[NOT A REGRESSION] firmware: framebuffer-coreboot: duplicate device name "simple-framebuffer.0"

[Javier Martinez Canillas]

Julius Werner <jwerner at chromium.org> writes:

Hello Julius,

>> 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
>> it seems SeaBIOS payload can also provide a VGA mode in the boot params.
>>
>> [...]
>>
>> To prevent the issue, make the framebuffer_core driver to disable sysfb
>> if there is system framebuffer data in the Coreboot table. That way only
>> this driver will register a device and sysfb would not attempt to do it
>> (or remove its registered device if was already executed before).
>
> I wonder if the priority should be the other way around? coreboot's
> framebuffer is generally only valid when coreboot exits to the payload
> (e.g. SeaBIOS). Only if the payload doesn't touch the display
> controller or if there is no payload and coreboot directly hands off
> to a kernel does the kernel driver for LB_TAG_FRAMEBUFFER make sense.
> But if there is some other framebuffer information passed to the
> kernel from a firmware component running after coreboot, most likely
> that one is more up to date and the framebuffer described by the
> coreboot table doesn't work anymore (because the payload usually
> doesn't modify the coreboot tables again, even if it changes hardware
> state). So if there are two drivers fighting over which firmware
> framebuffer description is the correct one, the coreboot driver should
> probably give way.
>

That's a very good point. I'm actually not familiar with Coreboot and I
used an educated guess (in the case of DT for example, that's the main
source of truth and I didn't know if a Core table was in a similar vein).

Maybe something like the following (untested) patch then?

>From de1c32017006f4671d91b695f4d6b4e99c073ab2 Mon Sep 17 00:00:00 2001
From: Javier Martinez Canillas <javierm at redhat.com>
Date: Thu, 12 Sep 2024 18:31:55 +0200
Subject: [PATCH] firmware: coreboot: Don't register a pdev if screen_info data
 is available

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 this information to
the Linux kernel.

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

But later then the framebuffer_coreboot driver will try to do the same
framebuffer (using the information from the Coreboot table), which will
lead to an error due a simple-framebuffer.0 device already 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 the 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 at chromium.org>
Link: https://lore.kernel.org/all/ZuCG-DggNThuF4pj@b20ea791c01f/T/#ma7fb65acbc1a56042258adac910992bb225a20d2
Suggested-by: Julius Werner <jwerner at chromium.org>
Signed-off-by: Javier Martinez Canillas <javierm at redhat.com>
---
 drivers/firmware/google/framebuffer-coreboot.c | 16 ++++++++++++++++
 1 file changed, 16 insertions(+)

diff --git a/drivers/firmware/google/framebuffer-coreboot.c b/drivers/firmware/google/framebuffer-coreboot.c
index daadd71d8ddd..4e50da17cd7e 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"
 
@@ -27,6 +28,7 @@ static int framebuffer_probe(struct coreboot_device *dev)
 	int i;
 	u32 length;
 	struct lb_framebuffer *fb = &dev->framebuffer;
+	struct screen_info *si = &screen_info;
 	struct platform_device *pdev;
 	struct resource res;
 	struct simplefb_platform_data pdata = {
@@ -36,6 +38,20 @@ static int framebuffer_probe(struct coreboot_device *dev)
 		.format = NULL,
 	};
 
+	/*
+	 * If the global screen_info data has been filled, the Generic
+	 * System Framebuffers (sysfb) will already register a platform
+	 * and pass the screen_info as platform_data to a driver that
+	 * could scan-out using the system provided framebuffer.
+	 *
+	 * On Coreboot systems, the advertise LB_TAG_FRAMEBUFFER entry
+	 * in the Coreboot table should only be used if the payload did
+	 * not set video mode info and passed it to the Linux kernel.
+	 */
+	if (si->orig_video_isVGA == VIDEO_TYPE_VLFB ||
+            si->orig_video_isVGA == VIDEO_TYPE_EFI)
+		return -EINVAL;
+
 	if (!fb->physical_address)
 		return -ENODEV;
 
-- 
Best regards,

Javier Martinez Canillas
Core Platforms
Red Hat