Re: [PATCH v4 0/9] Driver core: Add faux bus devices

José Expósito
2025-02-28T11:27:59Z

Hi everyone,

Thanks for CCing me Louis,

On Thu, Feb 27, 2025 at 02:06:21PM +0100, Louis Chauvet wrote:
>
>
> Le 10/02/2025 à 13:30, Greg Kroah-Hartman a écrit :
> > For years/decades now, I've been complaining when I see people use
> > platform devices for things that are obviously NOT platform devices.
> > To finally fix this up, here is a "faux bus" that should be used instead
> > of a platform device for these tiny and "fake" devices that people
> > create all over the place.
> >
> > The api is even simpler than the normal platform device api, just two
> > functions, one to create a device and one to remove it. When a device
> > is created, if a probe/release callback is offered, they will be called
> > at the proper time in the device's lifecycle. When finished with the
> > device, just destroy it and all should be good.
> >
> > This simple api should also hopefully provide for a simple rust binding
> > to it given the simple rules and lifecycle of the pointer passed back
> > from the creation function (i.e. it is alive and valid for as long as
> > you have not called destroy on it.)
> >
> > I've also converted four different examples of platform device abuse, the
> > dummy regulator driver, the USB phy code, the x86 microcode dvice, and
> > the "regulator" device that wifi uses to load the firmware tables, to
> > use this api. In all cases, the logic either was identical, or became
> > simpler, than before, a good sign (side note, a bug was fixed in the usb
> > phy code that no one ever noticed before).
> >
> > Note, unless there are major objections, I'm leaning toward getting
> > patch 1 and 2 of this series merged during this -rc cycle so that all of
> > the individual driver subsystem cleanups can go through those subsystems
> > as needed, as well as allowing the rust developers to create a binding
> > and get that merged easier. Having patch 1 merged on its own isn't
> > going to cause any changes if no one uses it, so that should be fine.
>
> Hi all,
>
> I have a maybe dumb question regarding the patches 3..9: do they break the
> UAPI?

I made a quick test with drm_info [1] and VKMS is not listed after moving
it to the faux bus. As far as I can tell, the reason is that drmGetDevices()
doesn't return the devices in the faux bus.

However, as drm_info is designed to list information as close as it is provided
by libdrm, I'm not sure if this should be considered a regressions or not.

Just for reference, Mutter handles the bus change correctly, and it uses
udev [2]. I think I should do something similar in my IGT tests and use udev
instead of drmGetDevices().

[1] https://gitlab.freedesktop.org/emersion/drm_info
[2] https://gitlab.gnome.org/GNOME/mutter/-/blob/main/src/backends/meta-udev.c?ref_type=heads#L174

> With a platform device, the drivers appear under /sys/bus/platform, but with
> faux device, they appear under /sys/bus/faux.
>
> I ask because I found out that one (see my reply to [2]) of the main drm
> library expects to find all the devices under pci, usb, platform, virtio and
> host1x buses [1], so at least for the vgem and vkms driver, this library
> will be broken (it will not crash, but previously detected devices will
> suddenly disappear).
>
> I don't know what are the rules for /sys/bus, but changing a device from one
> bus to another seems to break userspace programs. How should we handle this
> situation? Should we fix the existing drivers? Or only new drivers should
> use it?
>
> +CC: José Expósito
>
> Thanks,
> Louis Chauvet
>
> [1]:https://gitlab.freedesktop.org/mesa/drm/-/blob/main/xf86drm.c#L4460-4515
> [2]:https://lore.kernel.org/all/20250218165011.9123-21-jose.exposito89@gmail.com/
>
> > Changes from v4:
> > - really dropped the internal name structure, remanants were left over
> > from the last patch series
> > - added the rust binding patch from Lyude (is this one of the first
> > patch series that adds a new kernel api AND the rust binding at the
> > same time?)
> > - added a parent pointer to the api so the devices can be in the tree
> > if the caller wants them
> > - made probe synchronous to prevent race when using the api (when the
> > create call returns the device is fully ready to go.) Thanks to
> > testing of the drm driver change to find this issue.
> > - documentation tweaks
> > - #include <linux/container_of.h> finally added to faux.h
> >
> >
> > Changes from v3:
> > - Dropped the USB phy porting, turned out to be incorrect, it really
> > did need a platform device
> > - converted more drivers to the faux_device api (tlclk, lis3lv02d,
> > vgem, and vkms)
> > - collected some reviewed-by
> > - lots of minor tweaks of the faux.c api, and documentation based on
> > review, see the changelog in patch 1 for details.
> >
> > Changes from v2:
> > - lots of cleanups to faux.c based on reviews, see patch 1 for details
> > - actually tested the destroy device path, it worked first try!
> > - added 3 more example drivers
> >
> >
> >
> > Greg Kroah-Hartman (8):
> > driver core: add a faux bus for use when a simple device/bus is needed
> > regulator: dummy: convert to use the faux device interface
> > x86/microcode: move away from using a fake platform device
> > wifi: cfg80211: move away from using a fake platform device
> > tlclk: convert to use faux_device
> > misc: lis3lv02d: convert to use faux_device
> > drm/vgem/vgem_drv convert to use faux_device
> > drm/vkms: convert to use faux_device

In regard of the VKMS patch, in Documentation/gpu/vkms.rst, "IGT_DEVICE" needs
to be updated to point to the new path. It can be done in this series or I can
send a follow up patch.

Best wishes,
Jose

> >
> > Lyude Paul (1):
> > rust/kernel: Add faux device bindings
> >
> > Documentation/driver-api/infrastructure.rst | 6 +
> > MAINTAINERS | 2 +
> > arch/x86/kernel/cpu/microcode/core.c | 14 +-
> > drivers/base/Makefile | 2 +-
> > drivers/base/base.h | 1 +
> > drivers/base/faux.c | 232 ++++++++++++++++++++
> > drivers/base/init.c | 1 +
> > drivers/char/tlclk.c | 32 +--
> > drivers/gpu/drm/vgem/vgem_drv.c | 30 +--
> > drivers/gpu/drm/vkms/vkms_drv.c | 28 +--
> > drivers/gpu/drm/vkms/vkms_drv.h | 4 +-
> > drivers/misc/lis3lv02d/lis3lv02d.c | 26 +--
> > drivers/misc/lis3lv02d/lis3lv02d.h | 4 +-
> > drivers/regulator/dummy.c | 37 +---
> > include/linux/device/faux.h | 69 ++++++
> > net/wireless/reg.c | 28 +--
> > rust/bindings/bindings_helper.h | 1 +
> > rust/kernel/faux.rs | 67 ++++++
> > rust/kernel/lib.rs | 1 +
> > samples/rust/Kconfig | 10 +
> > samples/rust/Makefile | 1 +
> > samples/rust/rust_driver_faux.rs | 29 +++
> > 22 files changed, 502 insertions(+), 123 deletions(-)
> > create mode 100644 drivers/base/faux.c
> > create mode 100644 include/linux/device/faux.h
> > create mode 100644 rust/kernel/faux.rs
> > create mode 100644 samples/rust/rust_driver_faux.rs
> >
>
> --
> Louis Chauvet, Bootlin
> Embedded Linux and Kernel engineering
> https://bootlin.com
>