mtd: rawnand: davinci: Add dummy read after sending command

Sometimes, writes fail because the tWB_max is not correctly observed
after sending PAGEPROG. It leads to the R/B pin to be read as in
the "ready" state right after sending the command, thus preventing the
normal tPROG delay to be actually observed. This happens because the
ndelay() that waits for tWB_max starts before the command reaches the
NAND chip.

Add a dummy read when a delay is requested at the end of the executed
instruction to make sure that the sent command is received by the NAND
before starting the short ndelay() (<1us but rounded up to 1us in
practice). This read is done on the control register area because
doing it on the Async Data area would change the NAND's RE pin state.
This is not perfect as the two areas are behind two different
devm_ioremap_resource() and could possibly be located on different
interconnects (I did not find more details). This means either the
additional latency due to the load operation is enough impacting, or it
has the expected behavior of ensuring the write has been received.

This has been tested on two platforms designed off of the
DAVINCI/OMAP-L138. The first uses a Toshiba NAND Flash (TC58NYG2S3EBAI5),
the other a Macronix one (MX30UF4G18AC).
Signed-off-by: Bastien Curutchet <bastien.curutchet@bootlin.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20240308074609.9056-1-bastien.curutchet@bootlin.com

drivers/mtd/nand/raw/davinci_nand.c

@@ -671,8 +671,11 @@ static int davinci_nand_exec_instr(struct davinci_nand_info *info,
 		break;
 	}
 
-	if (instr->delay_ns)
+	if (instr->delay_ns) {
+		/* Dummy read to be sure that command is sent before ndelay starts */
+		davinci_nand_readl(info, 0);
 		ndelay(instr->delay_ns);
+	}
 
 	return 0;
 }