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? -Frank > >>> 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. >
