linux-kernel - RE: [RFC 1/3] fbdev: hyperv_fb: Remove hyperv

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Message-ID:
 <SN6PR02MB41578C2984A84B4D0AA17943D408A@SN6PR02MB4157.namprd02.prod.outlook.com>
Date: Fri, 12 Sep 2025 03:24:55 +0000
From: Michael Kelley <mhklinux@...look.com>
To: Prasanna Kumar T S M <ptsm@...ux.microsoft.com>, Thomas Zimmermann
	<tzimmermann@...e.de>, "deller@....de" <deller@....de>, "arnd@...db.de"
	<arnd@...db.de>, "soci@....rulez.org" <soci@....rulez.org>,
	"gonzalo.silvalde@...il.com" <gonzalo.silvalde@...il.com>,
	"rdunlap@...radead.org" <rdunlap@...radead.org>,
	"bartosz.golaszewski@...aro.org" <bartosz.golaszewski@...aro.org>,
	"wei.liu@...nel.org" <wei.liu@...nel.org>, "ssengar@...ux.microsoft.com"
	<ssengar@...ux.microsoft.com>, "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>, "linux-fbdev@...r.kernel.org"
	<linux-fbdev@...r.kernel.org>, "dri-devel@...ts.freedesktop.org"
	<dri-devel@...ts.freedesktop.org>
Subject: RE: [RFC 1/3] fbdev: hyperv_fb: Remove hyperv_fb driver

From: Prasanna Kumar T S M <ptsm@...ux.microsoft.com> Sent: Thursday, September 11, 2025 9:28 AM
> 
> Hi Michael,
> 
> On 10-09-2025 20:55, Michael Kelley wrote:
> > From: Thomas Zimmermann <tzimmermann@...e.de> Sent: Wednesday, September
> 10, 2025 2:36 AM
> >>
> >> Hi
> >>
> >> Am 09.09.25 um 18:58 schrieb Prasanna Kumar T S M:
> >>> The Hyper-V DRM driver is available since kernel version 5.14 and
> >>> provides full KMS support along with fbdev emulation via the DRM fbdev
> >>> helpers. This makes the hyperv_fb driver redundant, remove it.
> >>
> >> I'm all for removing obsolete drivers. But hyperv_drm likely first needs
> >> to merge the patch at
> >> https://lore.kernel.org/dri-devel/20250904145806.430568-5-tzimmermann@suse.de/
> >> It's been tested and works well. If maintainers from Microsoft have a
> >> look at the patch first, we could possibly land it fairly soon.
> >
> > Thomas --
> >
> > My testing of your v3 patch series for vblank timers ended up getting a
> > WARN_ON after about 3 days of usage. See [1]. So I don't think it's 100%
> > ready yet.
> >
> > But I agree we need your synthetic vblank timer support to address the
> > Hyper-V DRM driver performance issue, before removing the Hyper-V
> > fbdev driver. (See [2] for a description of the performance issue.)
> >
> > Second, isn't it customary to mark a driver as deprecated for a period
> > of time, before removing it entirely? I don't see any documentation
> > on the deprecation process, but I've seen it done in other cases. If you
> > grep through all the kernel Kconfig files, you'll see entries tagged with
> > DEPRECATED. Also the driver should be updated to output a deprecated
> > message when it loads.
> 
> Is deprecating the driver a mandatory step?
> 

I'm not aware of a mandatory requirement, at least not in the sense
of it being documented in Documentation/process like other aspects
of the Linux kernel development process. So I would say it's up to
the Maintainers for Hyper-V and FBDEV as to whether the Hyper-V
FB driver should go through a deprecation phase before being
removed.

Of course, the purpose of the deprecation phase is to be "nice"
to users of the driver by giving them some warning that it is going
away. That gives them an opportunity to raise objections, and/or
to do any necessary migration to the replacement driver. I suspect
there aren't many (or any?) users of Hyper-V FB that can't just move
to the Hyper-V DRM driver, but who knows. We might be surprised.

Michael