linux-kernel - Re: [PATCH] drm/msm: dsi: fix PLL init in bonded mode

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Message-ID: <wg6ncd4zyoh3lrrxtpjm6bup7ymyy3v3notzvvdssoptveblxq@4g4shyi2xqar>
Date: Mon, 27 Oct 2025 15:20:21 +0200
From: Dmitry Baryshkov <dmitry.baryshkov@....qualcomm.com>
To: Neil Armstrong <neil.armstrong@...aro.org>
Cc: Rob Clark <robin.clark@....qualcomm.com>,
        Dmitry Baryshkov <lumag@...nel.org>,
        Abhinav Kumar <abhinav.kumar@...ux.dev>,
        Jessica Zhang <jessica.zhang@....qualcomm.com>,
        Sean Paul <sean@...rly.run>,
        Marijn Suijten <marijn.suijten@...ainline.org>,
        David Airlie <airlied@...il.com>, Simona Vetter <simona@...ll.ch>,
        Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>,
        linux-arm-msm@...r.kernel.org, dri-devel@...ts.freedesktop.org,
        freedreno@...ts.freedesktop.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] drm/msm: dsi: fix PLL init in bonded mode

On Mon, Oct 27, 2025 at 02:09:48PM +0100, Neil Armstrong wrote:
> When in bonded DSI mode, only one PLL in one DSI PHY is used for both
> DSI PHYs, meaning that parents of the secondary DSI PHY will use the
> primary DSI PHY PLL as parent.
> 
> In this case the primary DSI PHY PLL will be set even if the primary
> DSI PHY is not yet enabled. The DSI PHY code has support for this
> particular use-case and will handle the fact the PLL was already
> set when initializing the primary DSI PHY.
> 
> By introducing a protected variable pll_enable_cnt in the commit
> cb55f39bf7b1 ("drm/msm/dsi/phy: Fix reading zero as PLL rates when unprepared"),
> this variable is only initially set to 1 when the DSI PHY is initialized
> making it impossible to set the PLL before, breaking the bonded DSI
> use case by returning 0 when setting the PLL from the secondary DSI
> PHY driver and skipping the correct clocks initialization.
> 
> But since it was already possible to set the PLL without enabling
> the DSI PHY, just drop the pll_enable_cnt setting from the PHY
> enable/disable and simply increment/decrement the pll_enable_cnt
> variable from the dsi_pll_enable/disable_pll_bias to make sure any
> PLL operation is done with the PLL BIAS enabled.
> 
> Fixes: cb55f39bf7b1 ("drm/msm/dsi/phy: Fix reading zero as PLL rates when unprepared")
> Closes: https://lore.kernel.org/all/50a49d72-2b1e-471d-b0c4-d5a0b38b2a21@linaro.org/
> Tested-by: Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>
> Signed-off-by: Neil Armstrong <neil.armstrong@...aro.org>
> ---
>  drivers/gpu/drm/msm/dsi/phy/dsi_phy.h     |  1 -
>  drivers/gpu/drm/msm/dsi/phy/dsi_phy_7nm.c | 18 ++----------------
>  2 files changed, 2 insertions(+), 17 deletions(-)
> 

Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@....qualcomm.com>


-- 
With best wishes
Dmitry