[PATCH v2] drm/bridge: imx8mp-hdmi-tx: allow 0.5% margin with selected clock

[Dominique Martinet <asmadeus@codewreck.org>]

From: Dominique Martinet <dominique.martinet@atmark-techno.com>

This allows the hdmi driver to pick e.g. 64.8MHz instead of 65Mhz when we
cannot output the exact frequency, enabling the imx8mp HDMI output to
support more modes

Tested-by: Adam Ford <aford173@gmail.com> #imx8mp-beacon
Reviewed-by: Frieder Schrempf <frieder.schrempf@kontron.de>
Tested-by: Frieder Schrempf <frieder.schrempf@kontron.de>
Signed-off-by: Dominique Martinet <dominique.martinet@atmark-techno.com>
---
Changes in v2:
- Improve comment about the tolerance
- Link to v1: https://lore.kernel.org/r/20240904083103.1257480-1-dominique.martinet@atmark-techno.com
---
 drivers/gpu/drm/bridge/imx/imx8mp-hdmi-tx.c | 11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/bridge/imx/imx8mp-hdmi-tx.c b/drivers/gpu/drm/bridge/imx/imx8mp-hdmi-tx.c
index 13bc570c5473..200d65184159 100644
--- a/drivers/gpu/drm/bridge/imx/imx8mp-hdmi-tx.c
+++ b/drivers/gpu/drm/bridge/imx/imx8mp-hdmi-tx.c
@@ -23,6 +23,7 @@ imx8mp_hdmi_mode_valid(struct dw_hdmi *dw_hdmi, void *data,
 		       const struct drm_display_mode *mode)
 {
 	struct imx8mp_hdmi *hdmi = (struct imx8mp_hdmi *)data;
+	long round_rate;
 
 	if (mode->clock < 13500)
 		return MODE_CLOCK_LOW;
@@ -30,8 +31,14 @@ imx8mp_hdmi_mode_valid(struct dw_hdmi *dw_hdmi, void *data,
 	if (mode->clock > 297000)
 		return MODE_CLOCK_HIGH;
 
-	if (clk_round_rate(hdmi->pixclk, mode->clock * 1000) !=
-	    mode->clock * 1000)
+	round_rate = clk_round_rate(hdmi->pixclk, mode->clock * 1000);
+	/* imx8mp's pixel clock generator (fsl-samsung-hdmi) cannot generate
+	 * all possible frequencies, so allow some tolerance to support more
+	 * modes.
+	 * Allow 0.5% difference allowed in various standards (VESA, CEA861)
+	 * 0.5% = 5/1000 tolerance (mode->clock is 1/1000)
+	 */
+	if (abs(round_rate - mode->clock * 1000) > mode->clock * 5)
 		return MODE_CLOCK_RANGE;
 
 	/* We don't support double-clocked and Interlaced modes */

---
base-commit: 67784a74e258a467225f0e68335df77acd67b7ab
change-id: 20240907-hdmi-tolerance-4d83074a4517

Best regards,
-- 
Dominique Martinet | Asmadeus