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

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Message-ID: <CAHCN7xKocdiWOdmoWQV3POr84qte6WNt0QbQRAwxKSvU8COB_w@mail.gmail.com>
Date:   Fri, 27 Sep 2019 07:33:50 -0500
From:   Adam Ford <aford173@...il.com>
To:     Tomi Valkeinen <tomi.valkeinen@...com>
Cc:     Tony Lindgren <tony@...mide.com>, Tero Kristo <t-kristo@...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 Fri, Sep 27, 2019 at 2:55 AM Tomi Valkeinen <tomi.valkeinen@...com> wrote:
>
> (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.

Is there anything I can do to help troubleshoot this?  I could insert
a hack that checks if we're omap3 and if so make the divider equal to
4, but that seems like just a hack.
I can run more tests or insert code somewhere if you want.

adam
>
>   Tomi
>
> --
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