linux-kernel - Re: [PATCH v2 1/3] dt-bindings: PCI: brcmstb: Add two optional props

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Date:   Fri, 14 Apr 2023 15:14:13 -0500
From:   Bjorn Helgaas <helgaas@...nel.org>
To:     Jim Quinlan <jim2101024@...il.com>
Cc:     linux-pci@...r.kernel.org,
        Nicolas Saenz Julienne <nsaenz@...nel.org>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Lorenzo Pieralisi <lorenzo.pieralisi@....com>,
        Cyril Brulebois <kibi@...ian.org>,
        Phil Elwell <phil@...pberrypi.com>,
        bcm-kernel-feedback-list@...adcom.com, james.quinlan@...adcom.com,
        Florian Fainelli <f.fainelli@...il.com>,
        Lorenzo Pieralisi <lpieralisi@...nel.org>,
        Krzysztof Wilczyński <kw@...ux.com>,
        Rob Herring <robh@...nel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.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 v2 1/3] dt-bindings: PCI: brcmstb: Add two optional props

It'd be nice to mention the property names (maybe omit the "brcm,"
prefix if that helps) in the commit log so "git log --oneline" is more
useful:

  959e000f0463 ("dt-bindings: PCI: brcmstb: Add two optional props")
  ea372f45cfff ("dt-bindings: PCI: Add bindings for Brcmstb EP voltage regulators")
  504253e44a9d ("dt-bindings: PCI: Correct brcmstb interrupts, interrupt-map.")
  145790e55d82 ("dt-bindings: PCI: Add compatible string for Brcmstb 74[23]5 MIPs SOCs")
  5e8a7d26d935 ("dt-bindings: PCI: brcmstb: compatible is required")
  f435ce7ebf8c ("dt-bindings: PCI: brcmstb: add BCM4908 binding")

On Tue, Apr 11, 2023 at 12:59:16PM -0400, Jim Quinlan wrote:
> Regarding "brcm,enable-l1ss":
> 
>   The Broadcom STB/CM PCIe HW -- a core that is also used by RPi SOCs --
>   requires the driver probe() to deliberately place the HW one of three
>   CLKREQ# modes:
> 
>   (a) CLKREQ# driven by the RC unconditionally
>   (b) CLKREQ# driven by the EP for ASPM L0s, L1
>   (c) Bidirectional CLKREQ#, as used for L1 Substates (L1SS).
> 
>   The HW+driver can tell the difference between downstream devices that
>   need (a) and (b), but does not know when to configure (c).  Further, the
>   HW may cause a CPU abort on boot if guesses wrong regarding the need for
>   (c).  So we introduce the boolean "brcm,enable-l1ss" property to indicate
>   that (c) is desired.  Setting this property only makes sense when the
>   downstream device is L1SS-capable and the OS is configured to activate
>   this mode (e.g. policy==superpowersave).
> 
>   This property is already present in the Raspian version of Linux, but the
>   upstream driver implementaion that will follow adds more details and
>   discerns between (a) and (b).
> 
> Regarding "brcm,completion-timeout-us"
> 
>   Our HW will cause a CPU abort if the L1SS exit time is longer than the
>   PCIe transaction completion abort timeout.  We've been asked to make this
>   configurable, so we are introducing "brcm,completion-timeout-us".

Completion Timeout is a generic PCIe concept.  Do we want a generic
(non-brcm) name that would be documented elsewhere?  Rob?

> Signed-off-by: Jim Quinlan <jim2101024@...il.com>
> ---
>  .../devicetree/bindings/pci/brcm,stb-pcie.yaml   | 16 ++++++++++++++++
>  1 file changed, 16 insertions(+)
> 
> diff --git a/Documentation/devicetree/bindings/pci/brcm,stb-pcie.yaml b/Documentation/devicetree/bindings/pci/brcm,stb-pcie.yaml
> index 7e15aae7d69e..f7fc2f6561bb 100644
> --- a/Documentation/devicetree/bindings/pci/brcm,stb-pcie.yaml
> +++ b/Documentation/devicetree/bindings/pci/brcm,stb-pcie.yaml
> @@ -64,6 +64,22 @@ properties:
>  
>    aspm-no-l0s: true
>  
> +  brcm,enable-l1ss:
> +    description: Indicates that PCIe L1SS power savings
> +      are desired, the downstream device is L1SS-capable, and the
> +      OS has been configured to enable this mode.  Note that when
> +      in this mode, this particular HW may not meet the requirement
> +      that requires CLKREQ# assertion to clock active to be
> +      within 400ns.

Maybe a pointer to the source of the 400ns requirement?

"requirement that requires" is a little redundant, maybe "... may not
meet the requirement that Refclk be valid within 400ns of CLKREQ#
assertion"?

(I don't actually know whether this refers to Refclk or if that would
be a true statement; this is just a possible sentence structure.)