What does this fix? What uses this?

Non-present fields are zero initialized by default. So this is a no-op?

If you want to explicitly initialize those two fields, at least use .class = ..., .class_mask = ....

Should PCI_DEVICE() also explicitly initialize these fields?

Actually the "class" and "class_mask" fields in the struct are not used at present.

In sys/dev/mlx4/mlx4_core/mlx4_main.c:

static DEFINE_PCI_DEVICE_TABLE(mlx4_pci_table) = {
<... snipped... >

/* MT27500 Family [ConnectX-3 Virtual Function] */
{ PCI_VDEVICE(MELLANOX, 0x1004), MLX4_PCI_DEV_IS_VF },

Here MLX4_PCI_DEV_IS_VF should be assigned to the "driver_data" field in the struct pci_device_id -- without the patch, it's assigned to the "class_mask" field...

So in __mlx4_init_one() we get the error:

if (!(pci_dev_data & MLX4_PCI_DEV_IS_VF) &&
     !(pci_resource_flags(pdev, 0) & IORESOURCE_MEM)) {
         dev_err(&pdev->dev, "Missing DCS, aborting."
                 "(driver_data: 0x%x, pci_resource_flags(pdev, 0):0x%x)\n",
                 pci_dev_data, pci_resource_flags(pdev, 0));
         err = -ENODEV;
         goto err_disable_pdev;
 }

Yes, the style you suggested is better.
Here I'm trying to use the same style in the related Linux side. I suppose the linuxkpi code should be as close to the related Linux code as possible?

The only user of PCI_DEVICE() is ofed/drivers/infiniband/hw/mthca/mthca_main.c, which doesn't have a bug.

Personally I'd like to keep it as is, since the Linux side's related definition is the same.

I think you should also change the "mlx4_pci_table()" to use .driver_data = MLX4_PCI_DEV_IS_VF so that we are not dependent on the compiler.

In FreeBSD record initialization of structures is preferred. It makes updating the structures easier. The LinuxKPI doesn't have to be identical to Linux. It's quirks and caveats should be understood before using it with real Linux driver code :-)

Ok. So I should not change sys/compat/linuxkpi/common/include/linux/pci.h, and I should fix mlx4_pci_table.
Will update the patch.

Got it.

Yes, that's correct.