Re: [PATCH 07/12] arch/x86: Declare edid_info in <asm/screen_info.h>

Thomas Zimmermann <tzimmermann@xxxxxxx> · Wed, 5 Jul 2023 10:18:18 +0200

Hi Arnd

Am 30.06.23 um 13:53 schrieb Arnd Bergmann:

On Fri, Jun 30, 2023, at 09:46, Thomas Zimmermann wrote:

Am 29.06.23 um 15:21 schrieb Arnd Bergmann:

On Thu, Jun 29, 2023, at 15:01, Thomas Zimmermann wrote:

Am 29.06.23 um 14:35 schrieb Arnd Bergmann:

On Thu, Jun 29, 2023, at 13:45, Thomas Zimmermann wrote:

FIRMWARE_EDID is a user-selectable feature, while ARCH_HAS_EDID_INFO
announces an architecture feature. They do different things.

I still have trouble seeing the difference.

The idea here is that ARCH_HAS_ signals the architecture's support for
the feature.  Drivers set 'depends on' in their Kconfig.

Another Kconfig token, VIDEO_SCREEN_INFO or FIRMWARE_EDID, would then
actually enable the feature.  Drivers select VIDEO_SCREEN_INFO or
FIRMWARE_EDID and the architectures contains code like

Fair enough. In that case, I guess FIRMWARE_EDID will just depend on
ARCH_HAS_EDID_INFO, or possibly "depends on FIRMWARE_EDID || EFI"
after it starts calling into an EFI specific function, right?

#ifdef VIDEO_SCREEN_INFO
struct screen_info screen_info = {
	/* set values here */
}
#endif

This allows us to disable code that requires screen_info/edid_info, but
also disable screen_info/edid_info unless such code has been enabled in
the kernel config.

Some architectures currently mimic this by guarding screen_info with
ifdef CONFIG_VT or similar. I'd like to make this more flexible. The
cost of a few more internal Kconfig tokens seems negligible.

I definitely get it for the screen_info, which needs the complexity.
For ARCHARCH_HAS_EDID_INFO I would hope that it's never selected by
anything other than x86, so I would still go with just a dependency
on x86 for simplicity, but I don't mind having the extra symbol if that
keeps it more consistent with how the screen_info is handled.

Well, I'd like to add edid_info to platforms with EFI. What would be 

arm/arm64 and loongarch, I guess. See below for the future plans.

I suppose you could use FIRMWARE_EDID on EFI or OF systems without
the need for a global edid_info structure, but that would not
share any code with the current fb_firmware_edid() function.

The current code is build on top of screen_info and edid_info. I'd
preferably not replace that, if possible.

One way I could imagine this looking in the end would be
something like

struct screen_info *fb_screen_info(struct device *dev)
{
       struct screen_info *si = NULL;

       if (IS_ENABLED(CONFIG_EFI))
             si = efi_get_screen_info(dev);

       if (IS_ENABLED(CONFIG_ARCH_HAS_SCREEN_INFO) && !si)
             si = screen_info;

       return si;
}

corresponding to fb_firmware_edid(). With this, any driver
that wants to access screen_info would call this function
instead of using the global pointer, plus either NULL pointer
check or a CONFIG_ARCH_HAS_SCREEN_INFO dependency.

This way we could completely eliminate the global screen_info
on arm64, riscv, and loongarch but still use the efi and
hyperv framebuffer/drm drivers.

If possible, I'd like to remove global screen_info and edid_info 

entirely from fbdev and the various consoles.

We currently use screen_info to set up the generic framebuffer device in 

drivers/firmware/sysfb.c. I'd like to use edid_info here as well, so 

that the generic graphics drivers can get EDID information.

For the few fbdev drivers and consoles that require the global 

screen_info/edid_info, I'd rather provide lookup functions in sysfb 

(e.g., sysfb_get_screen_info(), sysfb_get_edid_info()). The global 

screen_info/edid_info state would then become an internal artifact of 

the sysfb code.

Hopefully that explains some of the decisions made in this patchset.

Best regards
Thomas

     Arnd

--
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)

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