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

[Shawn Lin]

在 2025/04/17 星期四 15:04, Niklas Cassel 写道:
> 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?

Actually it does:

https://github.com/torvalds/linux/blob/master/Documentation/admin-guide/kernel-parameters.txt#L4757

> Kind regards,
> Niklas