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From patchwork Thu Mar 26 12:20:01 2020
Content-Type: text/plain; charset="utf-8"
MIME-Version: 1.0
Content-Transfer-Encoding: 7bit
Subject: drm/vc4: Fix HDMI mode validation
From: Nicolas Saenz Julienne <nsaenzjulienne@suse.de>
X-Patchwork-Id: 358980
Message-Id: <20200326122001.22215-1-nsaenzjulienne@suse.de>
To: Eric Anholt <eric@anholt.net>,
Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Stefan Wahren <stefan.wahren@i2se.com>, f.fainelli@gmail.com,
Dave Stevenson <dave.stevenson@raspberrypi.com>,
David Airlie <airlied@linux.ie>, linux-kernel@vger.kernel.org,
dri-devel@lists.freedesktop.org, maxime@cerno.tech,
Nicolas Saenz Julienne <nsaenzjulienne@suse.de>,
linux-rpi-kernel@lists.infradead.org
Date: Thu, 26 Mar 2020 13:20:01 +0100
Current mode validation impedes setting up some video modes which should
be supported otherwise. Namely 1920x1200@60Hz.
Fix this by lowering the minimum HDMI state machine clock to pixel clock
ratio allowed.
Fixes: 32e823c63e90 ("drm/vc4: Reject HDMI modes with too high of clocks")
Reported-by: Stefan Wahren <stefan.wahren@i2se.com>
Suggested-by: Dave Stevenson <dave.stevenson@raspberrypi.com>
Signed-off-by: Nicolas Saenz Julienne <nsaenzjulienne@suse.de>
Reviewed-by: Maxime Ripard <mripard@kernel.org>
---
drivers/gpu/drm/vc4/vc4_hdmi.c | 20 ++++++++++++++++----
1 file changed, 16 insertions(+), 4 deletions(-)
diff --git a/drivers/gpu/drm/vc4/vc4_hdmi.c b/drivers/gpu/drm/vc4/vc4_hdmi.c
index cea18dc15f77..340719238753 100644
--- a/drivers/gpu/drm/vc4/vc4_hdmi.c
+++ b/drivers/gpu/drm/vc4/vc4_hdmi.c
@@ -681,11 +681,23 @@ static enum drm_mode_status
vc4_hdmi_encoder_mode_valid(struct drm_encoder *crtc,
const struct drm_display_mode *mode)
{
- /* HSM clock must be 108% of the pixel clock. Additionally,
- * the AXI clock needs to be at least 25% of pixel clock, but
- * HSM ends up being the limiting factor.
+ /*
+ * As stated in RPi's vc4 firmware "HDMI state machine (HSM) clock must
+ * be faster than pixel clock, infinitesimally faster, tested in
+ * simulation. Otherwise, exact value is unimportant for HDMI
+ * operation." This conflicts with bcm2835's vc4 documentation, which
+ * states HSM's clock has to be at least 108% of the pixel clock.
+ *
+ * Real life tests reveal that vc4's firmware statement holds up, and
+ * users are able to use pixel clocks closer to HSM's, namely for
+ * 1920x1200@60Hz. So it was decided to have leave a 1% margin between
+ * both clocks. Which, for RPi0-3 implies a maximum pixel clock of
+ * 162MHz.
+ *
+ * Additionally, the AXI clock needs to be at least 25% of
+ * pixel clock, but HSM ends up being the limiting factor.
*/
- if (mode->clock > HSM_CLOCK_FREQ / (1000 * 108 / 100))
+ if (mode->clock > HSM_CLOCK_FREQ / (1000 * 101 / 100))
return MODE_CLOCK_HIGH;
return MODE_OK;
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