[PATCH v2 2/3] drm/edid: Add drm_edid_get_preferred_size()

[Thomas Zimmermann <tzimmermann@suse.de>]

Add drm_edid_get_preferred_size() to extract the panel's preferred
display resolution from a given EDID. Required for DRM's panel
orientation quirks.

v2:
- handle EDID without pixel timing descriptor (Sashiko)
- fix checks for width and height pointers

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Ard Biesheuvel <ardb@kernel.org>
---
 drivers/gpu/drm/drm_edid.c | 46 ++++++++++++++++++++++++++++++++++++++
 include/drm/drm_edid.h     |  2 ++
 2 files changed, 48 insertions(+)

diff --git a/drivers/gpu/drm/drm_edid.c b/drivers/gpu/drm/drm_edid.c
index 9990a836d0b6..53c7eed7a73e 100644
--- a/drivers/gpu/drm/drm_edid.c
+++ b/drivers/gpu/drm/drm_edid.c
@@ -2872,6 +2872,52 @@ u32 drm_edid_get_panel_id(const struct drm_edid *drm_edid)
 }
 EXPORT_SYMBOL(drm_edid_get_panel_id);
 
+/**
+ * drm_edid_get_preferred_size - Get a panel's preferred size from EDID
+ * @drm_edid: EDID that contains panel ID.
+ * @width: Returns the panel's width in pixels per scanline, if given
+ * @height: Returns the panel's height in scanlines, if given
+ *
+ * This function detects the preferred size of a panel from the given
+ * EDID. There is no such information stored in the EDID block directly,
+ * but the preferred mode often corresponds to the panel's native geometry.
+ *
+ * Return: Zero on success, or a negative errno code otherwise.
+ */
+int drm_edid_get_preferred_size(const struct drm_edid *drm_edid,
+				unsigned int *width, unsigned int *height)
+{
+	const struct edid *edid = drm_edid->edid;
+	const struct detailed_timing *dt;
+	const struct detailed_pixel_timing *pt;
+
+	if (drm_edid->size < EDID_LENGTH)
+		return -EINVAL;
+
+	/*
+	 * Use display mode from the Preferred Timing Descriptor. For old
+	 * and obscure displays, we might need better heuristics.
+	 */
+
+	if (edid->revision < 4 && !(edid->features & DRM_EDID_FEATURE_PREFERRED_TIMING))
+		return -EINVAL; /* no Preferred Timing Descriptor */
+
+	dt = &edid->detailed_timings[0];
+
+	if (!le16_to_cpu(dt->pixel_clock))
+		return -EINVAL;
+
+	pt = &dt->data.pixel_data;
+
+	if (width)
+		*width = (pt->hactive_hblank_hi & 0xf0) << 4 | pt->hactive_lo;
+	if (height)
+		*height = (pt->vactive_vblank_hi & 0xf0) << 4 | pt->vactive_lo;
+
+	return 0;
+}
+EXPORT_SYMBOL(drm_edid_get_preferred_size);
+
 /**
  * drm_edid_read_base_block - Get a panel's EDID base block
  * @adapter: I2C adapter to use for DDC
diff --git a/include/drm/drm_edid.h b/include/drm/drm_edid.h
index 04f7a7f1f108..981aeeb6ddf9 100644
--- a/include/drm/drm_edid.h
+++ b/include/drm/drm_edid.h
@@ -487,6 +487,8 @@ void drm_edid_get_product_id(const struct drm_edid *drm_edid,
 void drm_edid_print_product_id(struct drm_printer *p,
 			       const struct drm_edid_product_id *id, bool raw);
 u32 drm_edid_get_panel_id(const struct drm_edid *drm_edid);
+int drm_edid_get_preferred_size(const struct drm_edid *drm_edid,
+				unsigned int *width, unsigned int *height);
 bool drm_edid_match(const struct drm_edid *drm_edid,
 		    const struct drm_edid_ident *ident);
 bool drm_edid_has_quirk(struct drm_connector *connector, enum drm_edid_quirk quirk);
-- 
2.55.0