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From: Ard Biesheuvel <ardb@kernel.org>
To: Nikhil Mahale <nmahale@nvidia.com>
Cc: Michael Kelley <mikelley@microsoft.com>,
	Arnd Bergmann <arnd@arndb.de>, Will Deacon <will@kernel.org>,
	Catalin Marinas <catalin.marinas@arm.com>,
	Mark Rutland <mark.rutland@arm.com>,
	Marc Zyngier <maz@kernel.org>,
	Linux ARM <linux-arm-kernel@lists.infradead.org>,
	gregkh <gregkh@linuxfoundation.org>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	"linux-hyperv@vger.kernel.org" <linux-hyperv@vger.kernel.org>,
	linux-efi <linux-efi@vger.kernel.org>,
	linux-arch <linux-arch@vger.kernel.org>,
	"olaf@aepfle.de" <olaf@aepfle.de>,
	Andy Whitcroft <apw@canonical.com>,
	vkuznets <vkuznets@redhat.com>, Jason Wang <jasowang@redhat.com>,
	"marcelo.cerri@canonical.com" <marcelo.cerri@canonical.com>,
	KY Srinivasan <kys@microsoft.com>,
	Sunil Muthuswamy <sunilmut@microsoft.com>,
	Boqun Feng <boqun.feng@gmail.com>
Subject: Re: [PATCH v6 09/10] arm64: efi: Export screen_info
Date: Mon, 18 May 2020 14:51:11 +0200
Message-ID: <CAMj1kXEpryfqk5eKxB5NrDcriEBRQKEHnDVZNBMfB4DY=708fw@mail.gmail.com> (raw)
In-Reply-To: <4202ea20-6e51-31d3-44b1-3861798a8158@nvidia.com>

On Mon, 18 May 2020 at 06:25, Nikhil Mahale <nmahale@nvidia.com> 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@arndb.de> Sent: Wednesday, March 18, 2020 2:27 AM
> >>>
> >>> On Wed, Mar 18, 2020 at 1:18 AM Michael Kelley <mikelley@microsoft.com> wrote:
> >>>> From: Arnd Bergmann <arnd@arndb.de>
> >>>>> On Sat, Mar 14, 2020 at 4:36 PM Michael Kelley <mikelley@microsoft.com> 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@microsoft.com>
> >>>>>
> >>>>> 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.

  reply index

Thread overview: 45+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-03-14 15:35 [PATCH v6 00/10] Subject: Enable Linux guests on Hyper-V on ARM64 Michael Kelley
2020-03-14 15:35 ` [PATCH v6 01/10] arm64: hyperv: Add core Hyper-V include files Michael Kelley
2020-03-15 17:31   ` Marc Zyngier
2020-03-18  0:10     ` Michael Kelley
2020-03-16  8:47   ` Arnd Bergmann
2020-03-18  0:12     ` Michael Kelley
2020-03-18 10:10       ` Arnd Bergmann
2020-03-19 21:31         ` Michael Kelley
2020-03-20 16:38           ` Arnd Bergmann
2020-03-14 15:35 ` [PATCH v6 02/10] arm/arm64: smccc-1.1: Add vendor specific owner definition Michael Kelley
2020-03-14 15:35 ` [PATCH v6 03/10] arm64: hyperv: Add hypercall and register access functions Michael Kelley
2020-03-14 15:35 ` [PATCH v6 04/10] arm64: hyperv: Add memory alloc/free functions for Hyper-V size pages Michael Kelley
2020-03-16  8:22   ` Arnd Bergmann
2020-03-16  8:30     ` gregkh
2020-03-16  8:30     ` Marc Zyngier
2020-03-16  8:32       ` Arnd Bergmann
2020-03-18  0:15         ` Michael Kelley
2020-03-18  9:57           ` Arnd Bergmann
2020-03-19 21:43             ` Michael Kelley
2020-03-20 16:28               ` Arnd Bergmann
2020-03-20 17:22                 ` Michael Kelley
2020-03-14 15:35 ` [PATCH v6 05/10] arm64: hyperv: Add interrupt handlers for VMbus and stimer Michael Kelley
2020-03-16  8:25   ` Arnd Bergmann
2020-03-18  0:16     ` Michael Kelley
2020-03-18  9:52       ` Arnd Bergmann
2020-03-14 15:35 ` [PATCH v6 06/10] arm64: hyperv: Add kexec and panic handlers Michael Kelley
2020-03-15 18:15   ` Marc Zyngier
2020-03-18  0:17     ` Michael Kelley
2020-03-14 15:35 ` [PATCH v6 07/10] arm64: hyperv: Initialize hypervisor on boot Michael Kelley
2020-03-16  8:29   ` Arnd Bergmann
2020-03-18  0:17     ` Michael Kelley
2020-03-18  9:44       ` Arnd Bergmann
2020-03-14 15:35 ` [PATCH v6 08/10] Drivers: hv: vmbus: Add hooks for per-CPU IRQ Michael Kelley
2020-03-14 15:35 ` [PATCH v6 09/10] arm64: efi: Export screen_info Michael Kelley
2020-03-16  8:20   ` Arnd Bergmann
2020-03-18  0:18     ` Michael Kelley
2020-03-18  9:26       ` Arnd Bergmann
2020-03-19 21:46         ` Michael Kelley
2020-05-13 14:26           ` Nikhil Mahale
2020-05-18  4:25             ` Nikhil Mahale
2020-05-18 12:51               ` Ard Biesheuvel [this message]
2020-05-22 11:14                 ` Nikhil Mahale
2020-05-22 11:32                   ` Ard Biesheuvel
2020-03-14 15:35 ` [PATCH v6 10/10] Drivers: hv: Enable Hyper-V code to be built on ARM64 Michael Kelley
     [not found] ` <20200318031130.5476-1-hdanton@sina.com>
2020-03-19 21:04   ` [PATCH v6 03/10] arm64: hyperv: Add hypercall and register access functions Michael Kelley

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