drm/i915/pfit: Reject pfit downscaling for GMCH platforms

Gen2/3 pfit doesn't support downscaling at all, so reject it.

On i965+ downscaling is supported by the hardware (max scale
factor < 2.0), but as downscaling increases the effective
pixel rate we can't safely allow it unless
intel_crtc_compute_pixel_rate() gets fixed. Probably the
best solution would be to calculate (at least an
apporiximate) pfit destination window and use
ilk_pipe_pixel_rate() for all platforms. For now reject
downscaling on all gmch platforms.

The intel ddx has a similar check for this in userspace,
modesetting ddx does not. And presumably wayland compositors
also do not make such assumptions in userspace.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20241016143134.26903-4-ville.syrjala@linux.intel.com
Acked-by: Jani Nikula <jani.nikula@intel.com>

diff --git a/drivers/gpu/drm/i915/display/intel_panel.c b/drivers/gpu/drm/i915/display/intel_panel.c
index fb7def7..89cac3b 100644 (file)
--- a/drivers/gpu/drm/i915/display/intel_panel.c
+++ b/drivers/gpu/drm/i915/display/intel_panel.c
@@ -681,6 +681,7 @@ static void i9xx_scale_aspect(struct intel_crtc_state *crtc_state,
 static int gmch_panel_fitting(struct intel_crtc_state *crtc_state,
                              const struct drm_connector_state *conn_state)
 {
+       struct intel_display *display = to_intel_display(crtc_state);
        struct intel_crtc *crtc = to_intel_crtc(crtc_state->uapi.crtc);
        struct drm_i915_private *dev_priv = to_i915(crtc->base.dev);
        u32 pfit_control = 0, pfit_pgm_ratios = 0, border = 0;
@@ -693,6 +694,25 @@ static int gmch_panel_fitting(struct intel_crtc_state *crtc_state,
            adjusted_mode->crtc_vdisplay == pipe_src_h)
                goto out;
 
+       /*
+        * TODO: implement downscaling for i965+. Need to account
+        * for downscaling in intel_crtc_compute_pixel_rate().
+        */
+       if (adjusted_mode->crtc_hdisplay < pipe_src_w) {
+               drm_dbg_kms(display->drm,
+                           "[CRTC:%d:%s] pfit horizontal downscaling (%d->%d) not supported\n",
+                           crtc->base.base.id, crtc->base.name,
+                           pipe_src_w, adjusted_mode->crtc_hdisplay);
+               return -EINVAL;
+       }
+       if (adjusted_mode->crtc_vdisplay < pipe_src_h) {
+               drm_dbg_kms(display->drm,
+                           "[CRTC:%d:%s] pfit vertical downscaling (%d->%d) not supported\n",
+                           crtc->base.base.id, crtc->base.name,
+                           pipe_src_h, adjusted_mode->crtc_vdisplay);
+               return -EINVAL;
+       }
+
        switch (conn_state->scaling_mode) {
        case DRM_MODE_SCALE_CENTER:
                /*