[U-Boot] [PATCH 06/11] efi_loader: Decouple EFI input/output from stdin/stdout

[Mark Kettenis]

> From: Rob Clark <robdclark at gmail.com>
> Date: Thu, 12 Oct 2017 08:48:39 -0400
> 
> On Thu, Oct 12, 2017 at 3:15 AM, Alexander Graf <agraf at suse.de> wrote:
> >
> >
> > On 12.10.17 00:07, Rob Clark wrote:
> >> On Wed, Oct 11, 2017 at 10:45 AM, Alexander Graf <agraf at suse.de> wrote:
> >>>
> >>>
> >>> On 10.10.17 14:23, Rob Clark wrote:
> >>>> In some cases, it is quite useful to have (for example) EFI on screen
> >>>> but u-boot on serial port.
> >>>>
> >>>> This adds two new optional environment variables, "efiin" and "efiout",
> >>>> which can be used to set EFI console input/output independently of
> >>>> u-boot's input/output.  If unset, EFI console will default to stdin/
> >>>> stdout as before.
> >>>>
> >>>> Signed-off-by: Rob Clark <robdclark at gmail.com>
> >>>
> >>> With this patch, we lose the ability to have the efi in/out go to both
> >>> graphical and serial console, right? This is critical functionality to
> >>> have, since we don't necessarily know which output/input a user ends up
> >>> using.

Seconded.  In many cases I'd want to continue using serial as the
console even if a display is connected.

> >> I'll think about how to support iomux.. but some things like console
> >> size are just not going to work properly there.  And as long as we fix
> >
> > Yeah, those probably would need to get special cased.
> >
> >> the stdout shenanigans (ie. what I was seeing w/ qemu-x86) you can
> >> simply not set efiout var and have things working as before, so you
> >> don't loose any existing functionality (although, like I said, if the
> >> two different consoles have different sizes things aren't going to
> >> work properly for anything other than simple cases).
> >>
> >> In most cases, the display driver should be able to detect whether a
> >> display is connected.. this is what I've done on dragonboard410c, so
> >> if no display plugged in, 'efiout=vidconsole' fails and you fall back
> >> to serial, else you get efi on screen like you would on a "real"
> >> computer.  For boards that have a display driver that isn't able to do
> >> the basic check of whether a cable is plugged in, just don't set
> >> "efiout" (or fix the display driver) ;-)

Display detection will always be somewhat fragile.  The old Sun
workstations used to base the decision on whether a keyboard was
connected.  With no keyboard detected the output would always go to
serial.

> > Are you sure that's what happens on a "real" computer? As far as I
> > remember from all ARM servers running edk2 based firmware that I've
> > touched so far, the default is always to display on serial *and*
> > graphical output at the same time.
> 
> Well, I suppose some of the arm64 server vendors have done a better
> job than others on firmware.. you'd hope they would probe EDID, and
> report correct console dimensions based on display resolution, etc.
> 
> Doing both serial and display works for simple stuff, but it goes
> badly once the efi payload starts trying to change the cursor position
> and write to specific parts of the screen (which both Shell.efi and
> grub try to do).

>From my point of view a bootloader should only do "simple stuff".  All
this fancy display stuff makes a serial console much harder to use.