Re: [PATCH 0/3] add dynamic PCI device of_node creation for overlay

[Andy Shevchenko]

On Mon, May 9, 2022 at 10:36 PM Frank Rowand <frowand.list@xxxxxxxxx> wrote:
>
> On 5/9/22 12:00, Andy Shevchenko wrote:
> > On Mon, May 09, 2022 at 06:09:17PM +0200, Clément Léger wrote:
> >> Le Mon, 9 May 2022 10:56:36 -0500,
> >> Frank Rowand <frowand.list@xxxxxxxxx> a écrit :
> >
> > ...
> >
> >>> On the surface, it appears that your need might be well met by having
> >>> a base devicetree that describes all of the pcie nodes, but with each
> >>> node having a status of "disabled" so that they will not be used.
> >>> Have a devicetree overlay describing the pcie card (as you proposed),
> >>> where the overlay also includes a status of "ok" for the pcie node.
> >>> Applying the overlay, with a method of redirecting the target to a
> >>> specific pcie node would change the status of the pcie node to enable
> >>> its use.  (You have already proposed a patch to modify
> >>> of_overlay_fdt_apply() to allow a modified target, so not a new
> >>> concept from me.)  My suggestion is to apply the overlay devicetree
> >>> to the base devicetree before the combined FDT devicetree is passed
> >>> to the kernel at boot.  The overlay apply could be done by several
> >>> different entities.  It could be before the bootloader executes, it
> >>> could be done by the bootloader, it could be done by a shim between
> >>> the bootloader and the kernel.  This method avoids all of the issues
> >>> of applying an overlay to a running system that I find problematic.
> >>> It is also a method used by the U-boot bootloader, as an example.
> >>
> >> Ok, that is actually possible on a system that is given a device-tree
> >> by the bootloader. But on a system that is desrcibed using ACPI (such
> >> as the x86), this is much more difficult (at least to my knowledge)...
> >> We want this feature to be easy to use for the end user. Adding such
> >> configuration which also differs between various architecture is
> >> clearly not so easy to setup.
> >>
> >> Moreover, since the PCI is meant to be "Plug and Play", such
> >> configuration would completely break that. If the user switches the
> >> PCIe card from one slot to another, the bootloader configuration will
> >> need to be modified. This seems a big no way for me (and for the user).
> >
> > The main problem here is that Linux does not support hotplugging for the
> > devices behind non-hotpluggable buses. You need to develop something to
> > say that the device tree (in terms of hardware) can morph at run-time
> > transparently to the user. I think the closest one is what FPGA does,
> > or at least should do.
>
> That is something I was not aware of yet.  Is the card in question a
> hotpluggable card?  Do the systems that you anticipate plugging the
> card into support hotplug?

Any PCIe card is potentially hotpluggable (seems nobody actually cares
in 90%+ drivers in the Linux kernel). But what I have heard in a
thread (not this one IIRC) is that the card may have pluggable modules
and it would be nice to change configuration and notify OS somehow. I
might be mistaken if it's the case here or not.

-- 
With Best Regards,
Andy Shevchenko