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@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. > 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
