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

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Message-ID: <1d201377996e16ce25acb640867e1214@codeaurora.org>
Date:   Tue, 11 Feb 2020 11:28:12 +0530
From:   harigovi@...eaurora.org
To:     Jeffrey Hugo <jeffrey.l.hugo@...il.com>
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 2020-02-07 19:40, Jeffrey Hugo wrote:
> 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.

This is not 10nm specific. It is applicable for other nms also.
Its specific to the frequency being set. If vco_recalc returns the same 
value as being set by vco_set_rate,
vco_set_rate will not be invoked second time onwards.

For example: Lets take below devices:

Cheza is based on SDM845 which is 10nm only.
Clk frequency:206016
dsi_pll_10nm_vco_set_rate - DSI PLL0 rate=1236096000
dsi_pll_10nm_vco_recalc_rate - DSI PLL0 returning vco rate = 1236095947

Trogdor is based on sc7180 which is also 10nm.
Clk frequency:69300
dsi_pll_10nm_vco_set_rate - DSI PLL0 rate=1663200000
dsi_pll_10nm_vco_recalc_rate - DSI PLL0 returning vco rate = 1663200000

In same trogdor device, we slightly changed the clock frequency and the 
values actually differ which will not cause any issue.
Clk frequency:69310
dsi_pll_10nm_vco_set_rate - DSI PLL0 rate=1663440000
dsi_pll_10nm_vco_recalc_rate - DSI PLL0 returning vco rate = 1663439941