linux-kernel - Re: [RFC 0/6] SimpleDRM Driver (was: dvbe driver)

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Date:	Fri, 28 Jun 2013 12:43:05 +0200
From:	David Herrmann <dh.herrmann@...il.com>
To:	Stephen Warren <swarren@...dotorg.org>
Cc:	"dri-devel@...ts.freedesktop.org" <dri-devel@...ts.freedesktop.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Dave Airlie <airlied@...il.com>, linux-fbdev@...r.kernel.org,
	Stephen Warren <swarren@...dia.com>,
	Olof Johansson <olof@...om.net>
Subject: Re: [RFC 0/6] SimpleDRM Driver (was: dvbe driver)

Hi

On Wed, Jun 26, 2013 at 11:30 PM, Stephen Warren <swarren@...dotorg.org> wrote:
> On 06/24/2013 04:27 PM, David Herrmann wrote:
>> Hi
>>
>> This is my second revision of the dvbe driver. I renamed it to SimpleDRM to
>> show the resemblence with the recently introduced simplefb.c fbdev driver. The
>> driver is supposed to be the most basic DRM driver similar to efifb.c, vesafb.c,
>> offb.c, simplefb.c, ...
>> It provides a single virtual CRTC+encoder+connector and allows user-space to
>> create one dumb-buffer at a time and attach it.
>>
>> The setup changed slightly. It no longer uses shadow buffers but instead maps
>> the framebuffer directly into userspace. Furthermore, a new infrastructure is
>> used to unload firmware drivers during real hardware drivers probe cycles. Only
>> nouveau was changed to use it, yet.
>>
>> I still have an odd problem when unloading DRM drivers (not just SimpleDRM) with
>> an fbdev fallback. If I call printk() directly after unregister_framebufer(), I
>> get a NULL-deref somewhere in the VT layer (most times hide_cursor()). I haven't
>> figured out exactly where that happens, but I am also very reluctant to spend
>> more time debugging the VT layer.
>
> I tested this on a Tegra ARM system, and it basically worked.

Thanks a lot for the feedback!

> I have one question: With the simplefb driver, and console=tty1 on the
> kernel command-line, I see both the penguins logo and Linux's boot
> messages on the LCD panel that's hooked up through simplefb. However,
> with simpledrm, I only see the penguins logo, but no boot messages. Is
> that expected? How would I solve that if so?

No idea what is going wrong there. Somehow the simpledrm-fbdev device
is not picked up as primary device. I only got a black-cursor on
black-background (visible if painting something else on the fb0
device). And I get NULL-derefs in cursor_hide() during unregistration.
I digged through fbcon.c to find out what's going wrong but without
any success. I will see what I can do.

However, X-server or other apps work perfectly fine with it.

> Note: I needed to apply the following patch to get it to compile:
>
> diff --git a/drivers/gpu/drm/simpledrm/simpledrm_fbdev.c
> b/drivers/gpu/drm/simpledrm/simpledrm_fbdev.c
> index 40a2696..39885c8 100644
> --- a/drivers/gpu/drm/simpledrm/simpledrm_fbdev.c
> +++ b/drivers/gpu/drm/simpledrm/simpledrm_fbdev.c
> @@ -159,7 +159,7 @@ void sdrm_fbdev_cleanup(struct sdrm_device *sdrm)
>  {
>         struct fb_info *info;
>
> -       if (!sdrm->info)
> +       if (!sdrm->fbdev)
>                 return;

Ugh, embarrassing, sorry. I fixed it up. It was a late fix to a avoid
fbcon from panicking.

Thanks!
David

>
>         dev_info(sdrm->ddev->dev, "fbdev cleanup\n");
>
>
>
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