mtd: spi-nand: Add continuous read support

A regular page read consist in:
- Asking one page of content from the NAND array to be loaded in the
  chip's SRAM,
- Waiting for the operation to be done,
- Retrieving the data (I/O phase) from the chip's SRAM.

When reading several sequential pages, the above operation is repeated
over and over. There is however a way to optimize these accesses, by
enabling continuous reads. The feature requires the NAND chip to have a
second internal SRAM area plus a bit of additional internal logic to
trigger another internal transfer between the NAND array and the second
SRAM area while the I/O phase is ongoing. Once the first I/O phase is
done, the host can continue reading more data, continuously, as the chip
will automatically switch to the second SRAM content (which has already
been loaded) and in turns trigger the next load into the first SRAM area
again.

From an instruction perspective, the command op-codes are different, but
the same cycles are required. The only difference is that after a
continuous read (which is stopped by a CS deassert), the host must
observe a delay of tRST. However, because there is no guarantee in Linux
regarding the actual state of the CS pin after a transfer (in order to
speed-up the next transfer if targeting the same device), it was
necessary to manually end the continuous read with a configuration
register write operation.

Continuous reads have two main drawbacks:
* They only work on full pages (column address ignored)
* Only the main data area is pulled, out-of-band bytes are not
  accessible. Said otherwise, the feature can only be useful with on-die
  ECC engines.

Performance wise, measures have been performed on a Zynq platform using
Macronix SPI-NAND controller with a Macronix chip (based on the
flash_speed tool modified for testing sequential reads):
- 1-1-1 mode: performances improved from +3% (2-pages) up to +10% after
              a dozen pages.
- 1-1-4 mode: performances improved from +15% (2-pages) up to +40% after
              a dozen pages.

This series is based on a previous work from Macronix engineer Jaime
Liao.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Reviewed-by: Pratyush Yadav <pratyush@kernel.org>
Link: https://lore.kernel.org/linux-mtd/20240826101412.20644-5-miquel.raynal@bootlin.com

1 files changed, 16 insertions, 0 deletions

diff --git a/include/linux/mtd/spinand.h b/include/linux/mtd/spinand.h
index 5c19ead60499..14dce347dc46 100644
--- a/include/linux/mtd/spinand.h
+++ b/include/linux/mtd/spinand.h
@@ -336,6 +336,7 @@ struct spinand_ondie_ecc_conf {
  * @op_variants.update_cache: variants of the update-cache operation
  * @select_target: function used to select a target/die. Required only for
  *		   multi-die chips
+ * @set_cont_read: enable/disable continuous cached reads
  *
  * Each SPI NAND manufacturer driver should have a spinand_info table
  * describing all the chips supported by the driver.
@@ -354,6 +355,8 @@ struct spinand_info {
 	} op_variants;
 	int (*select_target)(struct spinand_device *spinand,
 			     unsigned int target);
+	int (*set_cont_read)(struct spinand_device *spinand,
+			     bool enable);
 };
 
 #define SPINAND_ID(__method, ...)					\
@@ -379,6 +382,9 @@ struct spinand_info {
 #define SPINAND_SELECT_TARGET(__func)					\
 	.select_target = __func,
 
+#define SPINAND_CONT_READ(__set_cont_read)				\
+	.set_cont_read = __set_cont_read,
+
 #define SPINAND_INFO(__model, __id, __memorg, __eccreq, __op_variants,	\
 		     __flags, ...)					\
 	{								\
@@ -422,6 +428,12 @@ struct spinand_dirmap {
  *		passed in spi_mem_op be DMA-able, so we can't based the bufs on
  *		the stack
  * @manufacturer: SPI NAND manufacturer information
+ * @cont_read_possible: Field filled by the core once the whole system
+ *		configuration is known to tell whether continuous reads are
+ *		suitable to use or not in general with this chip/configuration.
+ *		A per-transfer check must of course be done to ensure it is
+ *		actually relevant to enable this feature.
+ * @set_cont_read: Enable/disable the continuous read feature
  * @priv: manufacturer private data
  */
 struct spinand_device {
@@ -451,6 +463,10 @@ struct spinand_device {
 	u8 *scratchbuf;
 	const struct spinand_manufacturer *manufacturer;
 	void *priv;
+
+	bool cont_read_possible;
+	int (*set_cont_read)(struct spinand_device *spinand,
+			     bool enable);
 };
 
 /**