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

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Message-ID: <854f6130-c8a8-81cb-aa76-4830f218ae54@ti.com>
Date:   Fri, 27 Sep 2019 10:55:13 +0300
From:   Tomi Valkeinen <tomi.valkeinen@...com>
To:     Adam Ford <aford173@...il.com>, Tony Lindgren <tony@...mide.com>,
        Tero Kristo <t-kristo@...com>,
        Linux-OMAP <linux-omap@...r.kernel.org>
CC:     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

(dropping folks who're probably not interested...)

On 26/09/2019 17:12, Adam Ford wrote:
> On Thu, Sep 26, 2019 at 1:55 AM Tomi Valkeinen <tomi.valkeinen@...com> wrote:
>>
>> On 25/09/2019 23:51, Adam Ford wrote:
>>
>>>> Has anyone debugged why the hang is happening?
>>> I started to debug this, but I got distracted when I noticed the LCD
>>> did't work at all on modern kernels.  I have that fixed now, so I can
>>> go back to investigating this.

I dont' have the same board, but I was testing with omap3 beagle xm. I 
can reproduce rather similar issue, although I don't get a hang but 
instead sync lost and underflow flood (which makes the device unusable).

It looks like a bug in omap clock handling.

DSS uses dss1_alwon_fck_3430es2 as fclk. dss1_alwon_fck_3430es2 comes 
from dpll4_ck, and there's a divider after the PLL, dpll4_m4_ck.

When the DSS driver sets dss1_alwon_fck_3430es2 rate to 27000000 or 
27870967, which can be created with m4 dividers 32 and 31, it looks like 
the divider goes to bypass, or to a very small value. DSS gets a very 
high clock rate and breaks down.

  Tomi

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