[PATCH v2 3/3] mtd: rawnand: Support for sequential cache reads

[Miquel Raynal]

On Thu, 2023-01-12 at 09:36:37 UTC, Miquel Raynal wrote:
> From: JaimeLiao <jaimeliao.tw at gmail.com>
> 
> Add support for sequential cache reads for controllers using the generic
> core helpers for their fast read/write helpers.
> 
> Sequential reads may reduce the overhead when accessing physically
> continuous data by loading in cache the next page while the previous
> page gets sent out on the NAND bus.
> 
> The ONFI specification provides the following additional commands to
> handle sequential cached reads:
> 
> * 0x31 - READ CACHE SEQUENTIAL:
>   Requires the NAND chip to load the next page into cache while keeping
>   the current cache available for host reads.
> * 0x3F - READ CACHE END:
>   Tells the NAND chip this is the end of the sequential cache read, the
>   current cache shall remain accessible for the host but no more
>   internal cache loading operation is required.
> 
> On the bus, a multi page read operation is currently handled like this:
> 
> 	00 -- ADDR1 -- 30 -- WAIT_RDY (tR+tRR) -- DATA1_IN
> 	00 -- ADDR2 -- 30 -- WAIT_RDY (tR+tRR) -- DATA2_IN
> 	00 -- ADDR3 -- 30 -- WAIT_RDY (tR+tRR) -- DATA3_IN
> 
> Sequential cached reads may instead be achieved with:
> 
> 	00 -- ADDR1 -- 30 -- WAIT_RDY (tR) -- \
> 		       31 -- WAIT_RDY (tRCBSY+tRR) -- DATA1_IN \
> 		       31 -- WAIT_RDY (tRCBSY+tRR) -- DATA2_IN \
> 		       3F -- WAIT_RDY (tRCBSY+tRR) -- DATA3_IN
> 
> Below are the read speed test results with regular reads and
> sequential cached reads, on NXP i.MX6 VAR-SOM-SOLO in mapping mode with
> a NAND chip characterized with the following timings:
> * tR: 20 µs
> * tRCBSY: 5 µs
> * tRR: 20 ns
> and the following geometry:
> * device size: 2 MiB
> * eraseblock size: 128 kiB
> * page size: 2 kiB
> 
> ============= Normal read @ 33MHz =================
> mtd_speedtest: eraseblock read speed is 15633 KiB/s
> mtd_speedtest: page read speed is 15515 KiB/s
> mtd_speedtest: 2 page read speed is 15398 KiB/s
> ===================================================
> 
> ========= Sequential cache read @ 33MHz ===========
> mtd_speedtest: eraseblock read speed is 18285 KiB/s
> mtd_speedtest: page read speed is 15875 KiB/s
> mtd_speedtest: 2 page read speed is 16253 KiB/s
> ===================================================
> 
> We observe an overall speed improvement of about 5% when reading
> 2 pages, up to 15% when reading an entire block. This is due to the
> ~14us gain on each additional page read (tR - (tRCBSY + tRR)).
> 
> Co-developed-by: Miquel Raynal <miquel.raynal at bootlin.com>
> Signed-off-by: Miquel Raynal <miquel.raynal at bootlin.com>
> Signed-off-by: JaimeLiao <jaimeliao.tw at gmail.com>
> Tested-by: Liao Jaime <jaimeliao.tw at gmail.com>

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

Miquel