Re: [PATCH v7 2/8] PCI/VGA: Deal only with VGA class devices

From: Sui Jingfeng
Date: Sun Jun 18 2023 - 08:11:34 EST

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Hi,

On 2023/6/16 22:34, Alex Deucher wrote:

On Fri, Jun 16, 2023 at 10:22 AM Sui Jingfeng <suijingfeng@xxxxxxxxxxx> wrote:

On 2023/6/16 21:41, Alex Deucher wrote:

On Fri, Jun 16, 2023 at 3:11 AM Sui Jingfeng <suijingfeng@xxxxxxxxxxx> wrote:

Hi,

On 2023/6/16 05:11, Alex Deucher wrote:

On Wed, Jun 14, 2023 at 6:50 AM Sui Jingfeng <suijingfeng@xxxxxxxxxxx> wrote:

Hi,

On 2023/6/13 11:01, Sui Jingfeng wrote:

From: Sui Jingfeng <suijingfeng@xxxxxxxxxxx>

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@xxxxxxxxxx>
Signed-off-by: Sui Jingfeng <suijingfeng@xxxxxxxxxxx>
---
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.

But couldn't the boot device be determined via what whatever resources
were used by the pre-OS console?

I don't know what you are refer to by saying pre-OS console, UEFI
SHELL, UEFI GOP or something like that.

Right. Before the OS loads the platform firmware generally sets up
something for display. That could be GOP or vesa or some other
platform specific protocol.

If you are referring to the framebuffer driver which light up the screen
before the Linux kernel is loaded .

Then, what you have said is true, the boot device is determined by the
pre-OS console.

But the problem is how does the Linux kernel(vgaarb) could know which
one is the default boot device

on a multiple GPU machine. Relaying on the firmware fb's address and
size is what the mechanism

we already in using.

Right. It shouldn't need to depend on vgaarb.

I feel like that should be separate from vgaarb.

Emm, this really deserved another patch, please ?

vgaarb should handle PCI VGA routing and some other
mechanism should be used to determine what device provided the pre-OS
console.

If the new mechanism need the firmware changed, then this probably break
the old machine.

Also, this probably will get all arch involved. to get the new mechanism
supported.

The testing pressure and review power needed is quite large.

drm/amdgpu and drm/radeon already being used on X86, ARM64, Mips and
more arch...

The reviewing process will became quite difficult then.

vgaarb is really what we already in use, and being used more than ten
years ...

This base class is defined for all types of display controllers.

For VGA devices (Sub-Class 00h), the Programming Interface byte is divided into a bit field that identifies additional video controller compatibilities.

A device can support multiple interfaces by using the bit map to indicate which interfaces are supported.

For the XGA devices (Sub-Class 01h), only the standard XGA interface is defined.

Sub-Class 02h is for controllers that have hardware support for 3D operations and are not VGA compatible.

Yes, it works for x86 (and a few other platforms) today because of the
VGA legacy, so we can look at VGA routing to determine this. But even
today, we don't need VGA routing to determine what was the primary
display before starting the OS. We could probably have a platform
independent way to handle this by looking at the bread crumbs leftover
from the pre-OS environment.

Yes, I agree with you.

E.g., for pre-UEFI platforms, we can
look at VGA routing. For UEFI platforms we can look at what GOP left
us. For various non-UEFI ARM/PPC/MIPS/etc. platforms we can look at
whatever breadcrumbs those pre-OS environments left. That way when
VGA goes away, we can have a clean break and you won't need vgaarb if
the platform has no VGA devices.

Yes, I agree with you.

Then, move vga_is_firmware_default() function to video/aperture.c also ?

Because this function shouldn't be platform dependent,

can be usable as long as the PCI resource relocation don't happen (The vram bar don't move),

And screen_info is more about video specifci thing.

Yes your are right, it seems that the selection of the default boot device shouldn't depend on vgaarb.

As vgaarb is only for PCI vga compatible display controller.

It seems that platform display controller device should also participated in the arbitration.

Emm, but that may a bit difficult. Because we rely on the PCI notifier to snoop the bound between the drm driver and device,

call back to use if successful. So, what should I do next? as a first step?

Alex

Alex

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

--
Jingfeng

--
Jingfeng

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In reply to: Sui Jingfeng: "Re: [PATCH v7 2/8] PCI/VGA: Deal only with VGA class devices"
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