Re: [PATCH 00/12] Recover sysfb after DRM probe failure

Christian König
2026-01-15T15:58:33Z

On 1/15/26 15:54, Thomas Zimmermann wrote:
> Hi
>
> Am 15.01.26 um 15:39 schrieb Christian König:
>> Sorry to being late, but I only now realized what you are doing here.
>>
>> On 1/15/26 12:02, Thomas Zimmermann wrote:
>>> Hi,
>>>
>>> apologies for the delay. I wanted to reply and then forgot about it.
>>>
>>> Am 10.01.26 um 05:52 schrieb Zack Rusin:
>>>> On Fri, Jan 9, 2026 at 5:34 AM Thomas Zimmermann <tzimmermann@suse.de> wrote:
>>>>> Hi
>>>>>
>>>>> Am 29.12.25 um 22:58 schrieb Zack Rusin:
>>>>>> Almost a rite of passage for every DRM developer and most Linux users
>>>>>> is upgrading your DRM driver/updating boot flags/changing some config
>>>>>> and having DRM driver fail at probe resulting in a blank screen.
>>>>>>
>>>>>> Currently there's no way to recover from DRM driver probe failure. PCI
>>>>>> DRM driver explicitly throw out the existing sysfb to get exclusive
>>>>>> access to PCI resources so if the probe fails the system is left without
>>>>>> a functioning display driver.
>>>>>>
>>>>>> Add code to sysfb to recever system framebuffer when DRM driver's probe
>>>>>> fails. This means that a DRM driver that fails to load reloads the system
>>>>>> framebuffer driver.
>>>>>>
>>>>>> This works best with simpledrm. Without it Xorg won't recover because
>>>>>> it still tries to load the vendor specific driver which ends up usually
>>>>>> not working at all. With simpledrm the system recovers really nicely
>>>>>> ending up with a working console and not a blank screen.
>>>>>>
>>>>>> There's a caveat in that some hardware might require some special magic
>>>>>> register write to recover EFI display. I'd appreciate it a lot if
>>>>>> maintainers could introduce a temporary failure in their drivers
>>>>>> probe to validate that the sysfb recovers and they get a working console.
>>>>>> The easiest way to double check it is by adding:
>>>>>>     /* XXX: Temporary failure to test sysfb restore - REMOVE BEFORE COMMIT */
>>>>>>     dev_info(&pdev->dev, "Testing sysfb restore: forcing probe failure\n");
>>>>>>     ret = -EINVAL;
>>>>>>     goto out_error;
>>>>>> or such right after the devm_aperture_remove_conflicting_pci_devices .
>>>>> Recovering the display like that is guess work and will at best work
>>>>> with simple discrete devices where the framebuffer is always located in
>>>>> a confined graphics aperture.
>>>>>
>>>>> But the problem you're trying to solve is a real one.
>>>>>
>>>>> What we'd want to do instead is to take the initial hardware state into
>>>>> account when we do the initial mode-setting operation.
>>>>>
>>>>> The first step is to move each driver's remove_conflicting_devices call
>>>>> to the latest possible location in the probe function. We usually do it
>>>>> first, because that's easy. But on most hardware, it could happen much
>>>>> later.
>>>> Well, some drivers (vbox, vmwgfx, bochs and currus-qemu) do it because
>>>> they request pci regions which is going to fail otherwise. Because
>>>> grabbining the pci resources is in general the very first thing that
>>>> those drivers need to do to setup anything, we
>>>> remove_conflicting_devices first or at least very early.
>>> To my knowledge, requesting resources is more about correctness than a hard requirement to use an I/O or memory range. Has this changed?
>> Nope that is not correct.
>>
>> At least for AMD GPUs remove_conflicting_devices() really early is necessary because otherwise some operations just result in a spontaneous system reboot.
>
> Here I was only talking about avoiding calls to request_resource() and similar interfaces.
>
>>
>> For example resizing the PCIe BAR giving access to VRAM or disabling VGA emulation (which AFAIK is used for EFI as well) is only possible when the VGA or EFI framebuffer driver is kicked out first.
>
> Yeah, that's what I expected.
>
>>
>> And disabling VGA emulation is among the absolutely first steps you do to take over the scanout config.
>
> Assuming the driver (or driver author) is careful, is it possible to only read state from AMD hardware at such an early time?

