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

[Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>]

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.


Best regards,
Krzysztof