linux-kernel - Re: [Freedreno] [v1] drm/msm/dsi/pll: call vco set rate explicitly

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Message-ID: <CAOCk7NoCH9p9gOd7as=ty-EMeerAAhQtKZa8f2wZrDeV2LtGrw@mail.gmail.com>
Date:   Fri, 7 Feb 2020 07:10:55 -0700
From:   Jeffrey Hugo <jeffrey.l.hugo@...il.com>
To:     Harigovindan P <harigovi@...eaurora.org>
Cc:     "open list:DRM PANEL DRIVERS" <dri-devel@...ts.freedesktop.org>,
        MSM <linux-arm-msm@...r.kernel.org>,
        freedreno <freedreno@...ts.freedesktop.org>,
        DTML <devicetree@...r.kernel.org>,
        lkml <linux-kernel@...r.kernel.org>,
        Rob Clark <robdclark@...il.com>, nganji@...eaurora.org,
        Sean Paul <seanpaul@...omium.org>, kalyan_t@...eaurora.org,
        "Kristian H. Kristensen" <hoegsberg@...omium.org>
Subject: Re: [Freedreno] [v1] drm/msm/dsi/pll: call vco set rate explicitly

On Fri, Feb 7, 2020 at 5:38 AM <harigovi@...eaurora.org> wrote:
>
> On 2020-02-06 20:29, Jeffrey Hugo wrote:
> > On Thu, Feb 6, 2020 at 2:13 AM Harigovindan P <harigovi@...eaurora.org>
> > wrote:
> >>
> >> For a given byte clock, if VCO recalc value is exactly same as
> >> vco set rate value, vco_set_rate does not get called assuming
> >> VCO is already set to required value. But Due to GDSC toggle,
> >> VCO values are erased in the HW. To make sure VCO is programmed
> >> correctly, we forcefully call set_rate from vco_prepare.
> >
> > Is this specific to certain SoCs? I don't think I've observed this.
>
> As far as Qualcomm SOCs are concerned, since pll is analog and the value
> is directly read from hardware if we get recalc value same as set rate
> value, the vco_set_rate will not be invoked. We checked in our idp
> device which has the same SOC but it works there since the rates are
> different.

This doesn't seem to be an answer to my question.  What Qualcomm SoCs
does this issue apply to?  Everything implementing the 10nm pll?  One
specific SoC?  I don't believe I've seen this on MSM8998, nor SDM845,
so I'm interested to know what is the actual impact here.  I don't see
an "IDP" SoC in the IP catalog, so I really have no idea what you are
referring to.