[PATCH v2 4/9] mtd: spi-nand: Add continuous read support

[Miquel Raynal]

On Mon, 2024-08-26 at 10:14:07 UTC, Miquel Raynal wrote:
> 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 at bootlin.com>
> Reviewed-by: Pratyush Yadav <pratyush at kernel.org>

Applied to https://git.kernel.org/pub/scm/linux/kernel/git/mtd/linux.git nand/next.

Miquel