drm/vmwgfx: Add an option to change assumed FB bpp

Offer an option for advanced users who want larger modes at 16bpp.

This becomes necessary after the fix: "Work around mode set
failure in 2D VMs."  Without this patch, there would be no way
for existing advanced users to get to a high res mode, and the
regression is they will likely get a black screen after a software
update on their current VM.
Signed-off-by: Sinclair Yeh <syeh@vmware.com>
Reviewed-by: Thomas Hellstrom <thellstrom@vmware.com>
Cc: <stable@vger.kernel.org>

drivers/gpu/drm/vmwgfx/vmwgfx_drv.c

@@ -233,6 +233,7 @@ static int vmw_force_iommu;
 static int vmw_restrict_iommu;
 static int vmw_force_coherent;
 static int vmw_restrict_dma_mask;
+static int vmw_assume_16bpp;
 
 static int vmw_probe(struct pci_dev *, const struct pci_device_id *);
 static void vmw_master_init(struct vmw_master *);
@@ -249,6 +250,8 @@ MODULE_PARM_DESC(force_coherent, "Force coherent TTM pages");
 module_param_named(force_coherent, vmw_force_coherent, int, 0600);
 MODULE_PARM_DESC(restrict_dma_mask, "Restrict DMA mask to 44 bits with IOMMU");
 module_param_named(restrict_dma_mask, vmw_restrict_dma_mask, int, 0600);
+MODULE_PARM_DESC(assume_16bpp, "Assume 16-bpp when filtering modes");
+module_param_named(assume_16bpp, vmw_assume_16bpp, int, 0600);
 
 
 static void vmw_print_capabilities(uint32_t capabilities)
@@ -660,6 +663,8 @@ static int vmw_driver_load(struct drm_device *dev, unsigned long chipset)
 	dev_priv->vram_start = pci_resource_start(dev->pdev, 1);
 	dev_priv->mmio_start = pci_resource_start(dev->pdev, 2);
 
+	dev_priv->assume_16bpp = !!vmw_assume_16bpp;
+
 	dev_priv->enable_fb = enable_fbdev;
 
 	vmw_write(dev_priv, SVGA_REG_ID, SVGA_ID_2);

drivers/gpu/drm/vmwgfx/vmwgfx_drv.h

@@ -386,6 +386,7 @@ struct vmw_private {
 	spinlock_t hw_lock;
 	spinlock_t cap_lock;
 	bool has_dx;
+	bool assume_16bpp;
 
 	/*
 	 * VGA registers.

drivers/gpu/drm/vmwgfx/vmwgfx_kms.c

@@ -1555,6 +1555,9 @@ int vmw_du_connector_fill_modes(struct drm_connector *connector,
 	int i;
 	u32 assumed_bpp = 4;
 
+	if (dev_priv->assume_16bpp)
+		assumed_bpp = 2;
+
 	if (dev_priv->active_display_unit == vmw_du_screen_target) {
 		max_width  = min(max_width,  dev_priv->stdu_max_width);
 		max_height = min(max_height, dev_priv->stdu_max_height);