Re: Panic booting qemu-system-sparc64 with bochs_drm

[Sam Ravnborg <sam@ravnborg.org>]

Hi Mark.

On Sat, Jul 04, 2020 at 03:16:47PM +0100, Mark Cave-Ayland wrote:
> On 04/07/2020 14:41, Sam Ravnborg wrote:
> 
> > I think what is happening is that the bochs driver request a shadow copy
> > for the frambuffer. And with the change to fbops we now use the cfb_
> > functions to write to the shadow framebuffer - which is not in any IO
> > space. So this does not work.
> > 
> > So maybe all we need is the fix in drm_fb_helper_dirty_blit_real().
> > If you try to undo the change so fbops is set to &drm_fbdev_fb_ops,
> > but keep the fix in drm_fb_helper_dirty_blit_real() how does it then
> > behave?
> 
> Bingo! I just tried that and the framebuffer is now working under qemu-system-sparc64
> again - thank you so much for the help! From what you said I guess
> drm_fb_helper_dirty_blit_real() is responsible syncing the shadow copy?
> 
> Below is the current working diff based upon your previous one: it certainly feels
> like the difference in memcpy() behaviour should be hidden away in fb_memcpy_tofb()
> or similar.

From your feedback so far I thnk the minimal fix would be like this:

> --- a/drivers/gpu/drm/drm_fb_helper.c
> +++ b/drivers/gpu/drm/drm_fb_helper.c
> .. static void drm_fb_helper_dirty_blit_real(struct drm_fb_helper *fb_helper,
>         size_t len = (clip->x2 - clip->x1) * cpp;
>         unsigned int y;
> 
>         for (y = clip->y1; y < clip->y2; y++) {
> -               memcpy(dst, src, len);
> +               fb_memcpy_tofb(dst, src, len);
>                 src += fb->pitches[0];
>                 dst += fb->pitches[0];
>         }

(Hand edited, patch s not a valid syntax)

But I need feedback from someone that know all this a bit better
to judge if this is an OK change.
For once - this will only work with shadow buffers.

	Sam



> diff --git a/include/drm/drm_mode_config.h b/include/drm/drm_mode_config.h
> index 6c3ef49b46b3..dce9adf7d189 100644
> --- a/include/drm/drm_mode_config.h
> +++ b/include/drm/drm_mode_config.h
> @@ -865,6 +865,15 @@ struct drm_mode_config {
>          */
>         bool prefer_shadow_fbdev;
> 
> +       /**
> +        * @use_cfb_for_fbdev:
> +        *
> +        * Use cfb variants of drm_fb_helper_cfb_{fillrect,copyarea,imageblit}
> +        * The cfb variants are required when the CPU do not allow direct
> +        * access to the framebuffer (for example sparc64)
> +        */
> +       bool use_cfb_for_fbdev;
> +
>         /**
>          * @quirk_addfb_prefer_xbgr_30bpp:
>          *
> 
> > I did not find time to follow your instructions to test this myself with
> > qemu - sorry.
> 
> No worries, I do appreciate that as a maintainer it can be hard to fit these things
> around life, family, job etc.
> 
> 
> ATB,
> 
> Mark.
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