linux-kernel - Re: [-next 1/5] PCI: Add the pci_is

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Message-ID: <42a15522-7bed-c5b1-1dc3-65446b65e348@loongson.cn>
Date:   Fri, 6 Oct 2023 19:40:44 +0800
From:   Sui Jingfeng <suijingfeng@...ngson.cn>
To:     Bjorn Helgaas <helgaas@...nel.org>,
        Sui Jingfeng <sui.jingfeng@...ux.dev>
Cc:     Bjorn Helgaas <bhelgaas@...gle.com>,
        Gerd Hoffmann <kraxel@...hat.com>,
        Gurchetan Singh <gurchetansingh@...omium.org>,
        Chia-I Wu <olvaffe@...il.com>, linux-pci@...r.kernel.org,
        linux-kernel@...r.kernel.org, dri-devel@...ts.freedesktop.org,
        virtualization@...ts.linux-foundation.org,
        "Maciej W. Rozycki" <macro@...am.me.uk>
Subject: Re: [-next 1/5] PCI: Add the pci_is_vga() helper

Hi,


On 2023/10/6 06:51, Bjorn Helgaas wrote:
> On Wed, Aug 30, 2023 at 07:15:28PM +0800, Sui Jingfeng wrote:
>> From: Sui Jingfeng <suijingfeng@...ngson.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@...am.me.uk>
>> Signed-off-by: Sui Jingfeng <suijingfeng@...ngson.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?

No, I write it according to the spec.
But I'm sure that the boot_vga will not be shown at sysfs for a PCI_CLASS_NOT_DEFINED_VGA device.


> 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.


Maciej mentioned that PCI_CLASS_NOT_DEFINED_VGA device should also be handled in the past.
see [1]. But if no one interested in PCI_CLASS_NOT_DEFINED_VGA nowaday, then I guess
the gains of this patch may not deserve the time and risk. But I don't mind if someone
would like pick it up for other purpose.

Thanks for the reviewing. :-)

[1] https://lkml.org/lkml/2023/6/18/315


>> +	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
>>