[RFC,drm-misc-next v4 3/9] drm/radeon: Implement .be_primary() callback

[suijingfeng]

Hi,


On 2023/9/5 13:50, Christian König wrote:
> Am 04.09.23 um 21:57 schrieb Sui Jingfeng:
>> From: Sui Jingfeng <suijingfeng at loongson.cn>
>>
>> On a machine with multiple GPUs, a Linux user has no control over 
>> which one
>> is primary at boot time.
>
> Question is why is that useful? Should we give users the ability to 
> control that?
>
> I don't see an use case for this.
>

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
work without the prerequisite setups done by firmware, This
is called as POST.

One of the use cases is to test if a specific DRM driver
would works properly, under the circumstance of not being
POST-ed, The ast drm driver is the first one which refused
to work if not being POST-ed by the firmware.

Before apply this series, I was unable make drm/ast as the
primary video card easily. The problem is that on a multiple
video card configuration, the monitor connected with my
AST2400 card not light up. While confusing, a naive programmer
may suspect the PRIME is not working.

After applied this series and passing ast.modeset=10 on the
kernel cmd line, I found that the monitor connected with my
ast2400 video card still black, It doesn't display and It
doesn't show image to me.

While in the process of study drm/ast, I know that drm/ast
driver has the POST code shipped, See the ast_post_gpu() function.
Then, I was wondering why this function doesn't works.

After a short-time (hasty) debugging, I found that the ast_post_gpu()
function didn't get run. Because it have something to do with the
ast->config_mode. Without thinking too much, I hardcoded the
ast->config_mode as ast_use_p2a, the key point is to force the
ast_post_gpu() function to run.


```

--- a/drivers/gpu/drm/ast/ast_main.c
+++ b/drivers/gpu/drm/ast/ast_main.c
@@ -132,6 +132,8 @@ static int ast_device_config_init(struct ast_device 
*ast)
                 }
         }

+       ast->config_mode = ast_use_p2a;
+
         switch (ast->config_mode) {
         case ast_use_defaults:
                 drm_info(dev, "Using default configuration\n");

```

Then, the monitor light up, it display the Ubuntu greeter to me. Therefore
my patch is useful, at least for the Linux drm driver tester and developer.
It allow programmers to test the specific part of a specific driver without
changing a line of the source code and without the need of sudo authority.

It improves the efficiency of the testing and patch verification. I know
the PrimaryGPU option of Xorg conf, but this approach will remember the
setup have been made, you need modify it with root authority each time
you want to switch the primary. But on the process of rapid developing
and/or testing for multiple video drivers, with only one computer hardware
resource available. What we really want is a one-shot command, as provided
by this series.  So, this is the first use case.


The second use case is that sometime the firmware is not reliable.
While there are thousands of ARM64, PowerPC and Mips servers machine,
Most of them don't have a good UEFI firmware support. I haven't test the
drm/amdgpu and drm/radeon at my ARM64 server yet. Because this ARM64
server always use the platform(BMC) integrated display controller as primary.
The UEFI firmware of it does not provide options menu to tune.
So, for the first time, the discrete card because useless, despite more powerful.
I will take time to carry on the testing, so I will be able to tell more
in the future.


Even on X86, when select the PEG as primary on the UEFI BIOS menu.
There is no way to tell the bios which one of my three
discrete video be the primary. Not to mention some old UEFI
firmware, which doesn't provide a setting at all.
While the benefit of my approach is the flexibility.
Yes the i915, amdgpu and radeon are good quality,
but there may have programmers want to try nouveau.


The third use case is that VGAARB is also not reliable, It will
select a wrong device as primary. Especially on Arm64, Loongarch
and mips arch etc. And the X server will use this wrong device
as primary and completely crash there. Either because of lacking
a driver or the driver has a bug which can not bear the graphic
environment up. VGAARB is firmware dependent.
My patch provide a temporary method to rescue.


The forth is probably the PRIME and reverse PRIME development
and developing driver for new video cards.