[PATCH] drm/tilcdc: Set preferred depth

[Kevin Hao]

The commit c91acda3a380 ("drm/gem: Check for valid formats") adds a
check for valid pixel formats on drm_gem_fb_create(), but this breaks
the X server on the beaglebone black board.

We have set 'DefaultDepth' to 16 in our xorg.conf. In the X modesetting
driver, the drmmode_get_default_bpp() is used to guess the default
depth/bpp. First it tries to get them via DRM_CAP_DUMB_PREFERRED_DEPTH
ioctl, and if it fail, then try to create a FB with 'depth = 24' and
'bpp = 32' to check whether this depth/dpp is a valid combo. Before the
kernel commit c91acda3a380, the FB always can be created successfully.
This will avoid the bpp to be set to 24 forcibly. But after kernel
commit c91acda3a380, the FB will not be created successfully due to the
check of the valid pixel format. Then the bpp is set to 24, but the
'depth = 16' and 'bpp = 24' combo is not a valid pixel format.

Fix this issue by explicitly setting the preferred_depth in this driver.
With this change, the modesetting driver would choose the correct
depth/bpp combo based on our setting in xorg.conf.

Fixes: c91acda3a380 ("drm/gem: Check for valid formats")
Cc: stable at vger.kernel.org
Signed-off-by: Kevin Hao <haokexin at gmail.com>
---
 drivers/gpu/drm/tilcdc/tilcdc_drv.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/drivers/gpu/drm/tilcdc/tilcdc_drv.c b/drivers/gpu/drm/tilcdc/tilcdc_drv.c
index cd5eefa06060..d4bd4ebeff78 100644
--- a/drivers/gpu/drm/tilcdc/tilcdc_drv.c
+++ b/drivers/gpu/drm/tilcdc/tilcdc_drv.c
@@ -258,6 +258,7 @@ static int tilcdc_init(const struct drm_driver *ddrv, struct device *dev)
 
 	pm_runtime_put_sync(dev);
 
+	ddev->mode_config.preferred_depth = 16;
 	if (priv->rev == 1) {
 		DBG("Revision 1 LCDC supports only RGB565 format");
 		priv->pixelformats = tilcdc_rev1_formats;
@@ -273,6 +274,7 @@ static int tilcdc_init(const struct drm_driver *ddrv, struct device *dev)
 			priv->num_pixelformats =
 				ARRAY_SIZE(tilcdc_crossed_formats);
 			bpp = 32; /* Choose bpp with RGB support for fbdef */
+			ddev->mode_config.preferred_depth = 24;
 		} else if (0 == strcmp(str, "straight")) {
 			DBG("Configured for straight blue and red wires");
 			priv->pixelformats = tilcdc_straight_formats;
-- 
2.44.0