linux-kernel - Re: [PATCH v6 1/2] PCI: Configure Root Port MPS during host probing

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Message-ID: <20251126235432.GA2726707@bhelgaas>
Date: Wed, 26 Nov 2025 17:54:32 -0600
From: Bjorn Helgaas <helgaas@...nel.org>
To: Hans Zhang <18255117159@....com>
Cc: lpieralisi@...nel.org, kwilczynski@...nel.org, bhelgaas@...gle.com,
	heiko@...ech.de, mani@...nel.org, yue.wang@...ogic.com,
	pali@...nel.org, neil.armstrong@...aro.org, robh@...nel.org,
	jingoohan1@...il.com, khilman@...libre.com, jbrunet@...libre.com,
	martin.blumenstingl@...glemail.com, cassel@...nel.org,
	linux-pci@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org,
	linux-amlogic@...ts.infradead.org,
	linux-rockchip@...ts.infradead.org
Subject: Re: [PATCH v6 1/2] PCI: Configure Root Port MPS during host probing

On Wed, Nov 05, 2025 at 12:51:24AM +0800, Hans Zhang wrote:
> Current PCIe initialization logic may leave Root Ports (root bridges)
> operating with non-optimal Maximum Payload Size (MPS) settings. Existing
> code in pci_configure_mps() returns early for devices without an upstream
> bridge (!bridge) which includes Root Ports, so their MPS values remain
> at firmware/hardware defaults. This fails to utilize the controller's full
> capabilities, leading to suboptimal data transfer efficiency across the
> PCIe hierarchy.
> 
> With this patch, during the host controller probing phase:
> - When PCIe bus tuning is enabled (not PCIE_BUS_TUNE_OFF), and
> - The device is a Root Port without an upstream bridge (!bridge),
> The Root Port's MPS is set to its hardware-supported maximum value
> (128 << dev->pcie_mpss).
> 
> Note that this initial maximum MPS setting may be reduced later, during
> downstream device enumeration, if any downstream device does not suppor
> the Root Port's maximum MPS.
> 
> This change ensures Root Ports are properly initialized before downstream
> devices negotiate MPS, while maintaining backward compatibility via the
> PCIE_BUS_TUNE_OFF check.

"Properly" is sort of a junk word for me because all it really says is
we were stupid before, and we're smarter now, but it doesn't explain
exactly *what* was wrong and why this new thing is "proper."

It's obvious that the Max_Payload_Size power-on default (128 bytes) is
suboptimal in some situations, so you don't even need to say that.
And I think 128 bytes *is* optimal in the PCIE_BUS_PEER2PEER case.

s/Root Ports (root bridges)/Root Ports/
s/bridge (!bridge)/bridge/     # a couple times
s/hardware-supported//         # unnecessary
s/(128 << dev->pcie_mpss)//    # we can read the spec
s/suppor/support/

> Suggested-by: Niklas Cassel <cassel@...nel.org>
> Suggested-by: Manivannan Sadhasivam <mani@...nel.org>
> Signed-off-by: Hans Zhang <18255117159@....com>
> ---
>  drivers/pci/probe.c | 12 ++++++++++++
>  1 file changed, 12 insertions(+)
> 
> diff --git a/drivers/pci/probe.c b/drivers/pci/probe.c
> index 0ce98e18b5a8..2459def3af9b 100644
> --- a/drivers/pci/probe.c
> +++ b/drivers/pci/probe.c
> @@ -2196,6 +2196,18 @@ static void pci_configure_mps(struct pci_dev *dev)
>  		return;
>  	}
>  
> +	/*
> +	 * Unless MPS strategy is PCIE_BUS_TUNE_OFF (don't touch MPS at all),
> +	 * start off by setting Root Ports' MPS to MPSS. This only applies to
> +	 * Root Ports without an upstream bridge (root bridges), as other Root
> +	 * Ports will have downstream bridges.

I can't parse this sentence.  *No* Root Port has an upstream bridge.
So I don't know what "other Root Ports" would be or why they would
have downstream bridges (any Root Port is likely to have downstream
endpoints or bridges).

> +	   ... Depending on the MPS strategy
> +	 * and MPSS of downstream devices, the Root Port's MPS may be
> +	 * overridden later.
> +	 */
> +	if (!bridge && pci_pcie_type(dev) == PCI_EXP_TYPE_ROOT_PORT &&
> +	    pcie_bus_config != PCIE_BUS_TUNE_OFF)
> +		pcie_set_mps(dev, 128 << dev->pcie_mpss);
> +
>  	if (!bridge || !pci_is_pcie(bridge))
>  		return;
>  
> -- 
> 2.34.1
>