mtd: rawnand: mtk: Correct low level time calculation of r/w cycle

commit e1884ffd upstream.

At present, the flow of calculating AC timing of read/write cycle in SDR
mode is that:
At first, calculate high hold time which is valid for both read and write
cycle using the max value between tREH_min and tWH_min.
Secondly, calculate WE# pulse width using tWP_min.
Thridly, calculate RE# pulse width using the bigger one between tREA_max
and tRP_min.

But NAND SPEC shows that Controller should also meet write/read cycle time.
That is write cycle time should be more than tWC_min and read cycle should
be more than tRC_min. Obviously, we do not achieve that now.

This patch corrects the low level time calculation to meet minimum
read/write cycle time required. After getting the high hold time, WE# low
level time will be promised to meet tWP_min and tWC_min requirement,
and RE# low level time will be promised to meet tREA_max, tRP_min and
tRC_min requirement.

Fixes: edfee361 ("mtd: nand: mtk: add ->setup_data_interface() hook")
Cc: stable@vger.kernel.org # v4.17+
Signed-off-by: Xiaolei Li <xiaolei.li@mediatek.com>
Reviewed-by: Miquel Raynal <miquel.raynal@bootlin.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/mtd/nand/raw/mtk_nand.c

@@ -509,7 +509,8 @@ static int mtk_nfc_setup_data_interface(struct mtd_info *mtd, int csline,
 {
 	struct mtk_nfc *nfc = nand_get_controller_data(mtd_to_nand(mtd));
 	const struct nand_sdr_timings *timings;
-	u32 rate, tpoecs, tprecs, tc2r, tw2r, twh, twst, trlt;
+	u32 rate, tpoecs, tprecs, tc2r, tw2r, twh, twst = 0, trlt = 0;
+	u32 thold;
 
 	timings = nand_get_sdr_timings(conf);
 	if (IS_ERR(timings))
@@ -545,11 +546,28 @@ static int mtk_nfc_setup_data_interface(struct mtd_info *mtd, int csline,
 	twh = DIV_ROUND_UP(twh * rate, 1000000) - 1;
 	twh &= 0xf;
 
-	twst = timings->tWP_min / 1000;
+	/* Calculate real WE#/RE# hold time in nanosecond */
+	thold = (twh + 1) * 1000000 / rate;
+	/* nanosecond to picosecond */
+	thold *= 1000;
+
+	/*
+	 * WE# low level time should be expaned to meet WE# pulse time
+	 * and WE# cycle time at the same time.
+	 */
+	if (thold < timings->tWC_min)
+		twst = timings->tWC_min - thold;
+	twst = max(timings->tWP_min, twst) / 1000;
 	twst = DIV_ROUND_UP(twst * rate, 1000000) - 1;
 	twst &= 0xf;
 
-	trlt = max(timings->tREA_max, timings->tRP_min) / 1000;
+	/*
+	 * RE# low level time should be expaned to meet RE# pulse time,
+	 * RE# access time and RE# cycle time at the same time.
+	 */
+	if (thold < timings->tRC_min)
+		trlt = timings->tRC_min - thold;
+	trlt = max3(trlt, timings->tREA_max, timings->tRP_min) / 1000;
 	trlt = DIV_ROUND_UP(trlt * rate, 1000000) - 1;
 	trlt &= 0xf;