Re: [PATCH v3 4/9] PCI/VGA: Improve the default VGA device selection

[Bjorn Helgaas <helgaas@kernel.org>]

[+cc linux-pci (please cc in the future since the bulk of this patch
is in drivers/pci/)]

On Wed, Jul 12, 2023 at 12:43:05AM +0800, Sui Jingfeng wrote:
> From: Sui Jingfeng <suijingfeng@loongson.cn>
> 
> Currently, the strategy of selecting the default boot on a multiple video
> card coexistence system is not perfect. Potential problems are:
> 
> 1) This function is a no-op on non-x86 architectures.

Which function in particular is a no-op for non-x86?

> 2) It does not take the PCI Bar may get relocated into consideration.
> 3) It is not effective for the PCI device without a dedicated VRAM Bar.
> 4) It is device-agnostic, thus it has to waste the effort to iterate all
>    of the PCI Bar to find the VRAM aperture.
> 5) It has invented lots of methods to determine which one is the default
>    boot device, but this is still a policy because it doesn't give the
>    user a choice to override.

I don't think we need a list of *potential* problems.  We need an
example of the specific problem this will solve, i.e., what currently
does not work?

The drm/ast and maybe drm/loongson patches are the only ones that use
the new callback, so I assume there are real problems with those
drivers.

CONFIG_DRM_AST is a tristate.  We're talking about identifying the
boot-time console device.  So if CONFIG_DRM_AST=m, I guess we don't
get the benefit of the new callback unless the module gets loaded?

> Also honor the comment: "Clients have *TWO* callback mechanisms they
> can use"

This refers to the existing vga_client_register() function comment:

   * vga_client_register - register or unregister a VGA arbitration client
   * @pdev: pci device of the VGA client
   * @set_decode: vga decode change callback
   *
   * Clients have two callback mechanisms they can use.
   *
   * @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.

and the fact that struct vga_device currently only contains *one*
callback function pointer:

  unsigned int (*set_decode)(struct pci_dev *pdev, bool decode);

Adding the .is_primary_gpu() callback does mean there will now be two
callbacks, as the comment says, but I think it's just confusing to
mention this in the commit log, so I would just remove it.

> @@ -1509,13 +1543,24 @@ static int pci_notify(struct notifier_block *nb, unsigned long action,
>  	 * cases of hotplugable vga cards.
>  	 */
>  
> -	if (action == BUS_NOTIFY_ADD_DEVICE)
> +	switch (action) {
> +	case BUS_NOTIFY_ADD_DEVICE:
>  		notify = vga_arbiter_add_pci_device(pdev);
> -	else if (action == BUS_NOTIFY_DEL_DEVICE)
> +		if (notify)
> +			vga_arbiter_notify_clients();
> +		break;
> +	case BUS_NOTIFY_DEL_DEVICE:
>  		notify = vga_arbiter_del_pci_device(pdev);
> +		if (notify)
> +			vga_arbiter_notify_clients();
> +		break;
> +	case BUS_NOTIFY_BOUND_DRIVER:
> +		vga_arbiter_do_arbitration(pdev);
> +		break;
> +	default:
> +		break;
> +	}

Changing from if/else to switch makes the patch bigger than necessary
for no real benefit and obscures what is really changing.

Bjorn