mtd: rawnand: Fix and simplify again the continuous read derivations

We need to avoid the first page if we don't read it entirely.
We need to avoid the last page if we don't read it entirely.
While rather simple, this logic has been failed in the previous
fix. This time I wrote about 30 unit tests locally to check each
possible condition, hopefully I covered them all.
Reported-by: Christophe Kerello <christophe.kerello@foss.st.com>
Closes: https://lore.kernel.org/linux-mtd/20240221175327.42f7076d@xps-13/T/#m399bacb10db8f58f6b1f0149a1df867ec086bb0aSuggested-by: Christophe Kerello <christophe.kerello@foss.st.com>
Fixes: 828f6df1 ("mtd: rawnand: Clarify conditions to enable continuous reads")
Cc: stable@vger.kernel.org
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Tested-by: Christophe Kerello <christophe.kerello@foss.st.com>
Link: https://lore.kernel.org/linux-mtd/20240223115545.354541-2-miquel.raynal@bootlin.com

drivers/mtd/nand/raw/nand_base.c

@@ -3466,30 +3466,36 @@ static void rawnand_enable_cont_reads(struct nand_chip *chip, unsigned int page,
 				      u32 readlen, int col)
 {
 	struct mtd_info *mtd = nand_to_mtd(chip);
-	unsigned int end_page, end_col;
+	unsigned int first_page, last_page;
 
 	chip->cont_read.ongoing = false;
 
 	if (!chip->controller->supported_op.cont_read)
 		return;
 
-	end_page = DIV_ROUND_UP(col + readlen, mtd->writesize);
-	end_col = (col + readlen) % mtd->writesize;
+	/*
+	 * Don't bother making any calculations if the length is too small.
+	 * Side effect: avoids possible integer underflows below.
+	 */
+	if (readlen < (2 * mtd->writesize))
+		return;
 
+	/* Derive the page where continuous read should start (the first full page read) */
+	first_page = page;
 	if (col)
-		page++;
-
-	if (end_col && end_page)
-		end_page--;
+		first_page++;
 
-	if (page + 1 > end_page)
-		return;
-
-	chip->cont_read.first_page = page;
-	chip->cont_read.last_page = end_page;
-	chip->cont_read.ongoing = true;
+	/* Derive the page where continuous read should stop (the last full page read) */
+	last_page = page + ((col + readlen) / mtd->writesize) - 1;
 
-	rawnand_cap_cont_reads(chip);
+	/* Configure and enable continuous read when suitable */
+	if (first_page < last_page) {
+		chip->cont_read.first_page = first_page;
+		chip->cont_read.last_page = last_page;
+		chip->cont_read.ongoing = true;
+		/* May reset the ongoing flag */
+		rawnand_cap_cont_reads(chip);
+	}
 }
 
 static void rawnand_cont_read_skip_first_page(struct nand_chip *chip, unsigned int page)