linux-kernel - Re: [PATCH] drm/omap: Migrate minimum FCK/PCK ratio from Kconfig to dts

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Message-ID: <6cd7c9f0-cb26-588b-dfd1-faf462732549@ti.com>
Date:   Mon, 30 Sep 2019 20:48:20 +0300
From:   Tero Kristo <t-kristo@...com>
To:     Adam Ford <aford173@...il.com>,
        Tomi Valkeinen <tomi.valkeinen@...com>
CC:     "H. Nikolaus Schaller" <hns@...delico.com>,
        Tony Lindgren <tony@...mide.com>,
        Linux-OMAP <linux-omap@...r.kernel.org>,
        Adam Ford <adam.ford@...icpd.com>,
        Benoît Cousson <bcousson@...libre.com>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        devicetree <devicetree@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] drm/omap: Migrate minimum FCK/PCK ratio from Kconfig to
 dts

On 30/09/2019 18:10, Adam Ford wrote:
> On Mon, Sep 30, 2019 at 9:27 AM Tomi Valkeinen <tomi.valkeinen@...com> wrote:
>>
>> On 30/09/2019 17:20, Tomi Valkeinen wrote:
>>
>>> Let's see what Tero says, but yeah, something is odd here. I expected
>>> the max divider to be 16 with Tero's patch, but I don't see it having
>>> that effect. I can get the div to 31.
>>>
>>> You can see this from the clock register 0x48004e40 (CM_CLKSEL_DSS). The
>>> lowest bits are the divider, 5 to 0. The TRM says max div is 32.
>>>
>>> Tero said for him the dividers > 16 didn't "stick" to the register. I'm
>>> now wondering if he has an old beagleboard with OMAP34xx, which has max
>>> div 16.
>>
>> So testing a bit more here, I can see the DSS working fine and fps as
>> expected when I write values directly to CM_CLKSEL_DSS:5:0, with
>> dividers up to 31. With 32, DSS breaks. The TRM (AM/DM37x) says value 32
>> is valid.
> 
> I wonder if it's somehow being masked with bits 4:0 instead of 5:0
> which could potentially make the divider 0 and that value doesn't
> appear to be valid.

Hmmh, after some testing, it seems there is bad stuff happening with the 
divider clock implementation, I am re-working it as of now. Basically 
what is wrong is that with a divider max value of say 16, the driver 
attempts to craft the max value into a mask, but this ends up being 
0x1f. If the max value is 15, it ends up into 0xf which is correct.

-Tero
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