Re: [PATCH v3 3/4] fbdev/deferred-io: Support contiguous kernel memory framebuffers

Thomas Zimmermann
2025-06-04T08:12:48Z

Hi

Am 03.06.25 um 19:50 schrieb Michael Kelley:
> From: Thomas Zimmermann <tzimmermann@suse.de> Sent: Monday, June 2, 2025 11:25 PM
>> Hi
>>
>> Am 03.06.25 um 03:49 schrieb Michael Kelley:
>> [...]
>>>> Will the VMA have VM_PFNMAP or VM_MIXEDMAP set? PFN_SPECIAL is a
>>>> horrible hack.
>>>>
>>>> In another thread, you mention that you use PFN_SPECIAL to bypass the
>>>> check in vm_mixed_ok(), so VM_MIXEDMAP is likely not set?
>>> The VMA has VM_PFNMAP set, not VM_MIXEDMAP. It seemed like
>>> VM_MIXEDMAP is somewhat of a superset of VM_PFNMAP, but maybe that's
>>> a wrong impression. vm_mixed_ok() does a thorough job of validating the
>>> use of __vm_insert_mixed(), and since what I did was allowed, I thought
>>> perhaps it was OK. Your feedback has set me straight, and that's what I
>>> needed. :-)
>>>
>>> But the whole approach is moot with Alistair Popple's patch set that
>>> eliminates pfn_t. Is there an existing mm API that will do mkwrite on a
>>> special PTE in a VM_PFNMAP VMA? I didn't see one, but maybe I missed
>>> it. If there's not one, I'll take a crack at adding it in the next version of my
>>> patch set.
>> What is the motivation behind this work? The driver or fbdev as a whole
>> does not have much of a future anyway.
>>
>> I'd like to suggest removing hyperv_fb entirely in favor of hypervdrm?
>>
> Yes, I think that's the longer term direction. A couple months ago I had an
> email conversation with Saurabh Sengar from the Microsoft Linux team where
> he raised this idea. I think the Microsoft folks will need to drive the deprecation
> process, as they need to coordinate with the distro vendors who publish
> images for running on local Hyper-V and in the Azure cloud. And my
> understanding is that the Linux kernel process would want the driver to
> be available but marked "deprecated" for a year or so before it actually
> goes away.

We (DRM upstream) recently considered moving some fbdev drivers to
drivers/staging or marking them with !DRM if a DRM driver is available.
Hyverv_fb would be a candidate.

At least at SUSE, we ship hypervdrm instead of hyperv_fb. This works
well on the various generations of the hyperv system. Much of our
userspace would not be able to use hyperv_fb anyway.

>
> I do have some concerns about the maturity of the hyperv_drm driver
> "around the edges". For example, somebody just recently submitted a
> patch to flush output on panic. I have less familiarity hyperv_drm vs.
> hyperv_fb, so some of my concern is probably due to that. We might
> need to do review of hyperv_drm and see if there's anything else to
> deal with before hyperv_fb goes away.

The panic output is a feature that we recently added to the kernel. It
allows a DRM driver to display a final error message in the case of a
kernel panic (think of blue screens on Windows). Drivers require a
minimum of support to make it work. That's what the hypervdrm patches
were about.

Best regards
Thomas

>
> This all got started when I was looking at a problem with hyperv_fb,
> and I found several other related problems, some of which also existed
> in hyperv_drm. You've seen several small'ish fixes from me and Saurabh
> as a result, and this issue with mmap()'ing /dev/fb0 is the last one of that
> set. This fix is definitely a bit bigger, but it's the right fix. On the flip side,
> if we really get on a path to deprecate hyperv_fb, there are hack fixes for
> the mmap problem that are smaller and contained to hyperv_fb. I would
> be OK with a hack fix in that case.
>
> Michael

--
--
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Frankenstrasse 146, 90461 Nuernberg, Germany
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