Re: [Intel-gfx] [PATCH v3 4/4] PCI/VGA: introduce is_boot_device function callback to vga_client_register

[Bjorn Helgaas]

On Thu, Jun 08, 2023 at 07:43:22PM +0800, Sui Jingfeng wrote:
> From: Sui Jingfeng <suijingfeng@xxxxxxxxxxx>
> 
> The vga_is_firmware_default() function is arch-dependent, which doesn't
> sound right. At least, it also works on the Mips and LoongArch platforms.
> Tested with the drm/amdgpu and drm/radeon drivers. However, it's difficult
> to enumerate all arch-driver combinations. I'm wrong if there is only one
> exception.
> 
> With the observation that device drivers typically have better knowledge
> about which PCI bar contains the firmware framebuffer, which could avoid
> the need to iterate all of the PCI BARs.
> 
> But as a PCI function at pci/vgaarb.c, vga_is_firmware_default() is
> probably not suitable to make such an optimization for a specific device.
> 
> There are PCI display controllers that don't have a dedicated VRAM bar,
> this function will lose its effectiveness in such a case. Luckily, the
> device driver can provide an accurate workaround.
> 
> Therefore, this patch introduces a callback that allows the device driver
> to tell the VGAARB if the device is the default boot device. This patch
> only intends to introduce the mechanism, while the implementation is left
> to the device driver authors. Also honor the comment: "Clients have two
> callback mechanisms they can use"

s/bar/BAR/ (several)

Nothing here uses the callback.  I don't want to merge this until we
have a user.

I'm not sure why the device driver should know whether its device is
the default boot device.

