On 04/09/2023 16:29, Thomas Zimmermann wrote:
Hi Jocelyn,
thanks for moving this effort forward. It's much appreciated. I looked
through the patches and tried the patchset on my test machine.
Thanks for taking the time to review and test it.
Am 09.08.23 um 21:17 schrieb Jocelyn Falempe:
This introduces a new drm panic handler, which displays a message when
a panic occurs.
So when fbcon is disabled, you can still see a kernel panic.
This is one of the missing feature, when disabling VT/fbcon in the
kernel:
https://www.reddit.com/r/linux/comments/10eccv9/config_vtn_in_2023/
Fbcon can be replaced by a userspace kms console, but the panic screen
must be done in the kernel.
This is a proof of concept, and works only with simpledrm, using the
drm_client API.
This implementation with the drm client API, allocates new
framebuffers, and looks a bit too complex to run in a panic handler.
Maybe we should add an API to "steal" the current framebuffer instead,
because in a panic handler user-space is already stopped.
Yes, that was also my first thought. I'd use an extra callback in struct
drm_driver, like this:
struct drm_driver {
int (*get_scanout_buffer)(/* return HW scanout */)
}
The scanout buffer would be described by kernel virtual address address,
resolution, color format and scanline pitch. And that's what the panic
handler uses.
Thanks, I will try this solution. It shouldn't be too hard for simpledrm.
Any driver implementing this interface would support the panic handler.
If there's a concurrent display update, we'd have to synchronize.
Normally in the panic handler, the kernel is already single-threaded, so
there won't be another task doing things in parallel. (But the GPU might
still be busy doing other stuff, if we're in the middle of a game).
I think this drm_panic should be a "best effort", we can't guarantee the
user will see the panic screen in all panic situations.
To test it, make sure you're using the simpledrm driver, and trigger a
panic:
echo c > /proc/sysrq-trigger
The penguin was cute. :)
This only works if the display is already running. I had to start Gnome
to set a display mode. Then let the panic handler take over the output.
oh, I didn't expect this limitation. I will try to test that too. It
might also get fixed by using the get_scanout_buffer() above.
But with simpledrm, we could even display a message without an output,
as the framebuffer is always there.
There is one thing I don't know how to do, is to unregister the
drm_panic when the graphic driver is unloaded.
drm_client_register() says it will automatically unregister on driver
unload. But then I don't know how to remove it from my linked list,
and free the drm_client_dev struct.
Unregistering wouldn't be necessary with this proposed
get_scanout_buffer. In the case of a panic, just remain silent if
there's no driver that provides such a callback.
Is there a way to loop over all drm_devices ?
otherwise drm_panic may still call get_scanout_buffer() on a freed
device, which would be problematic.
This is a first draft, so let me know what do you think about it.
One thing that will need serious work is the raw output. The current
blitting for XRGB8888 is really just a quick-and-dirty hack.
I think we should try to reuse fbdev's blitting code, if possible. The
fbdev core, helpers and console come with all the features we need. We
really only need to make them work without the struct fb_info, which is
a full fbdev device.
I'm a bit reluctant to re-use the fbdev code, for a few reasons:
* I want drm_panic to work if CONFIG_FB and CONFIG_DRM_FBDEV_EMULATION
are not set.
* drm_panic only needs two things, to clear the framebuffer, and then
draw white pixels where needed. As the frame is static, and the amount
of white pixels is low, that should be good enough. So copy_area() or
image_blit() are not that useful.
* For this particular use-case, performances are not a priority, it
doesn't matter if it takes 10us or 100ms to draw the panic screen, as it
will stay the same until the user reboots.
* Some aggressive optimizations might cause issues in a panic handler,
like if you use workqueue/tasklet.
On the other hand, writing the code for all supported formats is a bit
tedious. drm_log [1] did it in ~300 lines, which should keep code
duplication low.
In struct fb_ops, there are callbacks for modifying the framebuffer. [1]
They are used by fbcon foir drawing. But they operate on fb_info.
For a while I've been thinking about using something like a drawable to
provide some abstractions:
struct drawable {
/* store buffer parameters here */
...
struct drawable_funcs *funcs;
};
struct drawable_funcs {
/* have function pointers similar to struct fb_ops */
fill_rect()
copy_area()
image_blit()
};
We cannot rewrite all the existing fbdev drivers. To make this work with
fbdev, we'd need adapter code that converts from drawable to fb_info and
forwards to the existing helpers in fb_ops.
But for DRM's panic output, drawable_funcs would have to point to the
scanout buffer and compatible callback funcs, for which we have
implementations in fbdev.
We might be able to create console-like output that is independent from
the fb_info. Hence, we could possible reuse a good chunk of the current
panic output.
I think that was the goal of drm_log, but this can be done better in
userspace, for example there is work ongoing to make plymouth display
them during the boot [2].
For the panic screen, only the kernel can do it. I also think the
current fbcon/kernel log is good for developer, but too technical for
most end-user.
Best regards,
--
Jocelyn
[1]
https://lore.kernel.org/all/1394131242-29567-1-git-send-email-dh.herrmann@xxxxxxxxx/
[2] https://gitlab.freedesktop.org/plymouth/plymouth/-/merge_requests/224
Best regards
Thomas
[1] https://elixir.bootlin.com/linux/v6.5.1/source/include/linux/fb.h#L273
Best regards,
Jocelyn Falempe (2):
drm/panic: Add a drm panic handler
drm/simpledrm: Add drm_panic support
drivers/gpu/drm/Kconfig | 11 ++
drivers/gpu/drm/Makefile | 1 +
drivers/gpu/drm/drm_drv.c | 3 +
drivers/gpu/drm/drm_panic.c | 286 +++++++++++++++++++++++++++++++
drivers/gpu/drm/tiny/simpledrm.c | 2 +
include/drm/drm_panic.h | 26 +++
6 files changed, 329 insertions(+)
create mode 100644 drivers/gpu/drm/drm_panic.c
create mode 100644 include/drm/drm_panic.h
base-commit: 6995e2de6891c724bfeb2db33d7b87775f913ad1
