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

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Message-ID: <4c2a94b4-e483-426f-b7d8-ed98ac474c63@163.com>
Date: Thu, 17 Apr 2025 17:48:04 +0800
From: Hans Zhang <18255117159@....com>
To: Niklas Cassel <cassel@...nel.org>
Cc: Shawn Lin <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



On 2025/4/17 16:39, Niklas Cassel wrote:
> On Thu, Apr 17, 2025 at 04:07:51PM +0800, Hans Zhang wrote:
>> On 2025/4/17 15:48, Niklas Cassel wrote:
>>
>> Hi Niklas and Shawn,
>>
>> Thank you very much for your discussion and reply.
>>
>> I tested it on RK3588 and our platform. By setting pci=pcie_bus_safe, the
>> maximum MPS will be automatically matched in the end.
>>
>> So is my patch no longer needed? For RK3588, does the customer have to
>> configure CONFIG_PCIE_BUS_SAFE or pci=pcie_bus_safe?
>>
>> Also, for pci-meson.c, can the meson_set_max_payload be deleted?
> 
> I think the only reason why this works is because
> pcie_bus_configure_settings(), in the case of
> pcie_bus_config == PCIE_BUS_SAFE, will walk the bus and set MPS in
> the bridge to the lowest of the downstream devices:
> https://github.com/torvalds/linux/blob/v6.15-rc2/drivers/pci/probe.c#L2994-L2999
> 
> 
> So Hans, if you look at lspci for the other RCs/bridges that don't
> have any downstream devices connected, do they also show DevCtl.MPS 256B
> or do they still show 128B ?
> 

Hi Niklas,

It will show DevCtl.MPS 256B.


oot@...efly:~# lspci
00:00.0 PCI bridge: Fuzhou Rockchip Electronics Co., Ltd Device 3588 
(rev 01)
root@...efly:~# lspci -vvv
00:00.0 PCI bridge: Fuzhou Rockchip Electronics Co., Ltd Device 3588 
(rev 01) (prog-if 00 [Normal decode])
         Control: I/O+ Mem+ BusMaster+ SpecCycle- MemWINV- VGASnoop- 
ParErr- Stepping- SERR+ FastB2B- DisINTx+
         Status: Cap+ 66MHz- UDF- FastB2B- ParErr- DEVSEL=fast >TAbort- 
<TAbort- <MAbort- >SERR- <PERR- INTx-
         Latency: 0
         Interrupt: pin A routed to IRQ 79
         Bus: primary=00, secondary=01, subordinate=ff, sec-latency=0
         I/O behind bridge: 0000f000-00000fff [disabled]
         Memory behind bridge: fff00000-000fffff [disabled]
         Prefetchable memory behind bridge: 
00000000fff00000-00000000000fffff [disabled]
         Secondary status: 66MHz- FastB2B- ParErr- DEVSEL=fast >TAbort- 
<TAbort- <MAbort- <SERR- <PERR-
         Expansion ROM at f0200000 [virtual] [disabled] [size=64K]
         BridgeCtl: Parity- SERR+ NoISA- VGA- VGA16- MAbort- >Reset- 
FastB2B-
                 PriDiscTmr- SecDiscTmr- DiscTmrStat- DiscTmrSERREn-
         Capabilities: [40] Power Management version 3
                 Flags: PMEClk- DSI- D1+ D2+ AuxCurrent=375mA 
PME(D0+,D1+,D2-,D3hot+,D3cold-)
                 Status: D0 NoSoftRst+ PME-Enable- DSel=0 DScale=0 PME-
         Capabilities: [50] MSI: Enable+ Count=16/32 Maskable+ 64bit+
                 Address: 00000000fe670040  Data: 0000
                 Masking: fffffeff  Pending: 00000000
         Capabilities: [70] Express (v2) Root Port (Slot-), MSI 08
                 DevCap: MaxPayload 256 bytes, PhantFunc 0
                         ExtTag+ RBE+
                 DevCtl: CorrErr- NonFatalErr- FatalErr- UnsupReq-
                         RlxdOrd+ ExtTag+ PhantFunc- AuxPwr- NoSnoop-
                         MaxPayload 256 bytes, MaxReadReq 512 bytes

Best regards,
Hans

> 
> One could argue that for all policies (execept for maybe PCIE_BUS_TUNE_OFF),
> pcie_bus_configure_settings() should start off by initializing DevCtl.MPS to
> DevCap.MPS (for the bridge itself), and after that pcie_bus_configure_settings()
> can override it depending on policy, e.g. set MPS to 128B in case of
> pcie_bus_config == PCIE_BUS_PEER2PEER, or walk the bus in case of
> pcie_bus_config == PCIE_BUS_SAFE.
> 
> That way, we should be able to remove the setting for pci-meson.c as well.
> 
> Bjorn, thoughts?
> 
> 
> Kind regards,
> Niklas