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Date:   Thu, 5 Oct 2023 17:51:01 -0500
From:   Bjorn Helgaas <helgaas@kernel.org>
To:     Sui Jingfeng <sui.jingfeng@linux.dev>
Cc:     Bjorn Helgaas <bhelgaas@google.com>,
        Gerd Hoffmann <kraxel@redhat.com>,
        Gurchetan Singh <gurchetansingh@chromium.org>,
        Chia-I Wu <olvaffe@gmail.com>,
        Sui Jingfeng <suijingfeng@loongson.cn>,
        linux-pci@vger.kernel.org, linux-kernel@vger.kernel.org,
        dri-devel@lists.freedesktop.org,
        virtualization@lists.linux-foundation.org,
        "Maciej W. Rozycki" <macro@orcam.me.uk>
Subject: Re: [-next 1/5] PCI: Add the pci_is_vga() helper
Message-ID: <20231005225101.GA792747@bhelgaas>
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On Wed, Aug 30, 2023 at 07:15:28PM +0800, Sui Jingfeng wrote:
> From: Sui Jingfeng <suijingfeng@loongson.cn>
> 
> The PCI code and ID assignment specification defined four types of
> display controllers for the display base class(03h), and the devices
> with 0x00h sub-class code are VGA devices. VGA devices with programming

I can update this with the spec details (PCI Code and Assignment spec
r1.15, secs 1.1 and 1.4).

> interface 0x00 is VGA-compatible, VGA devices with programming interface
> 0x01 are 8514-compatible controllers. Besides, PCI_CLASS_NOT_DEFINED_VGA
> is defined to provide backward compatibility for devices that were built
> before the class code field was defined. Hence, introduce the pci_is_vga()
> helper, let it handle the details for us. It returns true if the PCI(e)
> device being tested belongs to the VGA devices category.
>
> Cc: "Maciej W. Rozycki" <macro@orcam.me.uk>
> Signed-off-by: Sui Jingfeng <suijingfeng@loongson.cn>
> ---
>  include/linux/pci.h | 27 +++++++++++++++++++++++++++
>  1 file changed, 27 insertions(+)
> 
> diff --git a/include/linux/pci.h b/include/linux/pci.h
> index cf6e0b057752..ace727001911 100644
> --- a/include/linux/pci.h
> +++ b/include/linux/pci.h
> @@ -713,6 +713,33 @@ static inline bool pci_is_bridge(struct pci_dev *dev)
>  		dev->hdr_type == PCI_HEADER_TYPE_CARDBUS;
>  }
>  
> +/**
> + * The PCI code and ID assignment specification defined four types of
> + * display controllers for the display base class(03h), and the devices
> + * with 0x00h sub-class code are VGA devices. VGA devices with programming
> + * interface 0x00 is VGA-compatible, VGA devices with programming interface
> + * 0x01 are 8514-compatible controllers. Besides, PCI_CLASS_NOT_DEFINED_VGA
> + * is defined to provide backward compatibility for devices that were built
> + * before the class code field was defined. This means that it belong to the
> + * VGA devices category also.
> + *
> + * Returns:
> + * true if the PCI device is a VGA device, false otherwise.
> + */
> +static inline bool pci_is_vga(struct pci_dev *pdev)
> +{
> +	if (!pdev)
> +		return false;
> +
> +	if ((pdev->class >> 8) == PCI_CLASS_DISPLAY_VGA)
> +		return true;
> +
> +	if ((pdev->class >> 8) == PCI_CLASS_NOT_DEFINED_VGA)
> +		return true;

Are you seeing a problem that will be fixed by this series, i.e., a
PCI_CLASS_NOT_DEFINED_VGA device that we currently don't handle
correctly?

I think this makes sense per the spec, but there's always a risk of
breaking something, so it's nice if the change actually *fixes*
something to make that risk worthwhile.

> +	return false;
> +}
> +
>  #define for_each_pci_bridge(dev, bus)				\
>  	list_for_each_entry(dev, &bus->devices, bus_list)	\
>  		if (!pci_is_bridge(dev)) {} else
> -- 
> 2.34.1
>