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

[Shawn Lin <shawn.lin@rock-chips.com>]

在 2025/04/17 星期四 15:22, Niklas Cassel 写道:
> On Thu, Apr 17, 2025 at 03:08:34PM +0800, Shawn Lin wrote:
>> 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
> 
> If that is the case, then explain the before/after with Hans lspci output here:
> https://lore.kernel.org/linux-pci/bb40385c-6839-484c-90b2-d6c7ecb95ba9@163.com/
> 
> His patch changes the default value of DevCtl.MPS (from 128B to 256B), but if
> pci_configure_mps() can bump DevCtl.MPS to a higher value, his patch should not
> be needed, since the EP (an NVMe SSD in his case) has DevCap.MPS 512B, and the
> RC itself has DevCap.MPS 256B.
> 
> Seems like we are missing something here.

So Hans, could you please help set pci=pcie_bus_safe or
pci=pcie_bus_perf in your cmdline, and see how lspci dump different
without your patch?

> 
> 
> Kind regards,
> Niklas
> 
>