linux-kernel - Re: [PATCH v2 5/9] clk: qcom: rcg2: Stop hardcoding gfx3d pingpong parent numbers

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Date:   Thu, 11 Feb 2021 12:19:42 -0800
From:   Stephen Boyd <sboyd@...nel.org>
To:     AngeloGioacchino Del Regno 
        <angelogioacchino.delregno@...ainline.org>, agross@...nel.org
Cc:     bjorn.andersson@...aro.org, mturquette@...libre.com,
        robh+dt@...nel.org, linux-arm-msm@...r.kernel.org,
        linux-clk@...r.kernel.org, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org, phone-devel@...r.kernel.org,
        konrad.dybcio@...ainline.org, marijn.suijten@...ainline.org,
        martin.botka@...ainline.org,
        AngeloGioacchino Del Regno 
        <angelogioacchino.delregno@...ainline.org>
Subject: Re: [PATCH v2 5/9] clk: qcom: rcg2: Stop hardcoding gfx3d pingpong parent numbers

Quoting AngeloGioacchino Del Regno (2021-01-13 10:38:13)
> The function clk_gfx3d_determine_rate is selecting different PLLs
> to manage the GFX3D clock source in a special way: this one needs
> to be ping-pong'ed on different PLLs to ensure stability during
> frequency switching (set a PLL rate, let it stabilize, switch the
> RCG to the new PLL) and fast frequency transitions.
> 
> This technique is currently being used in the MSM8996 SoC and the
> function was assuming that the parents were always at a specific
> index in the parents list, which is TRUE, if we use this only on
> the MSM8996 MMCC.
> Unfortunately, MSM8996 is not the only SoC that needs to ping-pong
> the graphics RCG, so choices are:
> 1. Make new special ops just to hardcode *again* other indexes,
>    creating code duplication for (imo) no reason; or
> 2. Generalize this function, so that it becomes usable for a range
>    of SoCs with slightly different ping-pong configuration.
> 
> In this commit, the second road was taken: define a new "special"
> struct clk_rcg2_gfx3d, containing the ordered list of parents to
> ping-pong the graphics clock on, and the "regular" rcg2 clock
> structure in order to generalize the clk_gfx3d_determine_rate
> function and make it working for other SoCs.
> 
> As for the function itself it is left with the assumption that we
> need to ping-pong over three parents. The reasons for this are:
> 1. The initial model was MSM8996, which has 3 parents for the
>    graphics clock pingpong;
> 2. The other example that was taken into consideration is the
>    SDM630/636/660 SoC gpu clock controller, which is ping-ponging
>    over two dynamic clocked and one fixed clock PLL.
> 
> Signed-off-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@...ainline.org>
> ---

Applied to clk-next but I modified it a bunch. Let me know if it doesn't
work.