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

[Thomas Zimmermann <tzimmermann@suse.de>]

Hi Javier,

thanks for the patch.

Am 12.09.24 um 18:33 schrieb Javier Martinez Canillas:
> Julius Werner <jwerner@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@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@chromium.org>
> Link: https://lore.kernel.org/all/ZuCG-DggNThuF4pj@b20ea791c01f/T/#ma7fb65acbc1a56042258adac910992bb225a20d2
> Suggested-by: Julius Werner <jwerner@chromium.org>
> Signed-off-by: Javier Martinez Canillas <javierm@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;

Probably 'const'.

>   	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)

Rather call screen_info_video_type(si) [1] to get the type. If it 
returns 0, the screen_info is unset and the corebios code can handle the 
framebuffer. In any other case, the framebuffer went through a 
bootloader, which might have modified it. This also handles awkward 
cases, such as if the bootloader programs a VGA text mode.

[1] 
https://elixir.bootlin.com/linux/v6.10.10/source/include/linux/screen_info.h#L92

With these changes:

Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>

Best regards
Thomas

> +		return -EINVAL;
> +
>   	if (!fb->physical_address)
>   		return -ENODEV;
>   

-- 
--
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Frankenstrasse 146, 90461 Nuernberg, Germany
GF: Ivo Totev, Andrew Myers, Andrew McDonald, Boudien Moerman
HRB 36809 (AG Nuernberg)