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

[Niklas Cassel <cassel@kernel.org>]

On Thu, Apr 17, 2025 at 03:25:06PM +0800, Shawn Lin wrote:
> 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?

It seems that the default MPS strategy can be set using Kconfigs:
https://github.com/torvalds/linux/blob/v6.15-rc2/drivers/pci/pci.c#L126-L136
https://github.com/torvalds/linux/blob/v6.15-rc2/include/linux/pci.h#L1110-L1116

Note that the these Kconfigs are hidden behind CONFIG_EXPERT.
So unless you have explicitly set one of these Kconfigs, the default should be:
PCIE_BUS_DEFAULT,	/* Ensure MPS matches upstream bridge */


Kind regards,
Niklas