linux-kernel - Re: [PATCH 1/4] clk: qcom: gdsc: Add support to enable/disable the clocks with GDSC

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Date:   Mon, 30 Jul 2018 16:44:12 +0530
From:   Amit Nischal <anischal@...eaurora.org>
To:     Stephen Boyd <sboyd@...nel.org>
Cc:     Michael Turquette <mturquette@...libre.com>,
        Andy Gross <andy.gross@...aro.org>,
        David Brown <david.brown@...aro.org>,
        Rajendra Nayak <rnayak@...eaurora.org>,
        Odelu Kukatla <okukatla@...eaurora.org>,
        Taniya Das <tdas@...eaurora.org>,
        linux-arm-msm@...r.kernel.org, linux-soc@...r.kernel.org,
        linux-clk@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-clk-owner@...r.kernel.org
Subject: Re: [PATCH 1/4] clk: qcom: gdsc: Add support to enable/disable the
 clocks with GDSC

On 2018-07-25 12:22, Stephen Boyd wrote:
> Quoting Amit Nischal (2018-07-12 05:23:48)
>> On 2018-07-09 11:04, Stephen Boyd wrote:
>> > Quoting Amit Nischal (2018-06-06 04:41:45)
>> >> For some of the GDSCs, there is a requirement to enable/disable the
>> >> few clocks before turning on/off the gdsc power domain. Add support
>> >
>> > Why is there a requirement? Do the clks need to be in hw control mode
>> > or
>> > they can't be turned off when the GDSC is off? It's hard for me to
>> > understand with these vague statements.
>> >
>> 
>> This requirement is primarily to turn on the GPU GX GDSC and these
>> clocks
>> do not need to be in HW control mode and clock disable is not related
>> with the GDSC.
> 
> Ok that's good to know.
> 
>> 
>> To turn on the GX GDSC, root clock (GFX3D RCG) needs to be enabled 
>> first
>> before writing to SW_COLLAPSE bit of the GDSC. The CLK_ON signal from
>> RCG
>> would power up the GPU GX GDSC.
> 
> Can you please put this specific information in the commit text instead
> of making a vague statement about GDSC hardware configurations?
> 
> Does anything go wrong if the GDSC is enabled from genpd but doesn't
> actually turn on until the GFX3D RCG root bit is enabled or disabled by
> the clk enable call? I suppose we won't know if the GDSC is enabled or
> not until the clk is enabled? Maybe we should make the clk enable of 
> the
> RCG for GPU go check the GDSC status bit as well to make sure it's
> toggling on or off?

As per the current design, before turning on the GDSC(writing to the 
GDSCR
register) we are setting the ROOT_EN bit of RCG and GDSC's status will 
be
still off without clk_enable call to RCG even though we enable the GDSC.
clk_enable for RCG is CLK_ON signal for the GPU_GX_GDSC.

> 
> Also, does the RCG turn on when the GX GDSC is off? I think we may be
> able to rely on the GPU driver to "do the right thing" and enable the
> GPU CX GDSC first, then the RCG and branch for the GFX3D clk, and then
> the GPU GX GDSC for the core GPU logic. Then we don't need to do
> anything special in the GDSC code for this.

Its a GPU_GX_GDSC requirement to enable the RCG first and then GX_GDSC.
We want all of this sequence to be done by the GDSC driver so that 
client
only call for clk_apis for the clock branch. For clients, It will be
extra overhead to follow the below sequence.
GPU_CX_GDSC enable -> Enable RCG -> Enable GPU_GX_GDSC -> Enable Branch.