Re: [PATCH v6 09/10] arm64: efi: Export screen_info

[Ard Biesheuvel]

On Mon, 18 May 2020 at 06:25, Nikhil Mahale <nmahale@xxxxxxxxxx> wrote:
>
> On 5/13/20 7:56 PM, Nikhil Mahale wrote:
> > On 3/20/20 3:16 AM, Michael Kelley wrote:
> >> From: Arnd Bergmann <arnd@xxxxxxxx> Sent: Wednesday, March 18, 2020 2:27 AM
> >>>
> >>> On Wed, Mar 18, 2020 at 1:18 AM Michael Kelley <mikelley@xxxxxxxxxxxxx> wrote:
> >>>> From: Arnd Bergmann <arnd@xxxxxxxx>
> >>>>> On Sat, Mar 14, 2020 at 4:36 PM Michael Kelley <mikelley@xxxxxxxxxxxxx> wrote:
> >>>>>>
> >>>>>> The Hyper-V frame buffer driver may be built as a module, and
> >>>>>> it needs access to screen_info. So export screen_info.
> >>>>>>
> >>>>>> Signed-off-by: Michael Kelley <mikelley@xxxxxxxxxxxxx>
> >>>>>
> >>>>> Is there any chance of using a more modern KMS based driver for the screen
> >>>>> than the old fbdev subsystem? I had hoped to one day completely remove
> >>>>> support for the old CONFIG_VIDEO_FBDEV and screen_info from modern
> >>>>> architectures.
> >>>>>
> >>>>
> >>>> The current hyperv_fb.c driver is all we have today for the synthetic Hyper-V
> >>>> frame buffer device.  That driver builds and runs on both ARM64 and x86.
> >>>>
> >>>> I'm not knowledgeable about video/graphics drivers, but when you
> >>>> say "a more modern KMS based driver", are you meaning one based on
> >>>> DRM & KMS?  Does DRM make sense for a "dumb" frame buffer device?
> >>>> Are there any drivers that would be a good pattern to look at?
> >>>
> >>> It used to be a lot harder to write a DRM driver compared to an fbdev
> >>> driver, but this has changed to the opposite over the years.
> >>>
> >>> A fairly minimal example would be drivers/gpu/drm/pl111/pl111_drv.c
> >>> or anything in drivers/gpu/drm/tiny/, but you may want to look at the
> >>> other hypervisor platforms first, i.e drivers/gpu/drm/virtio/virtgpu_drv.c,
> >>> drivers/gpu/drm/vmwgfx/vmwgfx_drv.c, drivers/gpu/drm/xen/xen_drm_front.c,
> >>> drivers/gpu/drm/qxl/qxl_drv.c, and drivers/gpu/drm/bochs/bochs_drv.c.
> >>>
> >>
> >> Thanks for the pointers, especially for the other hypervisors.
> >>
> > Sorry if anybody in 'to' or 'cc' is receiving this reply multiple times.
> > I had configured by email client incorrectly to reply.
> >
> > screen_info is still useful with a modern KMS-based driver.  It exposes
> > the mode parameters that the GOP driver chose.  This information is
> > needed to implement seamless or glitchless boot, by both ensuring that
> > the scanout parameters don't change and being able to read back the
> > scanout image to populate the initial contents of the new surface.
> >
> > This works today on arches which implement (U)EFI and export
> > screen_info, including x86 and powerpc, but doesn't work on arm or
> > arm64.  As arm64 systems that implement UEFI with real GOP drivers
> > become more prevalent, it would be nice to be have these features there
> > as well.
>
> In addition to this, even if a driver doesn't implement a framebuffer
> console, or if it does but has an option to disable it, the driver still
> needs to know whether the EFI console is using resources on the GPU so
> it can avoid clobbering them. For example screen_info provides information
> like offset and size of EFI console, using this information driver can
> reserve memory used by console and prevent corruption on it.
>
> I think arm64 should export screen_info.
>

If there are reasons why KMS or fbdev drivers may need to access the
information in screen_info, it should be exported. I don't think that
is under debate here.