Drivers: hv: Always reserve framebuffer region for Gen1 VMs

vmbus_reserve_fb() tries reserving framebuffer region iff
'screen_info.lfb_base' is set. Gen2 VMs seem to have it set by EFI
and/or by the kernel EFI FB driver (or, in some edge cases like kexec,
the address where the buffer was moved, see
https://lore.kernel.org/all/20201014092429.1415040-1-kasong@redhat.com/)
but on Gen1 VM it depends on bootloader behavior. With grub, it depends
on 'gfxpayload=' setting but in some cases it is observed to be zero.
That being said, relying on 'screen_info.lfb_base' to reserve
framebuffer region is risky. For Gen1 VMs, it should always be
possible to get the address from the dedicated PCI device instead.

Check for legacy PCI video device presence and reserve the whole
region for framebuffer on Gen1 VMs.
Reviewed-by: Michael Kelley <mikelley@microsoft.com>
Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Link: https://lore.kernel.org/r/20220827130345.1320254-3-vkuznets@redhat.comSigned-off-by: Wei Liu <wei.liu@kernel.org>

drivers/hv/vmbus_drv.c

@@ -35,6 +35,7 @@
 #include <linux/kernel.h>
 #include <linux/syscore_ops.h>
 #include <linux/dma-map-ops.h>
+#include <linux/pci.h>
 #include <clocksource/hyperv_timer.h>
 #include "hyperv_vmbus.h"
 
@@ -2262,26 +2263,43 @@ static int vmbus_acpi_remove(struct acpi_device *device)
 
 static void vmbus_reserve_fb(void)
 {
-	int size;
+	resource_size_t start = 0, size;
+	struct pci_dev *pdev;
+
+	if (efi_enabled(EFI_BOOT)) {
+		/* Gen2 VM: get FB base from EFI framebuffer */
+		start = screen_info.lfb_base;
+		size = max_t(__u32, screen_info.lfb_size, 0x800000);
+	} else {
+		/* Gen1 VM: get FB base from PCI */
+		pdev = pci_get_device(PCI_VENDOR_ID_MICROSOFT,
+				      PCI_DEVICE_ID_HYPERV_VIDEO, NULL);
+		if (!pdev)
+			return;
+
+		if (pdev->resource[0].flags & IORESOURCE_MEM) {
+			start = pci_resource_start(pdev, 0);
+			size = pci_resource_len(pdev, 0);
+		}
+
+		/*
+		 * Release the PCI device so hyperv_drm or hyperv_fb driver can
+		 * grab it later.
+		 */
+		pci_dev_put(pdev);
+	}
+
+	if (!start)
+		return;
+
 	/*
 	 * Make a claim for the frame buffer in the resource tree under the
 	 * first node, which will be the one below 4GB.  The length seems to
 	 * be underreported, particularly in a Generation 1 VM.  So start out
 	 * reserving a larger area and make it smaller until it succeeds.
 	 */
-
-	if (screen_info.lfb_base) {
-		if (efi_enabled(EFI_BOOT))
-			size = max_t(__u32, screen_info.lfb_size, 0x800000);
-		else
-			size = max_t(__u32, screen_info.lfb_size, 0x4000000);
-
-		for (; !fb_mmio && (size >= 0x100000); size >>= 1) {
-			fb_mmio = __request_region(hyperv_mmio,
-						   screen_info.lfb_base, size,
-						   fb_mmio_name, 0);
-		}
-	}
+	for (; !fb_mmio && (size >= 0x100000); size >>= 1)
+		fb_mmio = __request_region(hyperv_mmio, start, size, fb_mmio_name, 0);
 }
 
 /**