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[PATCH v4 17/23] drm/rockchip: dw-mipi-dsi: improve PLL configuration

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The multiplication ratio for the PLL is required to be even due to the
use of a "by 2 pre-scaler".  Currently we are likely to end up with an
odd multiplier even though there is an equivalent set of parameters with
an even multiplier.

For example, using the 324MHz bit rate with a reference clock of 24MHz
we end up with M = 27, N = 2 whereas the example in the PHY databook
gives M = 54, N = 4 for this bit rate and reference clock.

By walking down through the available multiplier instead of up we are
more likely to hit an even multiplier.  With the above example we do now
get M = 54, N = 4 as given by the databook.

While doing this, change the loop limits to encode the actual limits on
the divisor, which are:

	40MHz >= (pllref / N) >= 5MHz

Signed-off-by: John Keeping <john@metanate.com>
Reviewed-by: Sean Paul <seanpaul@chromium.org>
---
v4:
- Add a comment explaining the limits so that it can be understood
  without finding the commit message above
- Add Sean's Reviewed-by
Unchanged in v3
Unchanged in v2
---
 drivers/gpu/drm/rockchip/dw-mipi-dsi.c | 13 ++++++++++++-
 1 file changed, 12 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/rockchip/dw-mipi-dsi.c b/drivers/gpu/drm/rockchip/dw-mipi-dsi.c
index 9b6a60deb69e..e6b52c7cb5e3 100644
--- a/drivers/gpu/drm/rockchip/dw-mipi-dsi.c
+++ b/drivers/gpu/drm/rockchip/dw-mipi-dsi.c
@@ -518,7 +518,18 @@ static int dw_mipi_dsi_get_lane_bps(struct dw_mipi_dsi *dsi,
 	pllref = DIV_ROUND_UP(clk_get_rate(dsi->pllref_clk), USEC_PER_SEC);
 	tmp = pllref;
 
-	for (i = 1; i < 6; i++) {
+	/*
+	 * The limits on the PLL divisor are:
+	 *
+	 *	5MHz <= (pllref / n) <= 40MHz
+	 *
+	 * we walk over these values in descreasing order so that if we hit
+	 * an exact match for target_mbps it is more likely that "m" will be
+	 * even.
+	 *
+	 * TODO: ensure that "m" is even after this loop.
+	 */
+	for (i = pllref / 5; i > (pllref / 40); i--) {
 		pre = pllref / i;
 		if ((tmp > (target_mbps % pre)) && (target_mbps / pre < 512)) {
 			tmp = target_mbps % pre;
-- 
2.12.0.rc0.230.gf625d4cdb9.dirty

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[PATCH v4 00/23] drm/rockchip: MIPI fixes & improvements John Keeping <john@metanate.com> - 2017-02-24 14:00 +0100
  [PATCH v4 18/23] drm/rockchip: dw-mipi-dsi: use specific poll helper John Keeping <john@metanate.com> - 2017-02-24 14:00 +0100
  [PATCH v4 13/23] drm/rockchip: dw-mipi-dsi: fix escape clock rate John Keeping <john@metanate.com> - 2017-02-24 14:00 +0100
  [PATCH v4 23/23] drm/rockchip: dw-mipi-dsi: add reset control John Keeping <john@metanate.com> - 2017-02-24 14:00 +0100
  [PATCH v4 12/23] drm/rockchip: dw-mipi-dsi: allow commands in panel_disable John Keeping <john@metanate.com> - 2017-02-24 14:00 +0100
  [PATCH v4 02/23] drm/rockchip: dw-mipi-dsi: pass mode in where needed John Keeping <john@metanate.com> - 2017-02-24 14:00 +0100
  [PATCH v4 07/23] drm/rockchip: dw-mipi-dsi: include bad value in error message John Keeping <john@metanate.com> - 2017-02-24 14:00 +0100
  [PATCH v4 21/23] drm/rockchip: dw-mipi-dsi: defer probe if panel is not loaded John Keeping <john@metanate.com> - 2017-02-24 14:00 +0100
  [PATCH v4 19/23] drm/rockchip: dw-mipi-dsi: use positive check for N{H,V}SYNC John Keeping <john@metanate.com> - 2017-02-24 14:00 +0100
  [PATCH v4 06/23] drm/rockchip: dw-mipi-dsi: avoid out-of-bounds read on tx_buf John Keeping <john@metanate.com> - 2017-02-24 14:00 +0100
  [PATCH v4 15/23] drm/rockchip: dw-mipi-dsi: configure PHY before enabling John Keeping <john@metanate.com> - 2017-02-24 14:00 +0100
  [PATCH v4 08/23] drm/rockchip: dw-mipi-dsi: respect message flags John Keeping <john@metanate.com> - 2017-02-24 14:00 +0100
  [PATCH v4 04/23] drm/rockchip: dw-mipi-dsi: fix command header writes John Keeping <john@metanate.com> - 2017-02-24 14:00 +0100
  [PATCH v4 10/23] drm/rockchip: dw-mipi-dsi: don't assume buffer is aligned John Keeping <john@metanate.com> - 2017-02-24 14:00 +0100
  [PATCH v4 11/23] drm/rockchip: dw-mipi-dsi: prepare panel after phy init John Keeping <john@metanate.com> - 2017-02-24 14:00 +0100
  [PATCH v4 17/23] drm/rockchip: dw-mipi-dsi: improve PLL configuration John Keeping <john@metanate.com> - 2017-02-24 14:00 +0100
  [PATCH v4 09/23] drm/rockchip: dw-mipi-dsi: only request HS clock when required John Keeping <john@metanate.com> - 2017-02-24 14:00 +0100
  [PATCH v4 03/23] drm/rockchip: dw-mipi-dsi: remove mode_set hook John Keeping <john@metanate.com> - 2017-02-24 14:00 +0100
  [PATCH v4 05/23] drm/rockchip: dw-mipi-dsi: fix generic packet status check John Keeping <john@metanate.com> - 2017-02-24 14:00 +0100
  [PATCH v4 01/23] drm/rockchip: dw-mipi-dsi: don't configure hardware in mode_set for MIPI John Keeping <john@metanate.com> - 2017-02-24 14:00 +0100
  [PATCH v4 16/23] drm/rockchip: dw-mipi-dsi: properly configure PHY timing John Keeping <john@metanate.com> - 2017-02-24 14:00 +0100
  [PATCH v4 22/23] drm/rockchip: dw-mipi-dsi: support non-burst modes John Keeping <john@metanate.com> - 2017-02-24 14:10 +0100
  Re: [PATCH v4 00/23] drm/rockchip: MIPI fixes & improvements Mark yao <mark.yao@rock-chips.com> - 2017-02-27 02:50 +0100
  Re: [PATCH v4 00/23] drm/rockchip: MIPI fixes & improvements Chris Zhong <zyw@rock-chips.com> - 2017-02-27 04:40 +0100

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