mtd: rawnand: Clarify conditions to enable continuous reads

The current logic is probably fine but is a bit convoluted. Plus, we
don't want partial pages to be part of the sequential operation just in
case the core would optimize the page read with a subpage read (which
would break the sequence). This may happen on the first and last page
only, so if the start offset or the end offset is not aligned with a
page boundary, better avoid them to prevent any risk.

Cc: [e-mail]
Fixes: 003fe4b9545b ("mtd: rawnand: Support for sequential cache reads")
Signed-off-by: Miquel Raynal <[e-mail]>
Tested-by: Martin Hundebøll <[e-mail]>
Link: https://lore.kernel.org/linux-mtd/[e-mail]

drivers/mtd/nand/raw/nand_base.c

···
 				      u32 readlen, int col)
 {
 	struct mtd_info *mtd = nand_to_mtd(chip);
+	unsigned int end_page, end_col;
+
+	chip->cont_read.ongoing = false;
 
 	if (!chip->controller->supported_op.cont_read)
 		return;
 
-	if ((col && col + readlen < (3 * mtd->writesize)) ||
-	    (!col && readlen < (2 * mtd->writesize))) {
-		chip->cont_read.ongoing = false;
-		return;
-	}
+	end_page = DIV_ROUND_UP(col + readlen, mtd->writesize);
+	end_col = (col + readlen) % mtd->writesize;
 
-	chip->cont_read.ongoing = true;
-	chip->cont_read.first_page = page;
 	if (col)
-		chip->cont_read.first_page++;
-	chip->cont_read.last_page = page + ((readlen >> chip->page_shift) & chip->pagemask);
+		page++;
+
+	if (end_col && end_page)
+		end_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;
+
 	rawnand_cap_cont_reads(chip);
 }