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 <xinliang.liu@linaro.org>
[s.christ@phytec.de: Picking patch from
                     https://lkml.org/lkml/2015/9/14/188]
Signed-off-by: Stefan Christ <s.christ@phytec.de>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Acked-by: Dave Airlie <airlied@gmail.com>
Tested-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/075ffb50cc16ab055b5d47b30163401bb356ab51.1487175046.git-series.maxime.ripard@free-electrons.com

drivers/gpu/drm/Kconfig

@@ -99,6 +99,15 @@ config DRM_FBDEV_EMULATION
 
 	  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

@@ -48,6 +48,12 @@ module_param_named(fbdev_emulation, drm_fbdev_emulation, bool, 0600);
 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);
 
@@ -1578,6 +1584,10 @@ static int drm_fb_helper_single_fb_probe(struct drm_fb_helper *fb_helper,
 		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);
 	if (ret < 0)