[RFC 0/3] drm: Add DRM text mode

[Noralf Trønnes]

Den 29.07.2016 10:23, skrev Daniel Vetter:
> Actually adding David.
> -Daniel
>
> On Fri, Jul 29, 2016 at 10:20:51AM +0200, Daniel Vetter wrote:
>> On Thu, Jul 28, 2016 at 04:15:04PM +0200, Noralf Trønnes wrote:
>>> This patchset explores the idea of adding a DRM text mode
>>> (like VGA text mode) to get an alternative to fbcon/fbdev.
>>>
>>> David Hermann has done alot of work on a fbcon replacement:
>>> - drm: add kernel-log renderer (Mar 2014)[1]
>>> - kmscon:
>>>    development started Nov 2011
>>>    added to systemd Oct 2014
>>>    removed from systemd Jul 2015[2]
>>>
>>> Since this work seems to have stalled, I propose an alternative solution
>>> to this problem that will also give VT console support while we're waiting
>>> for a userspace console.
>>>
>>> The idea is that the driver provides a modeset like with the fbdev emulation
>>> code, and from this a text buffer representation is made. The console code
>>> can now write to this text buffer and ask for the text buffer to be
>>> flushed/rendered to the pixel buffer.
>>>
>>> In this hack/implementation of the idea, I have just hijacked a CMA backed
>>> fbdev framebuffer to keep it simple.
>>> I have reused the default buffer format that VT uses.
>>> Getting panic handling to actually work, seems to be a challenge as Daniel
>>> describes on the DRMJanitors page[3].
>>>
>>> Is this idea of a DRM text mode worth developing further?
>> I guess some simpler drm console with vt support which would allow us to
>> get rid of fbdev could make sense. And there's definitely going to be a
>> lot of overlap with a full userspace solution using something like kmscon.
>>
>> But we can't just add a new drm text console, there's a pile of prep work
>> needed that David started for either approach:
>> - Nuking fbdev means no more fbdev drivers for firmware consoles (uboot,
>>    uefi, vga, ...). The simpledrm driver would cover this, would be great
>>    to get that landed (especially now that we have the simple pipe
>>    helpers!).

I have rebased simpledrm on drm_simple_display_pipe and have it working on
a Raspberry Pi. modetest gives me a wrong picture, but that is probably
because of some wrong format conversion since fbdev using the native
rgb565 format works fine.

I could finish this up and send a patch unless David is working on 
something?

I have used this version of simpledrm: [PATCH 09/11] drm: add SimpleDRM 
driver
https://lists.freedesktop.org/archives/dri-devel/2014-January/052594.html

>> - One nice benefit of fbdev is that it automatically gets rid of these
>>    firmware-based display drivers when the real driver loads. Some drivers
>>    do this properly even when fbdev isn't enabled (see i915_kick_out_vgacon
>>    and i915_kick_out_firmware_fb), but pretty much all others fail in some
>>    way or the other. David also had some code (iirc as part of simpledrm)
>>    to solve this issue.
>>
>> - I think we need more than rbg565 render support, iirc David also had
>>    some work in that area.
>>
>> - None of these approaches has a good answer yet to the configuration
>>    question. For a full VT I think we definitely should share the setup
>>    logic with the fbdev emulation code to make it useful, but as describe
>>    in the DRMJanitors page handlings panics is an entierly different
>>    problem.
>>
>> Definitely coordinate with David Herrmann here too, since if we do
>> something in this area it should be widely supported.
>>
>> Aside: Where's the pull request for your driver? ;-)

I have been working on tinydrm for 6 months straight now and it wasn't fun
anymore. So I figured this was a good time to take a break from it and do
something else for a while :-)


Noralf.

>> Cheers, Daniel
>>
>>>
>>> Noralf.
>>>
>>> [1] https://lwn.net/Articles/589858/
>>> [2] https://github.com/systemd/systemd/pull/747
>>> [3] https://www.x.org/wiki/DRMJanitors/#makepanichandlingwork
>>>
>>>
>>> Noralf Trønnes (3):
>>>    drm: Add support for text framebuffer
>>>    drm/text: Add panic and boot console support
>>>    drm/text: Add VT console support
>>>
>>>   drivers/gpu/drm/Makefile                    |   1 +
>>>   drivers/gpu/drm/drm-text/Makefile           |   5 +
>>>   drivers/gpu/drm/drm-text/drm-text-buffer.c  | 340 ++++++++++++++++++++++++++++
>>>   drivers/gpu/drm/drm-text/drm-text-console.c | 205 +++++++++++++++++
>>>   drivers/gpu/drm/drm-text/drm-text-debugfs.c | 295 ++++++++++++++++++++++++
>>>   drivers/gpu/drm/drm-text/drm-text-vt.c      | 197 ++++++++++++++++
>>>   drivers/gpu/drm/drm-text/drm-text.h         |  99 ++++++++
>>>   7 files changed, 1142 insertions(+)
>>>   create mode 100644 drivers/gpu/drm/drm-text/Makefile
>>>   create mode 100644 drivers/gpu/drm/drm-text/drm-text-buffer.c
>>>   create mode 100644 drivers/gpu/drm/drm-text/drm-text-console.c
>>>   create mode 100644 drivers/gpu/drm/drm-text/drm-text-debugfs.c
>>>   create mode 100644 drivers/gpu/drm/drm-text/drm-text-vt.c
>>>   create mode 100644 drivers/gpu/drm/drm-text/drm-text.h
>>>
>>> --
>>> 2.8.2
>>>
>>> _______________________________________________
>>> dri-devel mailing list
>>> dri-devel at lists.freedesktop.org
>>> https://lists.freedesktop.org/mailman/listinfo/dri-devel
>> -- 
>> Daniel Vetter
>> Software Engineer, Intel Corporation
>> http://blog.ffwll.ch