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

[Nikhil Mahale <nmahale@nvidia.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