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From: Nikhil Mahale <nmahale@nvidia.com>
To: Ard Biesheuvel <ardb@kernel.org>
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: Fri, 22 May 2020 16:44:52 +0530
Message-ID: <e8850c9b-965a-9ed4-fb22-f41de5f72b60@nvidia.com> (raw)
In-Reply-To: <CAMj1kXEpryfqk5eKxB5NrDcriEBRQKEHnDVZNBMfB4DY=708fw@mail.gmail.com>

On 5/18/20 6:21 PM, Ard Biesheuvel wrote:
> External email: Use caution opening links or attachments
> 
> 
> 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.
> 

Hi Ard, thanks for your feedback. If my understanding is correct,
you are agree to export screen_info. Can you provide guidance on how can
we proceed here? are you willing to accept this current patch as-is or
would you like me to re-submit the patch with the additional rationale
provided?

Thanks,
Nikhil Mahale

  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
2020-05-22 11:14                 ` Nikhil Mahale [this message]
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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