drm/fb-helper: Add multi buffer support for cma fbdev

This patch add a config to support to create multi buffer for cma fbdev.
Such as double buffer and triple buffer.

Cma fbdev is convient to add a legency fbdev. And still many Android
devices use fbdev now and at least double buffer is needed for these
Android devices, so that a buffer flip can be operated. It will need
some time for Android device vendors to abondon legency fbdev. So multi
buffer for fbdev is needed.

Signed-off-by: Xinliang Liu <[e-mail]>
[[e-mail]: Picking patch from
https://lkml.org/lkml/2015/9/14/188]
Signed-off-by: Stefan Christ <[e-mail]>
Signed-off-by: Maxime Ripard <[e-mail]>
Acked-by: Dave Airlie <[e-mail]>
Tested-by: Neil Armstrong <[e-mail]>
Signed-off-by: Daniel Vetter <[e-mail]>
Link: http://patchwork.freedesktop.org/patch/msgid/075ffb50cc16[e-mail]

drivers/gpu/drm/Kconfig

···
 
 	  If in doubt, say "Y".
 
+config DRM_FBDEV_OVERALLOC
+	int "Overallocation of the fbdev buffer"
+	depends on DRM_FBDEV_EMULATION
+	default 100
+	help
+	  Defines the fbdev buffer overallocation in percent. Default
+	  is 100. Typical values for double buffering will be 200,
+	  triple buffering 300.
+
 config DRM_LOAD_EDID_FIRMWARE
 	bool "Allow to specify an EDID data set instead of probing for it"
 	depends on DRM_KMS_HELPER

drivers/gpu/drm/drm_fb_helper.c

···
 MODULE_PARM_DESC(fbdev_emulation,
 		 "Enable legacy fbdev emulation [default=true]");
 
+static int drm_fbdev_overalloc = CONFIG_DRM_FBDEV_OVERALLOC;
+module_param(drm_fbdev_overalloc, int, 0444);
+MODULE_PARM_DESC(drm_fbdev_overalloc,
+		 "Overallocation of the fbdev buffer (%) [default="
+		 __MODULE_STRING(CONFIG_DRM_FBDEV_OVERALLOC) "]");
+
 static LIST_HEAD(kernel_fb_helper_list);
 static DEFINE_MUTEX(kernel_fb_helper_lock);
 
···
 		sizes.fb_width = sizes.surface_width = 1024;
 		sizes.fb_height = sizes.surface_height = 768;
 	}
+
+	/* Handle our overallocation */
+	sizes.surface_height *= drm_fbdev_overalloc;
+	sizes.surface_height /= 100;
 
 	/* push down into drivers */
 	ret = (*fb_helper->funcs->fb_probe)(fb_helper, &sizes);