drm/tegra: dc: Describe register copies

Most of the display controller's registers are double-buffered, a few of
them are triple-buffered. The ASSEMBLY shadow copy is latched intto the
ACTIVE copy for double-buffered registers. For triple-buffered registers
the ASSEMBLY copy is first latched into the ARM copy.

Latching into the ACTIVE copy happens immediately if the controller is
inactive. Otherwise the latching happens on the next frame boundary. The
latching of the ASSEMBLY into the ARM copy happens immediately. Latching
is controlled by a set of *_ACT_REQ and *_UPDATE bits in the
DC_CMD_STATE_CONTROL register.

Signed-off-by: Thierry Reding <treding@nvidia.com>

1 files changed, 12 insertions, 0 deletions

diff --git a/drivers/gpu/drm/tegra/dc.c b/drivers/gpu/drm/tegra/dc.c
index dbb2d00268b2..a56fccc428d5 100644
--- a/drivers/gpu/drm/tegra/dc.c
+++ b/drivers/gpu/drm/tegra/dc.c
@@ -53,6 +53,18 @@ static void tegra_dc_cursor_commit(struct tegra_dc *dc)
 	tegra_dc_writel(dc, CURSOR_ACT_REQ, DC_CMD_STATE_CONTROL);
 }
 
+/*
+ * Double-buffered registers have two copies: ASSEMBLY and ACTIVE. When the
+ * *_ACT_REQ bits are set the ASSEMBLY copy is latched into the ACTIVE copy.
+ * Latching happens mmediately if the display controller is in STOP mode or
+ * on the next frame boundary otherwise.
+ *
+ * Triple-buffered registers have three copies: ASSEMBLY, ARM and ACTIVE. The
+ * ASSEMBLY copy is latched into the ARM copy immediately after *_UPDATE bits
+ * are written. When the *_ACT_REQ bits are written, the ARM copy is latched
+ * into the ACTIVE copy, either immediately if the display controller is in
+ * STOP mode, or at the next frame boundary otherwise.
+ */
 static void tegra_dc_commit(struct tegra_dc *dc)
 {
 	tegra_dc_writel(dc, GENERAL_ACT_REQ << 8, DC_CMD_STATE_CONTROL);