> Signed-off-by: Sui Jingfeng <suijingfeng@xxxxxxxxxxx>
> ---
>  drivers/gpu/drm/amd/amdgpu/amdgpu_device.c |  2 +-
>  drivers/gpu/drm/i915/display/intel_vga.c   |  3 +--
>  drivers/gpu/drm/nouveau/nouveau_vga.c      |  2 +-
>  drivers/gpu/drm/radeon/radeon_device.c     |  2 +-
>  drivers/pci/vgaarb.c                       | 22 ++++++++++++++++++----
>  drivers/vfio/pci/vfio_pci_core.c           |  2 +-
>  include/linux/vgaarb.h                     |  8 +++++---
>  7 files changed, 28 insertions(+), 13 deletions(-)
> 
> diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_device.c b/drivers/gpu/drm/amd/amdgpu/amdgpu_device.c
> index 5c7d40873ee2..7a096f2d5c16 100644
> --- a/drivers/gpu/drm/amd/amdgpu/amdgpu_device.c
> +++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_device.c
> @@ -3960,7 +3960,7 @@ int amdgpu_device_init(struct amdgpu_device *adev,
>  	/* this will fail for cards that aren't VGA class devices, just
>  	 * ignore it */
>  	if ((adev->pdev->class >> 8) == PCI_CLASS_DISPLAY_VGA)
> -		vga_client_register(adev->pdev, amdgpu_device_vga_set_decode);
> +		vga_client_register(adev->pdev, amdgpu_device_vga_set_decode, NULL);
>  
>  	px = amdgpu_device_supports_px(ddev);
>  
> diff --git a/drivers/gpu/drm/i915/display/intel_vga.c b/drivers/gpu/drm/i915/display/intel_vga.c
> index 286a0bdd28c6..98d7d4dffe9f 100644
> --- a/drivers/gpu/drm/i915/display/intel_vga.c
> +++ b/drivers/gpu/drm/i915/display/intel_vga.c
> @@ -115,7 +115,6 @@ intel_vga_set_decode(struct pci_dev *pdev, bool enable_decode)
>  
>  int intel_vga_register(struct drm_i915_private *i915)
>  {
> -
>  	struct pci_dev *pdev = to_pci_dev(i915->drm.dev);
>  	int ret;
>  
> @@ -127,7 +126,7 @@ int intel_vga_register(struct drm_i915_private *i915)
>  	 * then we do not take part in VGA arbitration and the
>  	 * vga_client_register() fails with -ENODEV.
>  	 */
> -	ret = vga_client_register(pdev, intel_vga_set_decode);
> +	ret = vga_client_register(pdev, intel_vga_set_decode, NULL);
>  	if (ret && ret != -ENODEV)
>  		return ret;
>  
> diff --git a/drivers/gpu/drm/nouveau/nouveau_vga.c b/drivers/gpu/drm/nouveau/nouveau_vga.c
> index f8bf0ec26844..162b4f4676c7 100644
> --- a/drivers/gpu/drm/nouveau/nouveau_vga.c
> +++ b/drivers/gpu/drm/nouveau/nouveau_vga.c
> @@ -92,7 +92,7 @@ nouveau_vga_init(struct nouveau_drm *drm)
>  		return;
>  	pdev = to_pci_dev(dev->dev);
>  
> -	vga_client_register(pdev, nouveau_vga_set_decode);
> +	vga_client_register(pdev, nouveau_vga_set_decode, NULL);
>  
>  	/* don't register Thunderbolt eGPU with vga_switcheroo */
>  	if (pci_is_thunderbolt_attached(pdev))
> diff --git a/drivers/gpu/drm/radeon/radeon_device.c b/drivers/gpu/drm/radeon/radeon_device.c
> index afbb3a80c0c6..71f2ff39d6a1 100644
> --- a/drivers/gpu/drm/radeon/radeon_device.c
> +++ b/drivers/gpu/drm/radeon/radeon_device.c
> @@ -1425,7 +1425,7 @@ int radeon_device_init(struct radeon_device *rdev,
>  	/* if we have > 1 VGA cards, then disable the radeon VGA resources */
>  	/* this will fail for cards that aren't VGA class devices, just
>  	 * ignore it */
> -	vga_client_register(rdev->pdev, radeon_vga_set_decode);
> +	vga_client_register(rdev->pdev, radeon_vga_set_decode, NULL);
>  
>  	if (rdev->flags & RADEON_IS_PX)
>  		runtime = true;
> diff --git a/drivers/pci/vgaarb.c b/drivers/pci/vgaarb.c
> index b0bf4952a95d..d3dab61e0ef2 100644
> --- a/drivers/pci/vgaarb.c
> +++ b/drivers/pci/vgaarb.c
> @@ -53,6 +53,7 @@ struct vga_device {
>  	bool bridge_has_one_vga;
>  	bool is_firmware_default;	/* device selected by firmware */
>  	unsigned int (*set_decode)(struct pci_dev *pdev, bool decode);
> +	bool (*is_boot_device)(struct pci_dev *pdev);
>  };
>  
>  static LIST_HEAD(vga_list);
> @@ -614,10 +615,17 @@ static bool vga_is_boot_device(struct vga_device *vgadev)
