Message-ID: <3c1c86ab-96ea-aa1c-c9c5-9a4012644fd6@loongson.cn>
Date:   Fri, 16 Jun 2023 15:11:07 +0800
From:   Sui Jingfeng <suijingfeng@...ngson.cn>
To:     Alex Deucher <alexdeucher@...il.com>,
        Thomas Zimmermann <tzimmermann@...e.de>
Cc:     Sui Jingfeng <15330273260@....cn>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        linux-fbdev@...r.kernel.org, kvm@...r.kernel.org,
        nouveau@...ts.freedesktop.org, intel-gfx@...ts.freedesktop.org,
        linux-kernel@...r.kernel.org, dri-devel@...ts.freedesktop.org,
        amd-gfx@...ts.freedesktop.org, linux-pci@...r.kernel.org
Subject: Re: [PATCH v7 2/8] PCI/VGA: Deal only with VGA class devices

Hi,

On 2023/6/16 05:11, Alex Deucher wrote:
> On Wed, Jun 14, 2023 at 6:50 AM Sui Jingfeng <suijingfeng@...ngson.cn> wrote:
>> Hi,
>>
>> On 2023/6/13 11:01, Sui Jingfeng wrote:
>>> From: Sui Jingfeng <suijingfeng@...ngson.cn>
>>>
>>> Deal only with the VGA devcie(pdev->class == 0x0300), so replace the
>>> pci_get_subsys() function with pci_get_class(). Filter the non-PCI display
>>> device(pdev->class != 0x0300) out. There no need to process the non-display
>>> PCI device.
>>>
>>> Cc: Bjorn Helgaas <bhelgaas@...gle.com>
>>> Signed-off-by: Sui Jingfeng <suijingfeng@...ngson.cn>
>>> ---
>>>    drivers/pci/vgaarb.c | 22 ++++++++++++----------
>>>    1 file changed, 12 insertions(+), 10 deletions(-)
>>>
>>> diff --git a/drivers/pci/vgaarb.c b/drivers/pci/vgaarb.c
>>> index c1bc6c983932..22a505e877dc 100644
>>> --- a/drivers/pci/vgaarb.c
>>> +++ b/drivers/pci/vgaarb.c
>>> @@ -754,10 +754,6 @@ static bool vga_arbiter_add_pci_device(struct pci_dev *pdev)
>>>        struct pci_dev *bridge;
>>>        u16 cmd;
>>>
>>> -     /* Only deal with VGA class devices */
>>> -     if ((pdev->class >> 8) != PCI_CLASS_DISPLAY_VGA)
>>> -             return false;
>>> -
>> Hi, here is probably a bug fixing.
>>
>> For an example, nvidia render only GPU typically has 0x0380.
>>
>> as its PCI class number, but render only GPU should not participate in
>> the arbitration.
>>
>> As it shouldn't snoop the legacy fixed VGA address.
>>
>> It(render only GPU) can not display anything.
>>
>>
>> But 0x0380 >> 8 = 0x03, the filter  failed.
>>
>>
>>>        /* Allocate structure */
>>>        vgadev = kzalloc(sizeof(struct vga_device), GFP_KERNEL);
>>>        if (vgadev == NULL) {
>>> @@ -1500,7 +1496,9 @@ static int pci_notify(struct notifier_block *nb, unsigned long action,
>>>        struct pci_dev *pdev = to_pci_dev(dev);
>>>        bool notify = false;
>>>
>>> -     vgaarb_dbg(dev, "%s\n", __func__);
>>> +     /* Only deal with VGA class devices */
>>> +     if (pdev->class != PCI_CLASS_DISPLAY_VGA << 8)
>>> +             return 0;
>> So here we only care 0x0300, my initial intent is to make an optimization,
>>
>> nowadays sane display graphic card should all has 0x0300 as its PCI
>> class number, is this complete right?
>>
>> ```
>>
>> #define PCI_BASE_CLASS_DISPLAY        0x03
>> #define PCI_CLASS_DISPLAY_VGA        0x0300
>> #define PCI_CLASS_DISPLAY_XGA        0x0301
>> #define PCI_CLASS_DISPLAY_3D        0x0302
>> #define PCI_CLASS_DISPLAY_OTHER        0x0380
>>
>> ```
>>
>> Any ideas ?
> I'm not quite sure what you are asking about here.

To be honest, I'm worried about the PCI devices which has a

PCI_CLASS_DISPLAY_XGA as its PCI class number.

As those devices are very uncommon in the real world.


$ find . -name "*.c" -type f | xargs grep "PCI_CLASS_DISPLAY_XGA"


Grep the "PCI_CLASS_DISPLAY_XGA" in the linux kernel tree got ZERO,

there no code reference this macro. So I think it seems safe to ignore 
the XGA ?


PCI_CLASS_DISPLAY_3D and PCI_CLASS_DISPLAY_OTHER are used to annotate 
the render-only GPU.

And render-only GPU can't decode the fixed VGA address space, it is safe 
to ignore them.


>   For vga_arb, we
> only care about VGA class devices since those should be on the only
> ones that might have VGA routed to them.

>   However, as VGA gets deprecated,

We need the vgaarb for a system with multiple video card.

Not only because some Legacy VGA devices implemented

on PCI will typically have the same "hard-decoded" addresses;

But also these video card need to participate in the arbitration,

determine the default boot device.


Nowadays, the 'VGA devices' here is stand for the Graphics card

which is capable of display something on the screen.

We still need vgaarb to select the default boot device.


> you'll have more non VGA PCI classes for devices which
> could be the pre-OS console device.

Ah, we still want  do this(by applying this patch) first,

and then we will have the opportunity to see who will crying if 
something is broken. Will know more then.

But drop this patch or revise it with more consideration is also 
acceptable.


I asking about suggestion and/or review.

> Alex
>
>>>        /* For now we're only intereted in devices added and removed. I didn't
>>>         * test this thing here, so someone needs to double check for the
>>> @@ -1510,6 +1508,8 @@ static int pci_notify(struct notifier_block *nb, unsigned long action,
>>>        else if (action == BUS_NOTIFY_DEL_DEVICE)
>>>                notify = vga_arbiter_del_pci_device(pdev);
>>>
>>> +     vgaarb_dbg(dev, "%s: action = %lu\n", __func__, action);
>>> +
>>>        if (notify)
>>>                vga_arbiter_notify_clients();
>>>        return 0;
>>> @@ -1534,8 +1534,8 @@ static struct miscdevice vga_arb_device = {
>>>
>>>    static int __init vga_arb_device_init(void)
>>>    {
>>> +     struct pci_dev *pdev = NULL;
>>>        int rc;
>>> -     struct pci_dev *pdev;
>>>
>>>        rc = misc_register(&vga_arb_device);
>>>        if (rc < 0)
>>> @@ -1545,11 +1545,13 @@ static int __init vga_arb_device_init(void)
>>>
>>>        /* We add all PCI devices satisfying VGA class in the arbiter by
>>>         * default */
>>> -     pdev = NULL;
>>> -     while ((pdev =
>>> -             pci_get_subsys(PCI_ANY_ID, PCI_ANY_ID, PCI_ANY_ID,
>>> -                            PCI_ANY_ID, pdev)) != NULL)
>>> +     while (1) {
>>> +             pdev = pci_get_class(PCI_CLASS_DISPLAY_VGA << 8, pdev);
>>> +             if (!pdev)
>>> +                     break;
>>> +
>>>                vga_arbiter_add_pci_device(pdev);
>>> +     }
>>>
>>>        pr_info("loaded\n");
>>>        return rc;
>> --
>> Jingfeng
>>
-- 
Jingfeng

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