I'm not an expert for that particular stuff but I strongly don't think so.

Basically the VGA emulation is firmware which "owns" the CRTC registers and might modify them at any time unless it's turned off first.

So you can't even use data/index pairs of registers etc...

> We usually do remove_conflicting_devices() as the first thing in most driver's probe function. As a first step, it would be helpful to postpone itto a later point.

Well from what I knew that won't work in a lot of cases.

I mean what we could do on non-AMD HW is to remove the conflicting driver, play with the HW and if we find that this didn't worked reset the HW using a PCI function level reset and try to load the EFI or whatever driver again. But that has a rather low chance of working reliable I would say.

The problem with AMD GPUs is that the PCI function level reset is broken to begin with (which already caused us tons of headache in the case of pass through).

Regards,
Christian.

>
>>
>> So I absolutely clearly have to reject the amdgpu patch in this series, that will break tons of use cases.
>
> Don't worry, we're still in the early ideation phase.
>
> Best regards
> Thomas
>
>>
>> Regards,
>> Christian.
>>
>>>> I also don't think it's possible or even desirable by some drivers to
>>>> reuse the initial state, good example here is vmwgfx where by default
>>>> some people will setup their vm's with e.g. 8mb ram, when the vmwgfx
>>>> loads we allow scanning out from system memory, so you can set your vm
>>>> up with 8mb of vram but still use 4k resolutions when the driver
>>>> loads, this way the suspend size of the vm is very predictable (tiny
>>>> vram plus whatever ram was setup) while still allowing a lot of
>>>> flexibility.
>>> If there's no initial state to switch from, the first modeset can fail while leaving the display unusable. There's no way around that. Going back to the old state is not an option unless the driver has been written to support this.
>>>
>>> The case of vmwgfx is special, but does not effect the overall problem. For vmwgfx, it would be best to import that initial state and support a transparent modeset from vram to system memory (and back) at least during this initial state.
>>>
>>>
>>>> In general I think however this is planned it's two or three separate series:
>>>> 1) infrastructure to reload the sysfb driver (what this series is)
>>>> 2) making sure that drivers that do want to recover cleanly actually
>>>> clean out all the state on exit properly,
>>>> 3) abstracting at least some of that cleanup in some driver independent way
>>> That's really not going to work. For example, in the current series, you invoke devm_aperture_remove_conflicting_pci_devices_done() after drm_mode_reset(), drm_dev_register() and drm_client_setup(). Each of these calls can modify hardware state. In the case of _register() and _setup(), the DRM clients can perform a modeset, which destroys the initial hardware state. Patch 1 of this series removes the sysfb device/driver entirely. That should be a no-go as it significantly complicates recovery. For example, if the native drivers failed from an allocation failure, the sysfb device/driver is not likely to come back either. As the very first thing, the series should state which failures is is going to resolve, - failed hardware init, - invalid initial modesetting, - runtime errors (such ENOMEM, failed firmware loading), - others? And then specify how a recovery to sysfb could look in each supported scenario. In terms of implementation, make any transition between drivers
>>> gradually. The native driver needs to acquire the hardware resource (framebuffer and I/O apertures) without unloading the sysfb driver. Luckily there's struct drm_device.unplug, which does that. [1] Flipping this field disables hardware access for DRM drivers. All sysfb drivers support this. To get the sysfb drivers ready, I suggest dedicated helpers for each drivers aperture. The aperture helpers can use these callback to flip the DRM driver off and on again. For example, efidrm could do this as a minimum: int efidrm_aperture_suspend() { dev->unplug = true; remove_resource(/*framebuffer aperture*/) return 0 } int efidrm_aperture_resume() { insert_resource(/*framebuffer aperture*/) dev->unplug = false; return 0 } struct aperture_funcs efidrm_aperture_funcs { .suspend = efidrm_aperture_suspend, .resume = efidrm_aperture_resume, } Pass this struct when efidrm acquires the framebuffer aperture, so that the aperture helpers can control the behavior of efidrm. With this, a multi-
>>> step takeover from sysfb to native driver can be tried. It's still a massive effort that requires an audit of each driver's probing logic. There's no copy-paste pattern AFAICT. I suggest to pick one simple driver first and make a prototype. Let me also say that I DO like the general idea you're proposing. But if it was easy, we would likely have done it already. Best regards Thomas
>>>> z
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