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

[Bjorn Helgaas <helgaas@kernel.org>]

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.

Bjorn