linux-kernel - Re: [PATCH] PCI: dw-rockchip: Configure max payload size on host init

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Message-ID: <aACoEpueUHBLjgbb@ryzen>
Date: Thu, 17 Apr 2025 09:04:50 +0200
From: Niklas Cassel <cassel@...nel.org>
To: Hans Zhang <18255117159@....com>
Cc: lpieralisi@...nel.org, kw@...ux.com, bhelgaas@...gle.com,
	heiko@...ech.de, manivannan.sadhasivam@...aro.org, robh@...nel.org,
	jingoohan1@...il.com, thomas.richard@...tlin.com,
	linux-pci@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org,
	linux-rockchip@...ts.infradead.org,
	Shawn Lin <shawn.lin@...k-chips.com>
Subject: Re: [PATCH] PCI: dw-rockchip: Configure max payload size on host init

Hello Hans,

On Wed, Apr 16, 2025 at 11:19:26PM +0800, Hans Zhang wrote:
> The RK3588's PCIe controller defaults to a 128-byte max payload size,
> but its hardware capability actually supports 256 bytes. This results
> in suboptimal performance with devices that support larger payloads.

Patch looks good to me, but please always reference the TRM when you can.

Before this patch:
		DevCap: MaxPayload 256 bytes
		DevCtl: MaxPayload 128 bytes

As per rk3588 TRM, section "11.4.3.8 DSP_PCIE_CAP Detail Registers Description"

DevCap is per the register description of DSP_PCIE_CAP_DEVICE_CAPABILITIES_REG,
field PCIE_CAP_MAX_PAYLOAD_SIZE.
Which claims that the value after reset is 0x1 (256B).

DevCtl is per the register description of
DSP_PCIE_CAP_DEVICE_CONTROL_DEVICE_STATUS, field PCIE_CAP_MAX_PAYLOAD_SIZE_CS.
Which claims that the reset value is 0x0 (128B).

Both of these match the values above.

As per the description of PCIE_CAP_MAX_PAYLOAD_SIZE_CS:
"Permissible values that
can be programmed are indicated by the Max_Payload_Size
Supported field (PCIE_CAP_MAX_PAYLOAD_SIZE) in the Device
Capabilities (DEVICE_CAPABILITIES_REG) register (for more
details, see section 7.5.3.3 of PCI Express Base Specification)."

So your patch looks good.

I guess I'm mostly surprised that the e.g. pci_configure_mps() does not
already set DevCtl to the max(DevCap.MPS of the host, DevCap.MPS of the
endpoint).

Apparently pci_configure_mps() only decreases MPS from the reset values?
It never increases it?

Kind regards,
Niklas