>  	if (boot_vga && boot_vga->is_firmware_default)
>  		return false;
>  
> -	if (vga_is_firmware_default(pdev)) {
> -		vgadev->is_firmware_default = true;
> +	/*
> +	 * Ask the device driver first, if registered. Fallback to the
> +	 * default implement if the callback is non-exist.
> +	 */
> +	if (vgadev->is_boot_device)
> +		vgadev->is_firmware_default = vgadev->is_boot_device(pdev);
> +	else
> +		vgadev->is_firmware_default = vga_is_firmware_default(pdev);
> +
> +	if (vgadev->is_firmware_default)
>  		return true;
> -	}
>  
>  	/*
>  	 * A legacy VGA device has MEM and IO enabled and any bridges
> @@ -954,6 +962,10 @@ EXPORT_SYMBOL(vga_set_legacy_decoding);
>   * @set_decode callback: If a client can disable its GPU VGA resource, it
>   * will get a callback from this to set the encode/decode state.
>   *
> + * @is_boot_device: callback to the device driver, query if a client is the
> + * default boot device, as the device driver typically has better knowledge
> + * if specific device is the boot device. But this callback is optional.
> + *
>   * Rationale: we cannot disable VGA decode resources unconditionally, some
>   * single GPU laptops seem to require ACPI or BIOS access to the VGA registers
>   * to control things like backlights etc. Hopefully newer multi-GPU laptops do
> @@ -969,7 +981,8 @@ EXPORT_SYMBOL(vga_set_legacy_decoding);
>   * Returns: 0 on success, -1 on failure
>   */
>  int vga_client_register(struct pci_dev *pdev,
> -		unsigned int (*set_decode)(struct pci_dev *pdev, bool decode))
> +		unsigned int (*set_decode)(struct pci_dev *pdev, bool decode),
> +		bool (*is_boot_device)(struct pci_dev *pdev))
>  {
>  	int ret = -ENODEV;
>  	struct vga_device *vgadev;
> @@ -981,6 +994,7 @@ int vga_client_register(struct pci_dev *pdev,
>  		goto bail;
>  
>  	vgadev->set_decode = set_decode;
> +	vgadev->is_boot_device = is_boot_device;
>  	ret = 0;
>  
>  bail:
> diff --git a/drivers/vfio/pci/vfio_pci_core.c b/drivers/vfio/pci/vfio_pci_core.c
> index a5ab416cf476..2a8873a330ba 100644
> --- a/drivers/vfio/pci/vfio_pci_core.c
> +++ b/drivers/vfio/pci/vfio_pci_core.c
> @@ -2067,7 +2067,7 @@ static int vfio_pci_vga_init(struct vfio_pci_core_device *vdev)
>  	if (ret)
>  		return ret;
>  
> -	ret = vga_client_register(pdev, vfio_pci_set_decode);
> +	ret = vga_client_register(pdev, vfio_pci_set_decode, NULL);
>  	if (ret)
>  		return ret;
>  	vga_set_legacy_decoding(pdev, vfio_pci_set_decode(pdev, false));
> diff --git a/include/linux/vgaarb.h b/include/linux/vgaarb.h
> index d36225c582ee..66fe80ffad76 100644
> --- a/include/linux/vgaarb.h
> +++ b/include/linux/vgaarb.h
> @@ -50,7 +50,8 @@ struct pci_dev *vga_default_device(void);
>  void vga_set_default_device(struct pci_dev *pdev);
>  int vga_remove_vgacon(struct pci_dev *pdev);
>  int vga_client_register(struct pci_dev *pdev,
> -		unsigned int (*set_decode)(struct pci_dev *pdev, bool state));
> +		unsigned int (*set_decode)(struct pci_dev *pdev, bool state),
> +		bool (*is_boot_device)(struct pci_dev *pdev));
>  #else /* CONFIG_VGA_ARB */
>  static inline void vga_set_legacy_decoding(struct pci_dev *pdev,
>  		unsigned int decodes)
> @@ -76,7 +77,8 @@ static inline int vga_remove_vgacon(struct pci_dev *pdev)
>  	return 0;
>  }
>  static inline int vga_client_register(struct pci_dev *pdev,
> -		unsigned int (*set_decode)(struct pci_dev *pdev, bool state))
> +		unsigned int (*set_decode)(struct pci_dev *pdev, bool state),
> +		bool (*is_boot_device)(struct pci_dev *pdev))
>  {
>  	return 0;
>  }
> @@ -114,7 +116,7 @@ static inline int vga_get_uninterruptible(struct pci_dev *pdev,
>  
>  static inline void vga_client_unregister(struct pci_dev *pdev)
>  {
> -	vga_client_register(pdev, NULL);
> +	vga_client_register(pdev, NULL, NULL);
>  }
>  
>  #endif /* LINUX_VGA_H */
> -- 
> 2.25.1
>