Re: [PATCH v2 1/2] usb: xhci-pci: add generic auxiliary device interface

[Michal Pecio <michal.pecio@gmail.com>]

On Thu,  7 May 2026 05:40:33 +0900, Jihong Min wrote:
> Some xHCI PCI controllers expose controller-specific functionality that is
> not part of generic xHCI operation and is better handled by optional child
> drivers in other subsystems. Add a small auxiliary device registration path
> for selected xHCI PCI controllers.
> 
> The initial table creates an xhci_pci.hwmon auxiliary device for AMD
> 1022:43fd controllers. Store the created auxiliary device in devres so the
> xhci-pci remove path destroys it before HCD teardown. Use a
> PCI-domain-qualified id so auxiliary device names remain unique across PCI
> domains.
> 
> This keeps xhci-pci responsible only for publishing selected controller
> functions while allowing subsystem-specific drivers to bind through the
> auxiliary bus.
> 
> Assisted-by: Codex:gpt-5.5
> Signed-off-by: Jihong Min <hurryman2212@gmail.com>
> ---
>  drivers/usb/host/Kconfig    |  10 ++++
>  drivers/usb/host/xhci-pci.c | 114 ++++++++++++++++++++++++++++++++++++
>  2 files changed, 124 insertions(+)
> 
> diff --git a/drivers/usb/host/Kconfig b/drivers/usb/host/Kconfig
> index 0a277a07cf70..e0c2c7ac5c97 100644
> --- a/drivers/usb/host/Kconfig
> +++ b/drivers/usb/host/Kconfig
> @@ -42,6 +42,16 @@ config USB_XHCI_PCI
>  	depends on USB_PCI
>  	default y
>  
> +config USB_XHCI_PCI_AUXDEV
> +	bool "xHCI PCI auxiliary device support"
> +	depends on USB_XHCI_PCI
> +	select AUXILIARY_BUS
> +	help
> +	  This enables xHCI PCI support for registering auxiliary devices
> +	  for selected controllers. It is used by optional child drivers
> +	  that bind to xHCI PCI controller-specific functionality through
> +	  the auxiliary bus.
> +
>  config USB_XHCI_PCI_RENESAS
>  	tristate "Support for additional Renesas xHCI controller with firmware"
>  	depends on USB_XHCI_PCI
> diff --git a/drivers/usb/host/xhci-pci.c b/drivers/usb/host/xhci-pci.c
> index 585b2f3117b0..1ab27d2182eb 100644
> --- a/drivers/usb/host/xhci-pci.c
> +++ b/drivers/usb/host/xhci-pci.c
> @@ -8,6 +8,8 @@
>   * Some code borrowed from the Linux EHCI driver.
>   */
>  
> +#include <linux/auxiliary_bus.h>
> +#include <linux/device/devres.h>
>  #include <linux/pci.h>
>  #include <linux/slab.h>
>  #include <linux/module.h>
> @@ -103,6 +105,114 @@ static int xhci_pci_run(struct usb_hcd *hcd);
>  static int xhci_pci_update_hub_device(struct usb_hcd *hcd, struct usb_device *hdev,
>  				      struct usb_tt *tt, gfp_t mem_flags);
>  
> +#if IS_ENABLED(CONFIG_USB_XHCI_PCI_AUXDEV)
> +static const struct {
> +	struct pci_device_id id;
> +	const char *aux_dev_name;
> +} pci_ids_have_aux[] = {
> +	{
> +		.id = { PCI_DEVICE(PCI_VENDOR_ID_AMD, 0x43fd) }, /* PROM21 xHCI */
> +		.aux_dev_name = "hwmon",
> +	},
> +	{ /* end: all zeroes */ }
> +};
> +
> +struct xhci_pci_aux_devres {
> +	struct auxiliary_device *auxdev;
> +};
> +
> +static bool xhci_pci_aux_match_id(const struct pci_device_id *id,
> +				  const struct pci_dev *pdev)
> +{
> +	if (id->vendor != PCI_ANY_ID && id->vendor != pdev->vendor)
> +		return false;
> +	if (id->device != PCI_ANY_ID && id->device != pdev->device)
> +		return false;
> +	if (id->subvendor != PCI_ANY_ID &&
> +	    id->subvendor != pdev->subsystem_vendor)
> +		return false;
> +	if (id->subdevice != PCI_ANY_ID &&
