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

[Brian Norris]

Hi Javier,

On Thu, Sep 12, 2024 at 06:33:58PM +0200, Javier Martinez Canillas wrote:
> 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?

Julius is more familiar with the Coreboot + payload ecosystem than me,
but his explanations make sense to me, as does this patch.

> 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.

Normally, these sorts of "early" and "later" ordering descriptions would
set alarm bells when talking about independent drivers. But I suppose
the "early arch" code has better ordering guaranteeds than drivers, so
this should be fine.

> 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

Did you mean 'platform_device'?

> +	 * 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

s/advertise/advertised/ ?

> +	 * in the Coreboot table should only be used if the payload did
> +	 * not set video mode info and passed it to the Linux kernel.

s/passed/pass/

> +	 */
> +	if (si->orig_video_isVGA == VIDEO_TYPE_VLFB ||
> +            si->orig_video_isVGA == VIDEO_TYPE_EFI)

This line is using spaces for indentation. It should use a tab, and then
spaces for alignment. But presumably this will change based on Thomas's
suggestions anyway.

> +		return -EINVAL;

Is EINVAL right? IIUC, that will print a noisier error to the logs. I
believe the "expected" sorts of return codes are ENODEV or ENXIO. (See
call_driver_probe().) ENODEV seems like a fine choice, similar to
several of the other return codes already used here.

Anyway, this seems along the right track. Thanks for tackling, and feel
free to carry a:

Reviewed-by: Brian Norris <briannorris at chromium.org>

> +
>  	if (!fb->physical_address)
>  		return -ENODEV;
>  
> -- 
> Best regards,
> 
> Javier Martinez Canillas
> Core Platforms
> Red Hat
>