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Date:   Mon, 9 May 2022 20:00:35 +0300
From:   Andy Shevchenko <andriy.shevchenko@linux.intel.com>
To:     =?iso-8859-1?Q?Cl=E9ment_L=E9ger?= <clement.leger@bootlin.com>
Cc:     Frank Rowand <frowand.list@gmail.com>,
        Rob Herring <robh+dt@kernel.org>,
        Pantelis Antoniou <pantelis.antoniou@konsulko.com>,
        Bjorn Helgaas <bhelgaas@google.com>,
        Allan Nielsen <allan.nielsen@microchip.com>,
        Horatiu Vultur <horatiu.vultur@microchip.com>,
        Steen Hegelund <steen.hegelund@microchip.com>,
        Thomas Petazzoni <thomas.petazonni@bootlin.com>,
        Alexandre Belloni <alexandre.belloni@bootlin.com>,
        Mark Brown <broonie@kernel.org>,
        Jakub Kicinski <kuba@kernel.org>,
        Hans de Goede <hdegoede@redhat.com>,
        Andrew Lunn <andrew@lunn.ch>, devicetree@vger.kernel.org,
        linux-kernel@vger.kernel.org, linux-pci@vger.kernel.org
Subject: Re: [PATCH 0/3] add dynamic PCI device of_node creation for overlay
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 <20220509141634.16158c38@xps-bootlin>
 <f9b0cbf1-dde2-ff97-cca0-5d2895734f91@gmail.com>
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On Mon, May 09, 2022 at 06:09:17PM +0200, Cl?ment L?ger wrote:
> Le Mon, 9 May 2022 10:56:36 -0500,
> Frank Rowand <frowand.list@gmail.com> a ?crit :

...

> > 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.

> > The other big issue is mixing ACPI and devicetree on a single system.
> > Historically, the Linux devicetree community has not been receptive
> > to the ides of that mixture.  Your example might be a specific case
> > where the two can be isolated from each other, or maybe not.  (For
> > disclosure, I am essentially ACPI ignorant.)  I suspect that mixing
> > ACPI and devicetree is a recipe for disaster in the general case.
> 
> Agreed, on that fact, it did raised some eyebrows, and it was for that
> specific concern that initially, I proposed the fwnode solution.
> Honestly, the fwnode conversion represent a lot of work (hundreds of
> lines easily) + requires a conversion of all the subsystem that are not
> fwnode ready (spoiler: almost all of them are not ready). 

In either case you need to provide a format that would be suitable for
DT-based as well as ACPI-based platforms.

-- 
With Best Regards,
Andy Shevchenko