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

[Hans Zhang]

On 2025/4/18 00:52, Bjorn Helgaas wrote:
> On Thu, Apr 17, 2025 at 10:39:49AM +0200, 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 ?
> >
> > 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.
>
> Thanks, I came here to say basically the same thing.  Ideally I think
> the generic code in pcie_bus_configure_settings() should be able to
> increase MPS or decrease it such that neither meson_set_max_payload()
> nor rockchip_pcie_set_max_payload() is required.
>
> However, the requirement to pick a Kconfig setting makes it a mess.  I
> would love to get rid of those Kconfig symbols.  I don't like the
> command-line parameters either, but it would definitely be an
> improvement if we could nuke the Kconfig symbols and rely on the
> command-line parameters.
>
> It's also a problem when devices are hot-added after the hierarchy has
> already been set up because the new device might not work correctly in
> the existing config.
>
> It's a hard problem to solve.
>
> For new platforms without an install base, maybe it would be easier to
> rely on the command-line parameters since there aren't a bunch of
> users that would have to change the Kconfig.

Dear Bjorn,

Thanks your for reply. Niklas and I attempted to modify the relevant 
logic in drivers/pci/probe.c and found that there was a lot of code 
judging the global variable pcie_bus_config. At present, there is no 
good method. I will keep trying.

I wonder if you have any good suggestions? It seems that the code logic 
regarding pcie_bus_config is a little complicated and cannot be modified 
for the time being?

Best regards,
Hans