Re: [PATCH v3] PCI: imx6: Update MPLLB bandwidth to improve i.MX95 Gen3 PCIe stability

On Tue, Jul 14, 2026 at 10:41:07AM +0800, [email protected] wrote:
> From: Richard Zhu <[email protected]>
> 
> Bandwidth marginality was observed during i.MX95 Gen3 PCIe tests with
> the default MPLLB_BANDWIDTH value. This margin degradation worsens
> across voltage and temperature (VT) variations and different test
> matrices, potentially causing link stability issues.

What does "bandwidth marginality" mean?  And what does it mean to
observe it?  I guess it must mean some measurable thing is close to
being out of spec?

> Testing with MPLLB_BANDWIDTH value of 140 (0x8c) shows significant
> improvement in bandwidth margins across all VT conditions and test
> scenarios.
> 
> Implement PHY register write helper function and configure:
> - MPLLB_BW_OVRD_IN = 140 (0x8c) for improved bandwidth margin
> - MPLLB_BW_OVRD_EN to enable the override

AFAICT we don't know what MPLLB_BW_OVRD_IN was before, so we don't
know whether this increases or decreases the value.  And we don't
really even know what MPLLB_BW_OVRD_IN specifies.

> This ensures robust PCIe Gen3 performance across all operating
> conditions.
> 
> Signed-off-by: Richard Zhu <[email protected]>
> ---
> Changes in v3:
> Regarding Frank's comments, refine naming consistency and clarity.
> 
> Changes in v2:
> Update the register name and bit definitions.
> Don't move IMX95_PCIE_PHY_CR_PARA_SEL settings.
> ---
>  drivers/pci/controller/dwc/pci-imx6.c | 26 ++++++++++++++++++++++++++
>  1 file changed, 26 insertions(+)
> 
> diff --git a/drivers/pci/controller/dwc/pci-imx6.c b/drivers/pci/controller/dwc/pci-imx6.c
> index 53f3da6ab30d5..fadf14de10f7c 100644
> --- a/drivers/pci/controller/dwc/pci-imx6.c
> +++ b/drivers/pci/controller/dwc/pci-imx6.c
> @@ -80,6 +80,17 @@
>  #define IMX95_SID_MASK				GENMASK(5, 0)
>  #define IMX95_MAX_LUT				32
>  
> +#define IMX95_PCIE_PHY_REG_ADDR			0x3008
> +#define IMX95_PCIE_PHY_REG_EN			BIT(31)
> +#define IMX95_PCIE_PHY_REG_ADDR_MASK		GENMASK(15, 0)
> +#define IMX95_PCIE_PHY_REG_DATA			0x300c
> +#define IMX95_PCIE_PHY_MPLLB_OVRD_IN		0x2004
> +/* BIT(10): Override enable for mpllb_bandwidth[15:0] */
> +#define IMX95_PCIE_PHY_MPLLB_OVRD_BW_EN		0x400
> +/* Register offset: Override value for mpllb_bandwidth[15:0] */
> +#define IMX95_PCIE_PHY_MPLLB_BW_IN		0x2005
> +#define IMX95_PCIE_PHY_MPLLB_BW_VAL		0x8c
> +
>  #define IMX95_PCIE_RST_CTRL			0x3010
>  #define IMX95_PCIE_COLD_RST			BIT(0)
>  
> @@ -269,6 +280,16 @@ static int imx95_pcie_select_ref_clk_src(struct imx_pcie *imx_pcie)
>  	return 0;
>  }
>  
> +static void imx95_pcie_phy_write(struct imx_pcie *imx_pcie, int addr, u16 data)
> +{
> +	udelay(200);
> +	regmap_update_bits(imx_pcie->iomuxc_gpr, IMX95_PCIE_PHY_REG_ADDR,
> +			   IMX95_PCIE_PHY_REG_EN, IMX95_PCIE_PHY_REG_EN);
> +	regmap_update_bits(imx_pcie->iomuxc_gpr, IMX95_PCIE_PHY_REG_ADDR,
> +			   IMX95_PCIE_PHY_REG_ADDR_MASK, addr);
> +	regmap_write(imx_pcie->iomuxc_gpr, IMX95_PCIE_PHY_REG_DATA, data);
> +}
> +
>  static int imx95_pcie_init_phy(struct imx_pcie *imx_pcie)
>  {
>  	/*
> @@ -289,6 +310,11 @@ static int imx95_pcie_init_phy(struct imx_pcie *imx_pcie)
>  			IMX95_PCIE_PHY_CR_PARA_SEL,
>  			IMX95_PCIE_PHY_CR_PARA_SEL);
>  
> +	imx95_pcie_phy_write(imx_pcie, IMX95_PCIE_PHY_MPLLB_BW_IN,
> +			     IMX95_PCIE_PHY_MPLLB_BW_VAL);
> +	imx95_pcie_phy_write(imx_pcie, IMX95_PCIE_PHY_MPLLB_OVRD_IN,
> +			     IMX95_PCIE_PHY_MPLLB_OVRD_BW_EN);
> +
>  	return 0;
>  }
>  
> -- 
> 2.34.1
>