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

[Hongxing Zhu (OSS)]

> -----Original Message-----
> From: Manivannan Sadhasivam <mani@xxxxxxxxxx>
> Sent: Wednesday, July 29, 2026 9:58 PM
> To: Hongxing Zhu (OSS) <hongxing.zhu@xxxxxxxxxxx>
> Cc: Frank Li <frank.li@xxxxxxx>; l.stach@xxxxxxxxxxxxxx; lpieralisi@xxxxxxxxxx;
> kwilczynski@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 v3] PCI: imx6: Update MPLLB bandwidth to improve i.MX95
> Gen3 PCIe stability
> 
> On Tue, Jul 14, 2026 at 10:41:07AM +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_BANDWIDTH value. This margin degradation worsens
> > across voltage and temperature (VT) variations and different test
> > matrices, potentially causing link stability issues.
> >
> > 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
> >
> > This ensures robust PCIe Gen3 performance across all operating
> > conditions.
> >
> > Signed-off-by: Richard Zhu <hongxing.zhu@xxxxxxx>
> > ---
> > 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
> 
> Use BIT(10) please.
Okay, will change to BIT(10).

> 
> > +/* 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
> 
> Is it possible to define individual bits?
> 
This is a 16-bit bandwidth value (140 decimal = 0x8c) recommended by the IC
designers. It appears to be a complete value rather than a bit field, but
please let me know if you'd prefer a different representation.

> > +
> >  #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);
> 
> Why is this delay at the start of this function?
A proper delay is required between the assertion of IMX95_PCIE_PHY_CR_PARA_SEL
(in imx95_pcie_init_phy) and each subsequent PHY register write operation.
Without this delay, the values cannot be successfully written to the PHY
registers. Placing the delay at the start of the helper function ensures it's
applied before every PHY write.

Best Regards
Richard Zhu> 
> - Mani
> 
> --
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