linux-kernel - Re: [PATCH v6 08/17] drm/msm/dsi/phy: Fix reading zero as PLL rates when unprepared

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Message-ID: <24xkss4bw6ww43x2gbjchcm4gtmqhdecncmxopnnhf7y2tblc2@iibgqhuix5rm>
Date: Wed, 18 Jun 2025 16:07:09 +0300
From: Dmitry Baryshkov <dmitry.baryshkov@....qualcomm.com>
To: Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>
Cc: Abhinav Kumar <quic_abhinavk@...cinc.com>, Sean Paul <sean@...rly.run>,
        Marijn Suijten <marijn.suijten@...ainline.org>,
        David Airlie <airlied@...il.com>, Simona Vetter <simona@...ll.ch>,
        Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Maxime Ripard <mripard@...nel.org>,
        Thomas Zimmermann <tzimmermann@...e.de>, Rob Herring <robh@...nel.org>,
        Krzysztof Kozlowski <krzk+dt@...nel.org>,
        Conor Dooley <conor+dt@...nel.org>,
        Krishna Manikandan <quic_mkrishn@...cinc.com>,
        Jonathan Marek <jonathan@...ek.ca>,
        Kuogee Hsieh <quic_khsieh@...cinc.com>,
        Neil Armstrong <neil.armstrong@...aro.org>,
        Dmitry Baryshkov <lumag@...nel.org>,
        Bjorn Andersson <andersson@...nel.org>,
        Michael Turquette <mturquette@...libre.com>,
        Stephen Boyd <sboyd@...nel.org>,
        Rob Clark <robin.clark@....qualcomm.com>,
        linux-arm-msm@...r.kernel.org, dri-devel@...ts.freedesktop.org,
        freedreno@...ts.freedesktop.org, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-clk@...r.kernel.org,
        Abel Vesa <abel.vesa@...aro.org>,
        Srinivas Kandagatla <srini@...nel.org>
Subject: Re: [PATCH v6 08/17] drm/msm/dsi/phy: Fix reading zero as PLL rates
 when unprepared

On Wed, Jun 18, 2025 at 10:28:10AM +0200, Krzysztof Kozlowski wrote:
> On 13/06/2025 16:04, Dmitry Baryshkov wrote:
> > On 13/06/2025 17:02, Krzysztof Kozlowski wrote:
> >> On 13/06/2025 15:55, Dmitry Baryshkov wrote:
> >>>>   
> >>>> @@ -361,24 +373,47 @@ static int dsi_pll_7nm_lock_status(struct dsi_pll_7nm *pll)
> >>>>   
> >>>>   static void dsi_pll_disable_pll_bias(struct dsi_pll_7nm *pll)
> >>>>   {
> >>>> +	unsigned long flags;
> >>>>   	u32 data;
> >>>>   
> >>>> +	spin_lock_irqsave(&pll->pll_enable_lock, flags);
> >>>> +	--pll->pll_enable_cnt;
> >>>> +	if (pll->pll_enable_cnt < 0) {
> >>>> +		spin_unlock_irqrestore(&pll->pll_enable_lock, flags);
> >>>> +		DRM_DEV_ERROR_RATELIMITED(&pll->phy->pdev->dev,
> >>>> +					  "bug: imbalance in disabling PLL bias\n");
> >>>> +		return;
> >>>> +	} else if (pll->pll_enable_cnt > 0) {
> >>>> +		spin_unlock_irqrestore(&pll->pll_enable_lock, flags);
> >>>> +		return;
> >>>> +	} /* else: == 0 */
> >>>> +
> >>>>   	data = readl(pll->phy->base + REG_DSI_7nm_PHY_CMN_CTRL_0);
> >>>>   	data &= ~DSI_7nm_PHY_CMN_CTRL_0_PLL_SHUTDOWNB;
> >>>>   	writel(0, pll->phy->pll_base + REG_DSI_7nm_PHY_PLL_SYSTEM_MUXES);
> >>>>   	writel(data, pll->phy->base + REG_DSI_7nm_PHY_CMN_CTRL_0);
> >>>> +	spin_unlock_irqrestore(&pll->pll_enable_lock, flags);
> >>>>   	ndelay(250);
> >>>
> >>> What is this ndelay protecting? Is is to let the hardware to wind down
> >>> correctly? I'm worried about dsi_pll_disable_pll_bias() beng followed up
> >>> by dsi_pll_enable_pll_bias() in another thread, which would mean that
> >>> corresponding writes to the REG_DSI_7nm_PHY_CMN_CTRL_0 can come up
> >>> without any delay between them.
> >>>
> >>
> >> Great question, but why do you ask me? The code was there already and
> >> MSM DRM drivers are not something I know and could provide context about.
> > 
> > Because it's you who are changing the code as you've faced the issue 
> > with recalc_rate.
> > 
> Heh, the answer is then: I don't know. I think authors of the code could
> know.

The 10nm HPG documents a 250ns interval between enabling PLL bias and
and enabling the PLL via the CMN_PLL_CNTRL register. There is no extra
delay between disabling the PLL, disabling FIFO and remobing PLL bias.
Please adjust the code for 7nm and 10nm PHYs accordingly.


-- 
With best wishes
Dmitry