> +	    id->subdevice != pdev->subsystem_device)
> +		return false;
> +
> +	return !((id->class ^ pdev->class) & id->class_mask);
> +}
> +
> +static const char *xhci_pci_aux_dev_name(struct pci_dev *pdev)
> +{
> +	int i;
> +
> +	for (i = 0; pci_ids_have_aux[i].aux_dev_name; i++) {
> +		if (xhci_pci_aux_match_id(&pci_ids_have_aux[i].id, pdev))
> +			return pci_ids_have_aux[i].aux_dev_name;
> +	}
> +
> +	return NULL;
> +}
> +
> +static void xhci_pci_aux_devres_release(struct device *dev, void *res)
> +{
> +	struct xhci_pci_aux_devres *devres = res;
> +
> +	if (devres->auxdev)
> +		auxiliary_device_destroy(devres->auxdev);
> +}
> +
> +static void xhci_pci_try_add_aux_device(struct pci_dev *pdev)
> +{
> +	struct xhci_pci_aux_devres *devres;
> +	struct auxiliary_device *auxdev;
> +	const char *aux_dev_name;
> +
> +	aux_dev_name = xhci_pci_aux_dev_name(pdev);
> +	if (!aux_dev_name)
> +		return;
> +
> +	devres = devres_alloc(xhci_pci_aux_devres_release, sizeof(*devres),
> +			      GFP_KERNEL);
> +	if (!devres) {
> +		dev_warn(&pdev->dev,
> +			 "failed to allocate auxiliary device state\n");
> +		return;
> +	}
> +
> +	auxdev = auxiliary_device_create(&pdev->dev, KBUILD_MODNAME,
> +					 aux_dev_name, NULL,
> +					 (pci_domain_nr(pdev->bus) << 16) |
> +						 pci_dev_id(pdev));
> +	if (!auxdev) {
> +		devres_free(devres);
> +		dev_warn(&pdev->dev, "failed to add %s auxiliary device\n",
> +			 aux_dev_name);
> +		return;
> +	}
> +
> +	devres->auxdev = auxdev;
> +	devres_add(&pdev->dev, devres);
> +}
> +
> +static void xhci_pci_try_remove_aux_device(struct pci_dev *pdev)
> +{
> +	struct xhci_pci_aux_devres *devres;
> +
> +	devres = devres_find(&pdev->dev, xhci_pci_aux_devres_release, NULL,
> +			     NULL);
> +	if (!devres || !devres->auxdev)
> +		return;
> +
> +	auxiliary_device_destroy(devres->auxdev);
> +	devres->auxdev = NULL;
> +}
> +#else
> +static inline void xhci_pci_try_add_aux_device(struct pci_dev *pdev)
> +{
> +}
> +
> +static inline void xhci_pci_try_remove_aux_device(struct pci_dev *pdev)
> +{
> +}
> +#endif
> +
>  static const struct xhci_driver_overrides xhci_pci_overrides __initconst = {
>  	.reset = xhci_pci_setup,
>  	.start = xhci_pci_run,
> @@ -677,6 +787,8 @@ int xhci_pci_common_probe(struct pci_dev *dev, const struct pci_device_id *id)
>  	if (device_property_read_bool(&dev->dev, "ti,pwron-active-high"))
>  		pci_clear_and_set_config_dword(dev, 0xE0, 0, 1 << 22);
>  
> +	xhci_pci_try_add_aux_device(dev);

Is it considered acceptable to add

	if (IS_ENABLED(CONFIG_USB_XHCI_PCI_AUXDEV))

before this call, remove #ifdef around the definitions of the auxdev
functions and rely on dead code elimination to clean them up?

We already have a similar trick with CONFIG_USB_XHCI_PCI_RENESAS here
and it seemed to be working, though the amount of conditional code is
much lower and so is the potential risk.

The reason for doing it this way was because Greg doesn't like #ifdefs
in .c files, and neither do static analyzers like them.

> +
>  	return 0;
>  
>  put_usb3_hcd:
> @@ -713,6 +825,8 @@ void xhci_pci_remove(struct pci_dev *dev)
>  	xhci = hcd_to_xhci(pci_get_drvdata(dev));
>  	set_power_d3 = xhci->quirks & XHCI_SPURIOUS_WAKEUP;
>  
> +	xhci_pci_try_remove_aux_device(dev);
> +
>  	xhci->xhc_state |= XHCI_STATE_REMOVING;
>  
>  	if (pci_choose_state(dev, PMSG_SUSPEND) == PCI_D0)
> -- 
> 2.53.0
>