[PATCH v2] firmware: coreboot: Don't register a pdev if screen_info data is present

[Javier Martinez Canillas <javierm@xxxxxxxxxx>]

On Coreboot platforms, a system framebuffer may be provided to the Linux
kernel by filling a LB_TAG_FRAMEBUFFER entry in the Coreboot table. But
a Coreboot payload (e.g: SeaBIOS) could also provide its own framebuffer
information to the Linux kernel.

If that's the case, arch x86 boot code will fill the global screen_info
data and this used by the Generic System Framebuffers (sysfb) framework,
to register a platform device with pdata about the system's framebuffer.

But later, the framebuffer_coreboot driver will try to do the same and
attempt to register a "simple-framebuffer" platform device (using the
information from the Coreboot table), which will lead to an error due a
device with the same name already being registered:

    sysfs: cannot create duplicate filename '/bus/platform/devices/simple-framebuffer.0'
    ...
    coreboot: could not register framebuffer
    framebuffer coreboot8: probe with driver framebuffer failed with error -17

To prevent this issue, make the framebuffer_core driver to not register
a platform device if the global struct screen_info data has been filled.

Reported-by: Brian Norris <briannorris@xxxxxxxxxxxx>
Closes: https://lore.kernel.org/all/ZuCG-DggNThuF4pj@b20ea791c01f/T/#ma7fb65acbc1a56042258adac910992bb225a20d2
Suggested-by: Julius Werner <jwerner@xxxxxxxxxxxx>
Signed-off-by: Javier Martinez Canillas <javierm@xxxxxxxxxx>
Reviewed-by: Thomas Zimmermann <tzimmermann@xxxxxxx>
Reviewed-by: Brian Norris <briannorris@xxxxxxxxxxxx>
---

Changes in v2:
- Declare the struct screen_info as constant variable (Thomas Zimmermann).
- Use screen_info_video_type() instead of checking the screen_info video
  types directly (Thomas Zimmermann).
- Fix missing "device" word in a comment (Brian Norris).
- Fix some mispellings in a comment (Brian Norris).
- Change error code returned from -EINVAL to -ENODEV (Brian Norris).

 drivers/firmware/google/framebuffer-coreboot.c | 17 +++++++++++++++++
 1 file changed, 17 insertions(+)

diff --git a/drivers/firmware/google/framebuffer-coreboot.c b/drivers/firmware/google/framebuffer-coreboot.c
index daadd71d8ddd..49eeb13814e4 100644
--- a/drivers/firmware/google/framebuffer-coreboot.c
+++ b/drivers/firmware/google/framebuffer-coreboot.c
@@ -15,6 +15,7 @@
 #include <linux/module.h>
 #include <linux/platform_data/simplefb.h>
 #include <linux/platform_device.h>
+#include <linux/screen_info.h>
 
 #include "coreboot_table.h"
 
@@ -27,8 +28,10 @@ static int framebuffer_probe(struct coreboot_device *dev)
 	int i;
 	u32 length;
 	struct lb_framebuffer *fb = &dev->framebuffer;
+	const struct screen_info *si = &screen_info;
 	struct platform_device *pdev;
 	struct resource res;
+	unsigned int type;
 	struct simplefb_platform_data pdata = {
 		.width = fb->x_resolution,
 		.height = fb->y_resolution,
@@ -36,6 +39,20 @@ static int framebuffer_probe(struct coreboot_device *dev)
 		.format = NULL,
 	};
 
+	/*
+	 * On Coreboot systems, the advertised LB_TAG_FRAMEBUFFER entry
+	 * in the Coreboot table should only be used if the payload did
+	 * not pass a framebuffer information to the Linux kernel.
+	 *
+	 * If the global screen_info data has been filled, the Generic
+	 * System Framebuffers (sysfb) will already register a platform
+	 * device and pass that screen_info as platform_data to a driver
+	 * that can scan-out using the system provided framebuffer.
+	 */
+	type = screen_info_video_type(si);
+	if (type)
+		return -ENODEV;
+
 	if (!fb->physical_address)
 		return -ENODEV;
 
-- 
2.46.0