mtd: rawnand: Fix core interference with sequential reads

A couple of reports pointed at some strange failures happening a bit
randomly since the introduction of sequential page reads support. After
investigation it turned out the most likely reason for these issues was
the fact that sometimes a (longer) read might happen, starting at the
same page that was read previously. This is optimized by the raw NAND
core, by not sending the READ_PAGE command to the NAND device and just
reading out the data in a local cache. When this page is also flagged as
being the starting point for a sequential read, it means the page right
next will be accessed without the right instructions. The NAND chip will
be confused and will not output correct data. In order to avoid such
situation from happening anymore, we can however handle this case with a
bit of additional logic, to postpone the initialization of the read
sequence by one page.
Reported-by: Alexander Shiyan <eagle.alexander923@gmail.com>
Closes: https://lore.kernel.org/linux-mtd/CAP1tNvS=NVAm-vfvYWbc3k9Cx9YxMc2uZZkmXk8h1NhGX877Zg@mail.gmail.com/Reported-by: Måns Rullgård <mans@mansr.com>
Closes: https://lore.kernel.org/linux-mtd/yw1xfs6j4k6q.fsf@mansr.com/Reported-by: Martin Hundebøll <martin@geanix.com>
Closes: https://lore.kernel.org/linux-mtd/9d0c42fcde79bfedfe5b05d6a4e9fdef71d3dd52.camel@geanix.com/
Fixes: 003fe4b9 ("mtd: rawnand: Support for sequential cache reads")
Cc: stable@vger.kernel.org
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Tested-by: Martin Hundebøll <martin@geanix.com>
Link: https://lore.kernel.org/linux-mtd/20231215123208.516590-3-miquel.raynal@bootlin.com

drivers/mtd/nand/raw/nand_base.c

@@ -3478,6 +3478,18 @@ static void rawnand_enable_cont_reads(struct nand_chip *chip, unsigned int page,
 	rawnand_cap_cont_reads(chip);
 }
 
+static void rawnand_cont_read_skip_first_page(struct nand_chip *chip, unsigned int page)
+{
+	if (!chip->cont_read.ongoing || page != chip->cont_read.first_page)
+		return;
+
+	chip->cont_read.first_page++;
+	if (chip->cont_read.first_page == chip->cont_read.pause_page)
+		chip->cont_read.first_page++;
+	if (chip->cont_read.first_page >= chip->cont_read.last_page)
+		chip->cont_read.ongoing = false;
+}
+
 /**
  * nand_setup_read_retry - [INTERN] Set the READ RETRY mode
  * @chip: NAND chip object
@@ -3652,6 +3664,8 @@ static int nand_do_read_ops(struct nand_chip *chip, loff_t from,
 			buf += bytes;
 			max_bitflips = max_t(unsigned int, max_bitflips,
 					     chip->pagecache.bitflips);
+
+			rawnand_cont_read_skip_first_page(chip, page);
 		}
 
 		readlen -= bytes;