[PATCH v4] PCI: imx6: Update MPLLB bandwidth to improve i.MX95 Gen3 PCIe stability

[Hongxing Zhu (OSS)]

> -----Original Message-----
> From: Bjorn Helgaas <helgaas at kernel.org>
> Sent: Saturday, August 1, 2026 12:03 AM
> To: Hongxing Zhu (OSS) <hongxing.zhu at oss.nxp.com>
> Cc: Frank Li <frank.li at nxp.com>; l.stach at pengutronix.de; lpieralisi at kernel.org;
> kwilczynski at kernel.org; mani at kernel.org; robh at kernel.org;
> bhelgaas at google.com; s.hauer at pengutronix.de; kernel at pengutronix.de;
> festevam at gmail.com; linux-pci at vger.kernel.org; linux-arm-
> kernel at lists.infradead.org; imx at lists.linux.dev; linux-kernel at vger.kernel.org;
> Hongxing Zhu <hongxing.zhu at nxp.com>
> Subject: Re: [PATCH v4] PCI: imx6: Update MPLLB bandwidth to improve i.MX95
> Gen3 PCIe stability
> 
> On Fri, Jul 31, 2026 at 11:05:46AM +0800, hongxing.zhu at oss.nxp.com wrote:
> > From: Richard Zhu <hongxing.zhu at nxp.com>
> >
> > Bandwidth marginality was observed during i.MX95 Gen3 PCIe tests with
> > the default MPLLB_BANDWIDTH value (120). 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.
> 
> Thanks for including the previous and new MPLLB_BANDWIDTH values.
> 
> I'm fine with the patch, but I think "bandwidth marginality" is awkward wording
> because "marginality" doesn't seem to be commonly used in this context.  Maybe
> something like:
> 
>   We observed poor bandwidth margins during ...  The margins worsened
>   across ...
Understood. I'm totally fine with these updates.
Thanks for your kind review.

Best Regards
Richard Zhu