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: <f2654844-091f-46bf-88c6-6f6d4edc5533@linaro.org>
Date: Wed, 18 Jun 2025 15:34:20 +0200
From: Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>
To: Dmitry Baryshkov <dmitry.baryshkov@....qualcomm.com>
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 18/06/2025 15:07, Dmitry Baryshkov wrote:
> 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.
> 
> 

I can drop this 250 ns here, if that's what you ask me. But fixing
anything in 10nm is not relevant to this patchset. You were already
asking me for different fixes for some different things and I find it
not acceptable anymore. Stop blocking this patchset with every little
existing issue.

Or merge this code without this patch if a fix for reading PLL as zero
anyhow is questionable for you.

Best regards,
Krzysztof