> Note:
>> A better (if allowable) solution may be to represent clock divider in
>> LCDC IP as a basic divider clock - the one defined in common clock
>> framework. But for this to happen, all the platform's using this driver
>> should be using common clock framework (DaVinci is yet to be converted
>> to use common clock framework). And it has to be determined whether
>> common clock framework allows this kind of a clock modelling inside a
>> driver and for this to be part of clock tree. Advantage of doing so
>> would be better resolution for pixel clock, even though without this
>> existing use cases are working properly. Or another extreme alternative
>> would be to replicate clk-divider of common clock framework inside the
>> driver, but that probably is not preferred and not worth as it would be
>> duplication and without much advantage to existing users.
>>
> 
> Afzal,
> 
> Modeling the divider inside your IP block as a clock is supported in the
> common clock framework.  Linking up these sorts of clocks to the clock
> tree was one of the original design goals of CCF.
> 
> Regarding DaVinci: converting that platform over to use CCF would be the
> best approach.

This is work in progress. There are patches that have been posted. Work
has been slow on this though due to lack of bandwidth.

> An alternative would be that you could break
> single-image boot for AM335x and DaVinci, by having AM335x use CCF and
> DaVinci use the legacy clock framework.  From the LCDC driver's

Single image for DaVinci and AM335x is not possible anyway since ARMv5
and ARMv6+ cannot be supported in a single image.

> perspective this should not matter and is indeed the purpose of the
> clk.h api and clkdev interfaces, however looking at this driver I can
> see there would still be a lot ifdef-ery going on... better to just
> convert everything over to CCF.

Waiting for DaVinci CCF to complete will be too long a wait. Probably
convert to CCF just for AM335x ATM. There would be some ifdef'ry but
hopefully that need not be inside function bodies. Would have to see the
implementation, I guess.

Thanks,
Sekhar
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