linux-kernel - Re: [PATCH 1/1] RFC: dt bindings: Add property "brcm,gen3-eq-presets"

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Message-ID: <20241018190330.GA757230@bhelgaas>
Date: Fri, 18 Oct 2024 14:03:30 -0500
From: Bjorn Helgaas <helgaas@...nel.org>
To: Jim Quinlan <james.quinlan@...adcom.com>
Cc: linux-pci@...r.kernel.org, Nicolas Saenz Julienne <nsaenz@...nel.org>,
	Bjorn Helgaas <bhelgaas@...gle.com>,
	Lorenzo Pieralisi <lorenzo.pieralisi@....com>,
	bcm-kernel-feedback-list@...adcom.com, jim2101024@...il.com,
	Florian Fainelli <florian.fainelli@...adcom.com>,
	Lorenzo Pieralisi <lpieralisi@...nel.org>,
	Krzysztof Wilczyński <kw@...ux.com>,
	Manivannan Sadhasivam <manivannan.sadhasivam@...aro.org>,
	Rob Herring <robh@...nel.org>,
	Krzysztof Kozlowski <krzk+dt@...nel.org>,
	Conor Dooley <conor+dt@...nel.org>,
	"moderated list:BROADCOM BCM7XXX ARM ARCHITECTURE" <linux-arm-kernel@...ts.infradead.org>,
	"moderated list:BROADCOM BCM2711/BCM2835 ARM ARCHITECTURE" <linux-rpi-kernel@...ts.infradead.org>,
	"open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" <devicetree@...r.kernel.org>,
	open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/1] RFC: dt bindings: Add property "brcm,gen3-eq-presets"

On Fri, Oct 18, 2024 at 02:22:45PM -0400, Jim Quinlan wrote:
> Support configuration of the GEN3 preset equalization settings, aka the
> Lane Equalization Control Register(s) of the Secondary PCI Express
> Extended Capability.  These registers are of type HwInit/RsvdP and
> typically set by FW.  In our case they are set by our RC host bridge
> driver using internal registers.
> 
> Signed-off-by: Jim Quinlan <james.quinlan@...adcom.com>
> ---
>  .../devicetree/bindings/pci/brcm,stb-pcie.yaml       | 12 ++++++++++++
>  1 file changed, 12 insertions(+)
> 
> diff --git a/Documentation/devicetree/bindings/pci/brcm,stb-pcie.yaml b/Documentation/devicetree/bindings/pci/brcm,stb-pcie.yaml
> index 0925c520195a..f965ad57f32f 100644
> --- a/Documentation/devicetree/bindings/pci/brcm,stb-pcie.yaml
> +++ b/Documentation/devicetree/bindings/pci/brcm,stb-pcie.yaml
> @@ -104,6 +104,18 @@ properties:
>      minItems: 1
>      maxItems: 3
>  
> +  brcm,gen3-eq-presets:
> +    description: |
> +      A u16 array giving the GEN3 equilization presets, one for each lane.
> +      These values are destined for the 16bit registers known as the
> +      Lane Equalization Control Register(s) of the Secondary PCI Express
> +      Extended Capability.  In the array, lane 0 is first term, lane 1 next,
> +      etc. The contents of the entries reflect what is necessary for
> +      the current board and SoC, and the details of each preset are
> +      described in Section 7.27.4 of the PCI base spec, Revision 3.0.

s/equilization/equalization/

The spec citation ("PCI base spec r3.0") isn't quite right since
Conventional PCI doesn't have lanes.  These registers *are* defined in
PCIe r3.0, sec 7.27.4, but that's 14 years old.  It would be more
helpful to use a current spec version like PCIe r6.2, sec 7.7.3.4.

Since there's nothing about these registers that is brcm-specific
(other than the fact that they are typically set by firmware on
non-brcm platforms), it would be nice if we could give it a non-brcm
name.

Similarly, I think it would be nice to drop "gen3" from the name (and
the description).  The registers *were* added in PCIe r3.0, which also
added the 8 GT/s rate, and the description in PCIe r6.2, sec 7.7.3.4
does mention 8.0 GT/s specifically, but sec 4.2.4 says equalization
applies to "8.0 GT/s and higher data rates," so it's definitely not
limited to gen3.

Bjorn