Re: [PATCH v10 03/10] drm/i915/display/i9xx: Add a disable_tiling() for i9xx planes

["Ville Syrjälä" <ville.syrjala@linux.intel.com>]

On Wed, Jun 18, 2025 at 11:31:21AM +0200, Jocelyn Falempe wrote:
> drm_panic draws in linear framebuffer, so it's easier to re-use the
> current framebuffer, and disable tiling in the panic handler, to show
> the panic screen.
> This assumes that the alignment restriction is always smaller in
> linear than in tiled.
> It also assumes that the linear framebuffer size is always smaller
> than the tiled.
> 
> Signed-off-by: Jocelyn Falempe <jfalempe@redhat.com>
> ---
> 
> v7:
>  * Reword commit message about alignment/size when disabling tiling (Ville Syrjälä)
> 
>  drivers/gpu/drm/i915/display/i9xx_plane.c     | 23 +++++++++++++++++++
>  .../drm/i915/display/intel_display_types.h    |  2 ++
>  2 files changed, 25 insertions(+)
> 
> diff --git a/drivers/gpu/drm/i915/display/i9xx_plane.c b/drivers/gpu/drm/i915/display/i9xx_plane.c
> index 8f15333a4b07..0807fae12450 100644
> --- a/drivers/gpu/drm/i915/display/i9xx_plane.c
> +++ b/drivers/gpu/drm/i915/display/i9xx_plane.c
> @@ -905,6 +905,27 @@ static const struct drm_plane_funcs i8xx_plane_funcs = {
>  	.format_mod_supported_async = intel_plane_format_mod_supported_async,
>  };
>  
> +static void i9xx_disable_tiling(struct intel_plane *plane)
> +{
> +	struct intel_display *display = to_intel_display(plane);
> +	enum i9xx_plane_id i9xx_plane = plane->i9xx_plane;
> +	u32 dspcntr;
> +	u32 reg;
> +
> +	dspcntr = intel_de_read_fw(display, DSPCNTR(display, i9xx_plane));
> +	dspcntr &= ~DISP_TILED;
> +	intel_de_write_fw(display, DSPCNTR(display, i9xx_plane), dspcntr);
> +
> +	if (DISPLAY_VER(display) >= 4) {
> +		reg = intel_de_read_fw(display, DSPSURF(display, i9xx_plane));
> +		intel_de_write_fw(display, DSPSURF(display, i9xx_plane), reg);
> +
> +	} else {
> +		reg = intel_de_read_fw(display, DSPADDR(display, i9xx_plane));
> +		intel_de_write_fw(display, DSPADDR(display, i9xx_plane), reg);
> +	}
> +}

I thought I already shot this down before, but apparently this
got merged now :(

Just to reiterate why we don't want these 'disable tiling' hacks:
- different tiling formats have different stride/alignment/watermark
  requirements so one can't safely change from one tiling to another
- this completely fails to account for the TILEOFF vs. LINOFF stuff
- etc.

So IMO these hacks must be removed and instead the code must learn how
to propetly write the tiled data. igt has all the code for that btw
(twice over IIRC) so shouldn't be that hard.

I suppose the only hack we need to keep is to disable compression,
mainly because (IIRC) on flat CCS systems the CPU doesn't have access
to the AUX data to clear it manually.

I also wonder if there are actual igts for this? I think what is needed
is a test that sets random things (different panning, rotation, pixel
foramts, etc.) and triggers the dumper. Not quite sure how the test
could validate that the output is correct though. CRCs might be a bit
tricky since you need an identical reference image.

/me off to summer vacation. Good luck

-- 
Ville Syrjälä
Intel