[MTD] [NAND] subpage read feature as a way to increase performance.

This patch enables NAND subpage read functionality.
If upper layer drivers are requesting to read non page aligned data NAND
subpage-read functionality reads the only whose ECC regions which include
requested data when original code reads whole page.
This significantly improves performance in many cases.

Here are some digits :

UBI volume mount time
No subpage reads: 5.75 seconds
Subpage read patch: 2.42 seconds

Open/stat time for files on JFFS2 volume:
No subpage read  0m 5.36s
Subpage read     0m 2.88s

Signed-off-by Alexey Korolev <akorolev@infradead.org>
Acked-by: Artem Bityutskiy <dedekind@infradead.org>
Acked-by: Jörn Engel <joern@logfs.org>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>

2 files changed, 91 insertions, 1 deletions

diff --git a/drivers/mtd/nand/nand_base.c b/drivers/mtd/nand/nand_base.c
index ba1bdf787323..d1129bae6c27 100644
--- a/drivers/mtd/nand/nand_base.c
+++ b/drivers/mtd/nand/nand_base.c
@@ -798,6 +798,87 @@ static int nand_read_page_swecc(struct mtd_info *mtd, struct nand_chip *chip,
 }
 
 /**
+ * nand_read_subpage - [REPLACABLE] software ecc based sub-page read function
+ * @mtd:        mtd info structure
+ * @chip:       nand chip info structure
+ * @dataofs     offset of requested data within the page
+ * @readlen     data length
+ * @buf:        buffer to store read data
+ */
+static int nand_read_subpage(struct mtd_info *mtd, struct nand_chip *chip, uint32_t data_offs, uint32_t readlen, uint8_t *bufpoi)
+{
+        int start_step, end_step, num_steps;
+        uint32_t *eccpos = chip->ecc.layout->eccpos;
+        uint8_t *p;
+        int data_col_addr, i, gaps = 0;
+        int datafrag_len, eccfrag_len, aligned_len, aligned_pos;
+        int busw = (chip->options & NAND_BUSWIDTH_16) ? 2 : 1;
+
+        /* Column address wihin the page aligned to ECC size (256bytes). */
+        start_step = data_offs / chip->ecc.size;
+        end_step = (data_offs + readlen - 1) / chip->ecc.size;
+        num_steps = end_step - start_step + 1;
+
+        /* Data size aligned to ECC ecc.size*/
+        datafrag_len = num_steps * chip->ecc.size;
+        eccfrag_len = num_steps * chip->ecc.bytes;
+
+        data_col_addr = start_step * chip->ecc.size;
+        /* If we read not a page aligned data */
+        if (data_col_addr != 0)
+                chip->cmdfunc(mtd, NAND_CMD_RNDOUT, data_col_addr, -1);
+
+        p = bufpoi + data_col_addr;
+        chip->read_buf(mtd, p, datafrag_len);
+
+        /* Calculate  ECC */
+        for (i = 0; i < eccfrag_len ; i += chip->ecc.bytes, p += chip->ecc.size)
+                chip->ecc.calculate(mtd, p, &chip->buffers->ecccalc[i]);
+
+        /* The performance is faster if to position offsets
+           according to ecc.pos. Let make sure here that
+           there are no gaps in ecc positions */
+        for (i = 0; i < eccfrag_len - 1; i++) {
+                if (eccpos[i + start_step * chip->ecc.bytes] + 1 !=
+                        eccpos[i + start_step * chip->ecc.bytes + 1]) {
+                        gaps = 1;
+                        break;
+                }
+        }
+        if (gaps) {
+                chip->cmdfunc(mtd, NAND_CMD_RNDOUT, mtd->writesize, -1);
+                chip->read_buf(mtd, chip->oob_poi, mtd->oobsize);
+        } else {
+                /* send the command to read the particular ecc bytes */
+                /* take care about buswidth alignment in read_buf */
+                aligned_pos = eccpos[start_step * chip->ecc.bytes] & ~(busw - 1);
+                aligned_len = eccfrag_len;
+                if (eccpos[start_step * chip->ecc.bytes] & (busw - 1))
+                        aligned_len++;
+                if (eccpos[(start_step + num_steps) * chip->ecc.bytes] & (busw - 1))
+                        aligned_len++;
+
+                chip->cmdfunc(mtd, NAND_CMD_RNDOUT, mtd->writesize + aligned_pos, -1);
+                chip->read_buf(mtd, &chip->oob_poi[aligned_pos], aligned_len);
+        }
+
+        for (i = 0; i < eccfrag_len; i++)
+                chip->buffers->ecccode[i] = chip->oob_poi[eccpos[i + start_step * chip->ecc.bytes]];
+
+        p = bufpoi + data_col_addr;
+        for (i = 0; i < eccfrag_len ; i += chip->ecc.bytes, p += chip->ecc.size) {
+                int stat;
+
+                stat = chip->ecc.correct(mtd, p, &chip->buffers->ecccode[i], &chip->buffers->ecccalc[i]);
+                if (stat == -1)
+                        mtd->ecc_stats.failed++;
+                else
+                        mtd->ecc_stats.corrected += stat;
+        }
+        return 0;
+}
+
+/**
  * nand_read_page_hwecc - [REPLACABLE] hardware ecc based page read function
  * @mtd:        mtd info structure
  * @chip:       nand chip info structure
@@ -994,6 +1075,8 @@ static int nand_do_read_ops(struct mtd_info *mtd, loff_t from,
                         /* Now read the page into the buffer */
                         if (unlikely(ops->mode == MTD_OOB_RAW))
                                 ret = chip->ecc.read_page_raw(mtd, chip, bufpoi);
+                        else if (!aligned && NAND_SUBPAGE_READ(chip) && !oob)
+                                ret = chip->ecc.read_subpage(mtd, chip, col, bytes, bufpoi);
                         else
                                 ret = chip->ecc.read_page(mtd, chip, bufpoi);
                         if (ret < 0)
@@ -1001,7 +1084,8 @@ static int nand_do_read_ops(struct mtd_info *mtd, loff_t from,
 
