[PATCH v2] [RFC] rcar-vin: rcar-csi2: Correct the selection of hsfreqrange

[Suresh Udipi <sudipi@jp.adit-jv.com>]

hsfreqrange should be chosen based on the calculated mbps which
is closer to the default bit rate  and within the range as per
table[1]. But current calculation always selects first value which
is greater than or equal to the calculated mbps which may lead
to chosing a wrong range in some cases.

For example for 360 mbps for H3/M3N
Existing logic selects
Calculated value 360Mbps : Default 400Mbps Range [368.125 -433.125 mbps]

This hsfreqrange is out of range.

The logic is changed to get the default value which is closest to the
calculated value [1]

Calculated value 360Mbps : Default 350Mbps  Range [320.625 -380.625 mpbs]

[1] specs r19uh0105ej0200-r-car-3rd-generation.pdf [Table 25.9]

There is one exectpion value 227Mbps, which may cause out of
range.

Fixes: 769afd212b16 ("media: rcar-csi2: add Renesas R-Car MIPI CSI-2 receiver driver")

Signed-off-by: Suresh Udipi <sudipi@jp.adit-jv.com>
Signed-off-by: Kazuyoshi Akiyama <akiyama@nds-osk.co.jp>
---
Changes in v2:
  - Added the boundary check for the maximum bit rate.
  
  - Simplified the logic by remmoving range check 
    as only the closest default value covers most 
    of the use cases.

  - Aligning the commit message based on the above change

 drivers/media/platform/rcar-vin/rcar-csi2.c | 18 +++++++++++++-----
 1 file changed, 13 insertions(+), 5 deletions(-)

diff --git a/drivers/media/platform/rcar-vin/rcar-csi2.c b/drivers/media/platform/rcar-vin/rcar-csi2.c
index faa9fb2..6573625 100644
--- a/drivers/media/platform/rcar-vin/rcar-csi2.c
+++ b/drivers/media/platform/rcar-vin/rcar-csi2.c
@@ -199,6 +199,7 @@ static const struct rcsi2_mbps_reg phtw_mbps_v3m_e3[] = {
 /* PHY Frequency Control */
 #define PHYPLL_REG			0x68
 #define PHYPLL_HSFREQRANGE(n)		((n) << 16)
+#define PHYPLL_HSFREQRANGE_MAX		1500
 
 static const struct rcsi2_mbps_reg hsfreqrange_h3_v3h_m3n[] = {
 	{ .mbps =   80, .reg = 0x00 },
@@ -431,16 +432,23 @@ static int rcsi2_wait_phy_start(struct rcar_csi2 *priv)
 static int rcsi2_set_phypll(struct rcar_csi2 *priv, unsigned int mbps)
 {
 	const struct rcsi2_mbps_reg *hsfreq;
+	const struct rcsi2_mbps_reg *hsfreq_prev = NULL;
 
-	for (hsfreq = priv->info->hsfreqrange; hsfreq->mbps != 0; hsfreq++)
-		if (hsfreq->mbps >= mbps)
-			break;
-
-	if (!hsfreq->mbps) {
+	if (mbps > PHYPLL_HSFREQRANGE_MAX) {
 		dev_err(priv->dev, "Unsupported PHY speed (%u Mbps)", mbps);
 		return -ERANGE;
 	}
 
+	for (hsfreq = priv->info->hsfreqrange; hsfreq->mbps != 0; hsfreq++) {
+		if (hsfreq->mbps >= mbps)
+			break;
+		hsfreq_prev = hsfreq;
+	}
+
+	if (hsfreq_prev &&
+	    ((mbps - hsfreq_prev->mbps) <= (hsfreq->mbps - mbps)))
+		hsfreq = hsfreq_prev;
+
 	rcsi2_write(priv, PHYPLL_REG, PHYPLL_HSFREQRANGE(hsfreq->reg));
 
 	return 0;
-- 
2.7.4