[U-Boot] [PATCH 1/2] mtd: OMAP: Enable GPMC prefetch mode

[Daniel Mack]

Enable GPMC's prefetch feature for NAND access. This speeds up NAND read
access a lot by pre-fetching contents in the background and reading them
through the FIFO address.

The current implementation has two limitations:

 a) it only works for 16bit
 b) it only supports read access

Both is easily fixable by someone who has hardware to implement it.

Note that U-Boot code uses non word-aligned buffers to read data into, and
request read lengths that are not multiples of 4, so both partial buffers
(head and tail) have to be addressed.

Tested on AM335x hardware.

Signed-off-by: Daniel Mack <zonque at gmail.com>
---
 doc/README.nand               |   5 ++
 drivers/mtd/nand/omap_gpmc.c  | 115 +++++++++++++++++++++++++++++++++++++++++-
 include/linux/mtd/omap_gpmc.h |   6 ++-
 3 files changed, 123 insertions(+), 3 deletions(-)

diff --git a/doc/README.nand b/doc/README.nand
index 70cf768..6459f2a 100644
--- a/doc/README.nand
+++ b/doc/README.nand
@@ -292,6 +292,11 @@ Platform specific options
 		Thus BCH16 can be supported on 4K page NAND.
 
 
+    CONFIG_NAND_OMAP_PREFETCH
+	On OMAP platforms that use the GPMC controller (CONFIG_NAND_OMAP_GPMC),
+	this options enables the code that uses the prefetch mode to speed up
+	read operations.
+
 NOTE:
 =====
 
diff --git a/drivers/mtd/nand/omap_gpmc.c b/drivers/mtd/nand/omap_gpmc.c
index 1acf06b..e2d57bd 100644
--- a/drivers/mtd/nand/omap_gpmc.c
+++ b/drivers/mtd/nand/omap_gpmc.c
@@ -446,6 +446,113 @@ static int omap_correct_data_bch(struct mtd_info *mtd, uint8_t *dat,
 	return (err) ? err : error_count;
 }
 
