Re: [PATCH v6 4/4] PCI: qcom: Add RX margining settings for 16.0 GT/s

[Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>]

On Wed, Sep 04, 2024 at 11:53:42AM +0200, Johan Hovold wrote:
> On Wed, Sep 04, 2024 at 12:42:00PM +0530, Manivannan Sadhasivam via B4 Relay wrote:
> > From: Shashank Babu Chinta Venkata <quic_schintav@quicinc.com>
> > 
> > Add RX lane margining settings for 16.0 GT/s (GEN 4) data rate. These
> > settings improve link stability while operating at high date rates and
> > helps to improve signal quality.
> > 
> > Signed-off-by: Shashank Babu Chinta Venkata <quic_schintav@quicinc.com>
> > Reviewed-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
> > [mani: dropped the code refactoring and minor changes]
> > Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
> > ---
> >  drivers/pci/controller/dwc/pcie-designware.h  | 18 ++++++++++++++++
> >  drivers/pci/controller/dwc/pcie-qcom-common.c | 31 +++++++++++++++++++++++++++
> >  drivers/pci/controller/dwc/pcie-qcom-common.h |  1 +
> >  drivers/pci/controller/dwc/pcie-qcom-ep.c     |  4 +++-
> >  drivers/pci/controller/dwc/pcie-qcom.c        |  4 +++-
> >  5 files changed, 56 insertions(+), 2 deletions(-)
> > 
> > diff --git a/drivers/pci/controller/dwc/pcie-designware.h b/drivers/pci/controller/dwc/pcie-designware.h
> > index 51744ad25575..f5be99731f7e 100644
> > --- a/drivers/pci/controller/dwc/pcie-designware.h
> > +++ b/drivers/pci/controller/dwc/pcie-designware.h
> > @@ -209,6 +209,24 @@
> >  
> >  #define PCIE_PL_CHK_REG_ERR_ADDR			0xB28
> >  
> > +/*
> > + * 16.0 GT/s (GEN4) lane margining register definitions
> 
> nit: Gen 4?
> 
> > + */
> > +#define GEN4_LANE_MARGINING_1_OFF		0xb80
> 
> nit: upper case hex
> 
> > +#define MARGINING_MAX_VOLTAGE_OFFSET		GENMASK(29, 24)
> > +#define MARGINING_NUM_VOLTAGE_STEPS		GENMASK(22, 16)
> > +#define MARGINING_MAX_TIMING_OFFSET		GENMASK(13, 8)
> > +#define MARGINING_NUM_TIMING_STEPS		GENMASK(5, 0)
> > +
> > +#define GEN4_LANE_MARGINING_2_OFF		0xb84
> 
> Same here
> 
> > +#define MARGINING_IND_ERROR_SAMPLER		BIT(28)
> > +#define MARGINING_SAMPLE_REPORTING_METHOD	BIT(27)
> > +#define MARGINING_IND_LEFT_RIGHT_TIMING		BIT(26)
> > +#define MARGINING_IND_UP_DOWN_VOLTAGE		BIT(25)
> > +#define MARGINING_VOLTAGE_SUPPORTED		BIT(24)
> > +#define MARGINING_MAXLANES			GENMASK(20, 16)
> > +#define MARGINING_SAMPLE_RATE_TIMING		GENMASK(13, 8)
> > +#define MARGINING_SAMPLE_RATE_VOLTAGE		GENMASK(5, 0)
> >  /*
> >   * iATU Unroll-specific register definitions
> >   * From 4.80 core version the address translation will be made by unroll
> > diff --git a/drivers/pci/controller/dwc/pcie-qcom-common.c b/drivers/pci/controller/dwc/pcie-qcom-common.c
> > index dc7d93db9dc5..99b75e7f085d 100644
> > --- a/drivers/pci/controller/dwc/pcie-qcom-common.c
> > +++ b/drivers/pci/controller/dwc/pcie-qcom-common.c
> > @@ -43,3 +43,34 @@ void qcom_pcie_common_set_16gt_eq_settings(struct dw_pcie *pci)
> >  	dw_pcie_writel_dbi(pci, GEN3_EQ_CONTROL_OFF, reg);
> >  }
> >  EXPORT_SYMBOL_GPL(qcom_pcie_common_set_16gt_eq_settings);
> > +
> > +void qcom_pcie_common_set_16gt_rx_margining_settings(struct dw_pcie *pci)
> 
> I'd try to find a shorter symbol name here, "settings" seems redundant
> after "set". Perhaps just
> 
> 	qcom_pcie_common_enable_lane_margining()
> 
> or
> 
> 	qcom_pcie_common_enable_16gt_lane_margining()?
> 

This one looks better. Since lane margining is implemented in the receiver, we
don't really need 'rx' in the function name.

> if these settings are indeed specific to 16 GT/s. But perhaps it's
> better to let the helper honour pci->max_link_speed if different
> settings will later be needed for higher speeds:
> 
> 	if (pcie_link_speed[pci->max_link_speed] >= PCIE_SPEED_16_0GT)
> 		qcom_pcie_common_enable_lane_margining(pci)
> 

I did thought about it during the review, but this setting claims to be for 16
GT/s only. So I wouldn't recommend applying it to other speeds without checking 
with Qcom.

Unfortunately, I'm on vacation for 2 weeks and have limited access to Qcom
internal docs/chat. So won't be able to check it soon. If Shashank could check
it, it is fine. But on the conservative side, let's stick to 16 GT/s only?

- Mani

> >  void qcom_pcie_common_set_16gt_eq_settings(struct dw_pcie *pci);
> > +void qcom_pcie_common_set_16gt_rx_margining_settings(struct dw_pcie *pci);
> 
> And maybe something similar for the eq settings for symmetry.
> 
> Johan

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