RE: [PATCH v2] PCI: imx6: Update MPLLB bandwidth to improve i.MX95 Gen3 PCIe stability

[Hongxing Zhu (OSS)]

> -----Original Message-----
> From: Frank Li (OSS) <frank.li@xxxxxxxxxxx>
> Sent: Wednesday, July 8, 2026 11:19 PM
> To: Hongxing Zhu (OSS) <hongxing.zhu@xxxxxxxxxxx>
> Cc: Frank Li <frank.li@xxxxxxx>; l.stach@xxxxxxxxxxxxxx; lpieralisi@xxxxxxxxxx;
> kwilczynski@xxxxxxxxxx; mani@xxxxxxxxxx; robh@xxxxxxxxxx;
> bhelgaas@xxxxxxxxxx; s.hauer@xxxxxxxxxxxxxx; kernel@xxxxxxxxxxxxxx;
> festevam@xxxxxxxxx; linux-pci@xxxxxxxxxxxxxxx; linux-arm-
> kernel@xxxxxxxxxxxxxxxxxxx; imx@xxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx;
> Hongxing Zhu <hongxing.zhu@xxxxxxx>
> Subject: Re: [PATCH v2] PCI: imx6: Update MPLLB bandwidth to improve i.MX95
> Gen3 PCIe stability
> 
> On Wed, Jul 08, 2026 at 11:59:28AM +0800, hongxing.zhu@xxxxxxxxxxx wrote:
> > From: Richard Zhu <hongxing.zhu@xxxxxxx>
> >
> > Bandwidth marginality was observed during i.MX95 Gen3 PCIe tests with
> > the default MPLLB_BINDWIDTH value. This margin degradation worsens
> > across voltage and temperature (VT) variations and different test
> > matrices, potentially causing link stability issues.
> >
> > Testing with MPLLB_BINDWIDTH 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
> >
> > This ensures robust PCIe Gen3 performance across all operating
> > conditions.
> >
> > Signed-off-by: Richard Zhu <hongxing.zhu@xxxxxxx>
> > ---
> > 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 | 24 ++++++++++++++++++++++++
> >  1 file changed, 24 insertions(+)
> >
> > diff --git a/drivers/pci/controller/dwc/pci-imx6.c
> > b/drivers/pci/controller/dwc/pci-imx6.c
> > index 53f3da6ab30d5..5f75aa09e0377 100644
> > --- a/drivers/pci/controller/dwc/pci-imx6.c
> > +++ b/drivers/pci/controller/dwc/pci-imx6.c
> > @@ -80,6 +80,15 @@
> >  #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
> > +#define IMX95_PCIE_PHY_MPLLB_OVRD_BW_EN		0x400
> 
> > +#define IMX95_PCIE_PHY_MPLLB_OVRD_BW_IN		0x2005
> 
> This one IMX95_PCIE_PHY_MPLLB_BW_IN to keep naming consistent
> 
> write IMX95_PCIE_PHY_MPLLB_OVRD_BW_EN ->
> IMX95_PCIE_PHY_MPLLB_OVRD_IN write IMX95_PCIE_PHY_MPLLB_BW ->
> IMX95_PCIE_PHY_MPLLB_BW_IN
> 
> > +#define IMX95_PCIE_PHY_MPLLB_BW			0x8c
> 
> 0x8c only one special value.
> 
> IMX95_PCIE_PHY_MPLLB_BW_XXX put special name for 0x8c, if no means, you
> can put 0x8c direct at
> 
> imx95_pcie_phy_write(imx_pcie, IMX95_PCIE_PHY_MPLLB_OVRD_BW_IN,
> 0x8c)
> 
> We need knows what's macro means, like it is offset of regiser or it is field in
> register easily.
How about add some comments as below?
+/* BIT(10): Override enable for mpllb_bandwidth[15:0] */
 #define IMX95_PCIE_PHY_MPLLB_OVRD_BW_EN                0x400
-#define IMX95_PCIE_PHY_MPLLB_OVRD_BW_IN                0x2005
-#define IMX95_PCIE_PHY_MPLLB_BW                        0x8c
+/* 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)
@@ -309,8 +311,8 @@ 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_OVRD_BW_IN,
-                            IMX95_PCIE_PHY_MPLLB_BW);
+       imx95_pcie_phy_write(imx_pcie, IMX95_PCIE_PHY_MPLLB_BW_IN,
+                            IMX95_PCIE_PHY_MPLLB_BW_VAL);

Best Regards
Richard Zhu
> 
> Frank
> 
> > +
> >  #define IMX95_PCIE_RST_CTRL			0x3010
> >  #define IMX95_PCIE_COLD_RST			BIT(0)
> >
> > @@ -269,6 +278,16 @@ 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 +308,11 @@ 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_OVRD_BW_IN,
> > +			     IMX95_PCIE_PHY_MPLLB_BW);
> > +	imx95_pcie_phy_write(imx_pcie, IMX95_PCIE_PHY_MPLLB_OVRD_IN,
> > +			     IMX95_PCIE_PHY_MPLLB_OVRD_BW_EN);
> > +
> >  	return 0;
> >  }
> >
> > --
> > 2.34.1
> >
> >