+#ifdef CONFIG_NAND_OMAP_GPMC_PREFETCH
+
+#define PREFETCH_CONFIG1_CS_SHIFT	24
+#define PREFETCH_FIFOTHRESHOLD_MAX	0x40
+#define PREFETCH_FIFOTHRESHOLD(val)	((val) << 8)
+#define PREFETCH_STATUS_COUNT(val)	(val & 0x00003fff)
+#define PREFETCH_STATUS_FIFO_CNT(val)	((val >> 24) & 0x7F)
+#define ENABLE_PREFETCH			(1 << 7)
+
+/**
+ * omap_prefetch_enable - configures and starts prefetch transfer
+ * @fifo_th: fifo threshold to be used for read/ write
+ * @count: number of bytes to be transferred
+ * @is_write: prefetch read(0) or write post(1) mode
+ */
+static int omap_prefetch_enable(int fifo_th, unsigned int count, int is_write)
+{
+	uint32_t val;
+
+	if (fifo_th > PREFETCH_FIFOTHRESHOLD_MAX)
+		return -EINVAL;
+
+	if (readl(&gpmc_cfg->prefetch_control))
+		return -EBUSY;
+
+	/* Set the amount of bytes to be prefetched */
+	writel(count, &gpmc_cfg->prefetch_config2);
+
+	val = (cs << PREFETCH_CONFIG1_CS_SHIFT) | (is_write & 1) |
+		PREFETCH_FIFOTHRESHOLD(fifo_th) | ENABLE_PREFETCH;
+	writel(val, &gpmc_cfg->prefetch_config1);
+
+	/*  Start the prefetch engine */
+	writel(1, &gpmc_cfg->prefetch_control);
+
+	return 0;
+}
+
+/**
+ * omap_prefetch_reset - disables and stops the prefetch engine
+ */
+static void omap_prefetch_reset(void)
+{
+	writel(0, &gpmc_cfg->prefetch_control);
+	writel(0, &gpmc_cfg->prefetch_config1);
+}
+
+static int __read_prefetch_aligned(struct nand_chip *chip, uint32_t *buf, int len)
+{
+	int ret;
+	uint32_t cnt;
+
+	ret = omap_prefetch_enable(PREFETCH_FIFOTHRESHOLD_MAX, len, 0);
+	if (ret < 0)
+		return ret;
+
+	do {
+		int i;
+
+		cnt = readl(&gpmc_cfg->prefetch_status);
+		cnt = PREFETCH_STATUS_FIFO_CNT(cnt);
+
+		for (i = 0; i < cnt / 4; i++) {
+			*buf++ = readl(CONFIG_SYS_NAND_BASE);
+			len -= 4;
+		}
+	} while (len);
+
+	omap_prefetch_reset();
+
+	return 0;
+}
+
+static void omap_nand_read_prefetch8(struct mtd_info *mtd, uint8_t *buf, int len)
+{
+	int ret;
+	uint32_t head, tail;
+	struct nand_chip *chip = mtd->priv;
+
+	/*
+	 * If the destination buffer is unaligned, start with reading
+	 * the overlap byte-wise.
+	 */
+	head = ((uint32_t) buf) % 4;
+	if (head) {
+		nand_read_buf(mtd, buf, head);
+		buf += head;
+		len -= head;
+	}
+
+	/*
+	 * Only transfer multiples of 4 bytes in a pre-fetched fashion.
+	 * If there's a residue, care for it byte-wise afterwards.
+	 */
+	tail = len % 4;
+
+	ret = __read_prefetch_aligned(chip, (uint32_t *) buf, len - tail);
+	if (ret < 0) {
+		/* fallback in case the prefetch engine is busy */
+		nand_read_buf(mtd, buf, len);
+	} else if (tail) {
+		buf += len - tail;
+		nand_read_buf(mtd, buf, tail);
+	}
+}
+#endif /* CONFIG_NAND_OMAP_GPMC_PREFETCH */
+
 /**
  * omap_read_page_bch - hardware ecc based page read function
  * @mtd:	mtd info structure
@@ -883,11 +990,15 @@ int board_nand_init(struct nand_chip *nand)
 	if (err)
 		return err;
 
-#ifdef CONFIG_SPL_BUILD
+#ifdef CONFIG_NAND_OMAP_GPMC_PREFETCH
+	/* TODO: Implement for 16-bit bus width */
 	if (nand->options & NAND_BUSWIDTH_16)
 		nand->read_buf = nand_read_buf16;
 	else
-		nand->read_buf = nand_read_buf;
+		nand->read_buf = omap_nand_read_prefetch8;
+#endif
+
+#ifdef CONFIG_SPL_BUILD
 	nand->dev_ready = omap_spl_dev_ready;
 #endif
 
diff --git a/include/linux/mtd/omap_gpmc.h b/include/linux/mtd/omap_gpmc.h
index 9a86582..6cbae45 100644
--- a/include/linux/mtd/omap_gpmc.h
+++ b/include/linux/mtd/omap_gpmc.h
@@ -66,7 +66,11 @@ struct gpmc {
 	u32 status;		/* 0x54 */
 	u8 res5[0x8];		/* 0x58 */
 	struct gpmc_cs cs[8];	/* 0x60, 0x90, .. */
-	u8 res6[0x14];		/* 0x1E0 */
+	u32 prefetch_config1;	/* 0x1E0 */
+	u32 prefetch_config2;	/* 0x1E4 */
+	u32 res6;		/* 0x1E8 */
+	u32 prefetch_control;	/* 0x1EC */
+	u32 prefetch_status;	/* 0x1F0 */
 	u32 ecc_config;		/* 0x1F4 */
 	u32 ecc_control;	/* 0x1F8 */
 	u32 ecc_size_config;	/* 0x1FC */
-- 
1.9.3