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

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Message-ID: <85643fe4-c7df-4d64-e852-60b66892470a@rock-chips.com>
Date: Thu, 17 Apr 2025 15:08:34 +0800
From: Shawn Lin <shawn.lin@...k-chips.com>
To: Niklas Cassel <cassel@...nel.org>, Hans Zhang <18255117159@....com>
Cc: shawn.lin@...k-chips.com, 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
Subject: Re: [PATCH] PCI: dw-rockchip: Configure max payload size on host init

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
> 
>