                         /* Transfer not aligned data */
                         if (!aligned) {
-                                chip->pagebuf = realpage;
+                                if (!NAND_SUBPAGE_READ(chip) && !oob)
+                                        chip->pagebuf = realpage;
                                 memcpy(buf, chip->buffers->databuf + col, bytes);
                         }
 
@@ -2521,6 +2605,7 @@ int nand_scan_tail(struct mtd_info *mtd)
                 chip->ecc.calculate = nand_calculate_ecc;
                 chip->ecc.correct = nand_correct_data;
                 chip->ecc.read_page = nand_read_page_swecc;
+                chip->ecc.read_subpage = nand_read_subpage;
                 chip->ecc.write_page = nand_write_page_swecc;
                 chip->ecc.read_oob = nand_read_oob_std;
                 chip->ecc.write_oob = nand_write_oob_std;
diff --git a/include/linux/mtd/nand.h b/include/linux/mtd/nand.h
index 863e22a0ddb5..83f678702dff 100644
--- a/include/linux/mtd/nand.h
+++ b/include/linux/mtd/nand.h
@@ -177,6 +177,7 @@ typedef enum {
 #define NAND_MUST_PAD(chip) (!(chip->options & NAND_NO_PADDING))
 #define NAND_HAS_CACHEPROG(chip) ((chip->options & NAND_CACHEPRG))
 #define NAND_HAS_COPYBACK(chip) ((chip->options & NAND_COPYBACK))
+#define NAND_SUBPAGE_READ(chip) ((chip->ecc.mode == NAND_ECC_SOFT))
 
 /* Mask to zero out the chip options, which come from the id table */
 #define NAND_CHIPOPTIONS_MSK    (0x0000ffff & ~NAND_NO_AUTOINCR)
@@ -274,6 +275,10 @@ struct nand_ecc_ctrl {
         int                     (*read_page)(struct mtd_info *mtd,
                                              struct nand_chip *chip,
                                              uint8_t *buf);
+        int                     (*read_subpage)(struct mtd_info *mtd,
+                                             struct nand_chip *chip,
+                                             uint32_t offs, uint32_t len,
+                                             uint8_t *buf);
         void                    (*write_page)(struct mtd_info *mtd,
                                               struct nand_chip *chip,
                                               const uint8_t *buf);