[RFC][PATCH v6 0/5] drm/panic: Add a drm panic handler

[Maxime Ripard]

Hi,

Sorry for not taking a look at this sooner.

On Tue, Dec 05, 2023 at 10:48:22AM +0100, Jocelyn Falempe wrote:
> drm/panic: Add a drm panic handler
> 
> 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 with simpledrm and mgag200, using a new get_scanout_buffer() api
> 
> To test it, make sure you're using the simpledrm driver, and trigger a panic:
> echo c > /proc/sysrq-trigger
> 
> v2:
>  * Use get_scanout_buffer() instead of the drm client API. (Thomas Zimmermann)
>  * Add the panic reason to the panic message (Nerdopolis)
>  * Add an exclamation mark (Nerdopolis)
>  
> v3:
>  * Rework the drawing functions, to write the pixels line by line and
>  to use the drm conversion helper to support other formats.
>  (Thomas Zimmermann)
>  
> v4:
>  * Fully support all simpledrm formats using drm conversion helpers
>  * Rename dpanic_* to drm_panic_*, and have more coherent function name.
>    (Thomas Zimmermann)
>  * Use drm_fb_r1_to_32bit for fonts (Thomas Zimmermann)
>  * Remove the default y to DRM_PANIC config option (Thomas Zimmermann)
>  * Add foreground/background color config option
>  * Fix the bottom lines not painted if the framebuffer height
>    is not a multiple of the font height.
>  * Automatically register the driver to drm_panic, if the function
>    get_scanout_buffer() exists. (Thomas Zimmermann)
>  * Add mgag200 support.
>  
> v5:
>  * Change the drawing API, use drm_fb_blit_from_r1() to draw the font.
>    (Thomas Zimmermann)
>  * Also add drm_fb_fill() to fill area with background color.
>  * Add draw_pixel_xy() API for drivers that can't provide a linear buffer.
>  * Add a flush() callback for drivers that needs to synchronize the buffer.
>  * Add a void *private field, so drivers can pass private data to
>    draw_pixel_xy() and flush(). 
>  * Add ast support.
>  * Add experimental imx/ipuv3 support, to test on an ARM hw. (Maxime Ripard)
> 
> v6:
>  * Fix sparse and __le32 warnings
>  * Drop the IMX/IPUV3 experiment, it was just to show that it works also on ARM devices.
> 
> With mgag200 support, I was able to test that the xrgb8888 to rgb565 conversion is working.
> 
> IMX/IPUV3 support is not complete, I wasn't able to have etnaviv working on my board.

I'm not sure what you mean here, etnaviv shouldn't be involved in the
panic handling at all, so it's not super important that you couldn't get
it to work.

> But it shows that it can still work on ARM with DMA buffer in this
> case.

It simplifies the situation a bit, and looking at the v5 I think there's
a couple of things that we need to handle / document, so it would still
be very valuable.

I'll review that v5 patch

Maxime
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