Re: [Nouveau] [RFC, drm-misc-next v4 0/9] PCI/VGA: Allowing the user to select the primary video adapter at boot time

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Am 06.09.23 um 12:31 schrieb Sui Jingfeng:

On 2023/9/6 14:45, Christian König wrote:
Firmware framebuffer device already get killed by the drm_aperture_remove_conflicting_pci_framebuffers() function (or its siblings). So, this series is definitely not to interact with the firmware framebuffer (or more intelligent framebuffer drivers).  It is for user space program, such as X server and Wayland compositor. Its for Linux user or drm drivers testers, which allow them to direct graphic display server
using right hardware of interested as primary video card.

Also, I believe that X server and Wayland compositor are the best test examples.
If a specific DRM driver can't work with X server as a primary,
then there probably have something wrong.

But what's the use case for overriding this setting?

On a specific machine with multiple GPUs mounted,
only the primary graphics get POST-ed (initialized) by the firmware.
Therefore, the DRM drivers for the rest video cards, have to choose to
work without the prerequisite setups done by firmware, This is called as POST.

Well, you don't seem to understand the background here. This is perfectly normal behavior.

Secondary cards are posted after loading the appropriate DRM driver. At least for amdgpu this is done by calling the appropriate functions in the BIOS.

Well, thanks for you tell me this. You know more than me and definitely have a better understanding.

Are you telling me that the POST function for AMDGPU reside in the BIOS?
The kernel call into the BIOS?

Yes, exactly that.

Does the BIOS here refer to the UEFI runtime or ATOM BIOS or something else?

On dGPUs it's the VBIOS on a flashrom on the board, for iGPUs (APUs as AMD calls them) it's part of the system BIOS.

UEFI is actually just a small subsystem in the system BIOS which replaced the old interface used between system BIOS, video BIOS and operating system.

But the POST function for the drm ast, reside in the kernel space (in other word, in ast.ko).
Is this statement correct?

I don't know the ast driver well enough to answer that, but I assume they just read the BIOS and execute the appropriate functions.

I means that for ASpeed BMC chip, if the firmware not POST the display controller. Then we have to POST it at the kernel space before doing various modeset option.
We can only POST this chip by directly operate the various registers.
Am I correct for the judgement about ast drm driver?

Well POST just means Power On Self Test, but what you mean is initializing the hardware.

Some drivers can of course initialize the hardware without the help of the BIOS, but I don't think AST can do that. As far as I know it's a relatively simple driver.

BTW firmware is not the same as the BIOS (which runs the POST), firmware usually refers to something run on microcontrollers inside the ASIC while the (system or video) BIOS runs on the host CPU.


Thanks for your reviews.

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