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

[Niklas Cassel <cassel@kernel.org>]

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