[PATCH 1/5] mtd: rawnand: mtk: Correct low level time calculation of r/w cycle

[Xiaolei Li <xiaolei.li@mediatek.com>]

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: 93db446a424c ("mtd: nand: move raw NAND related code to the raw/ subdir")
Signed-off-by: Xiaolei Li <xiaolei.li@mediatek.com>
---
 drivers/mtd/nand/raw/mtk_nand.c | 14 +++++++++++---
 1 file changed, 11 insertions(+), 3 deletions(-)

diff --git a/drivers/mtd/nand/raw/mtk_nand.c b/drivers/mtd/nand/raw/mtk_nand.c
index b6b4602f5132..dd855f860a4b 100644
--- a/drivers/mtd/nand/raw/mtk_nand.c
+++ b/drivers/mtd/nand/raw/mtk_nand.c
@@ -508,7 +508,7 @@ static int mtk_nfc_setup_data_interface(struct nand_chip *chip, int csline,
 {
 	struct mtk_nfc *nfc = nand_get_controller_data(chip);
 	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;
 
 	timings = nand_get_sdr_timings(conf);
 	if (IS_ERR(timings))
@@ -544,11 +544,19 @@ static int mtk_nfc_setup_data_interface(struct nand_chip *chip, int csline,
 	twh = DIV_ROUND_UP(twh * rate, 1000000) - 1;
 	twh &= 0xf;
 
-	twst = timings->tWP_min / 1000;
+	/* Calculate min low level timing for write cycle */
+	if ((twh + 1) * 1000000 / rate < timings->tWC_min / 1000)
+		twst = (timings->tWC_min / 1000 - (twh + 1) * 1000000 / rate)
+			* 1000;
+	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;
+	/* Calculate min low level timing for read cycle */
+	if ((twh + 1) * 1000000 / rate < timings->tRC_min / 1000)
+		trlt = (timings->tRC_min / 1000 - (twh + 1) * 1000000 / rate)
+			* 1000;
+	trlt = max3(trlt, timings->tREA_max, timings->tRP_min) / 1000;
 	trlt = DIV_ROUND_UP(trlt * rate, 1000000) - 1;
 	trlt &= 0xf;
 
-- 
2.18.0


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