mtd: spi-nor: remove micron_quad_enable()

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Gitweb:     http://git.infradead.org/?p=mtd-2.6.git;a=commit;h=3b5394a3ccffbfa1d1d448d48742853a862822c4
Commit:     3b5394a3ccffbfa1d1d448d48742853a862822c4
Parent:     c08266794926a9f6c2940e8585fbcbdc51caa7ed
Author:     Cyrille Pitchen <cyrille.pitchen at atmel.com>
AuthorDate: Wed Feb 3 14:26:46 2016 +0100
Committer:  Brian Norris <computersforpeace at gmail.com>
CommitDate: Fri Feb 12 11:35:13 2016 -0800

    mtd: spi-nor: remove micron_quad_enable()
    
    This patch remove the micron_quad_enable() function which force the Quad
    SPI mode. However, once this mode is enabled, the Micron memory expect ALL
    commands to use the SPI 4-4-4 protocol. Hence a failure does occur when
    calling spi_nor_wait_till_ready() right after the update of the Enhanced
    Volatile Configuration Register (EVCR) in the micron_quad_enable() as
    the SPI controller driver is not aware about the protocol change.
    
    Since there is almost no performance increase using Fast Read 4-4-4
    commands instead of Fast Read 1-1-4 commands, we rather keep on using the
    Extended SPI mode than enabling the Quad SPI mode.
    
    Let's take the example of the pretty standard use of 8 dummy cycles during
    Fast Read operations on 64KB erase sectors:
    
    Fast Read 1-1-4 requires 8 cycles for the command, then 24 cycles for the
    3byte address followed by 8 dummy clock cycles and finally 65536*2 cycles
    for the read data; so 131112 clock cycles.
    
    On the other hand the Fast Read 4-4-4 would require 2 cycles for the
    command, then 6 cycles for the 3byte address followed by 8 dummy clock
    cycles and finally 65536*2 cycles for the read data. So 131088 clock
    cycles. The theorical bandwidth increase is 0.0%.
    
    Now using Fast Read operations on 512byte pages:
    Fast Read 1-1-4 needs 8+24+8+(512*2) = 1064 clock cycles whereas Fast
    Read 4-4-4 would requires 2+6+8+(512*2) = 1040 clock cycles. Hence the
    theorical bandwidth increase is 2.3%.
    Consecutive reads for non sequential pages is not a relevant use case so
    The Quad SPI mode is not worth it.
    
    mtd_speedtest seems to confirm these figures.
    
    Signed-off-by: Cyrille Pitchen <cyrille.pitchen at atmel.com>
    Fixes: 548cd3ab54da ("mtd: spi-nor: Add quad I/O support for Micron SPI NOR")
    Signed-off-by: Brian Norris <computersforpeace at gmail.com>
---
 drivers/mtd/spi-nor/spi-nor.c | 46 +------------------------------------------
 1 file changed, 1 insertion(+), 45 deletions(-)

diff --git a/drivers/mtd/spi-nor/spi-nor.c b/drivers/mtd/spi-nor/spi-nor.c
index 13e07ce..999b3ce5 100644
--- a/drivers/mtd/spi-nor/spi-nor.c
+++ b/drivers/mtd/spi-nor/spi-nor.c
@@ -1101,45 +1101,6 @@ static int spansion_quad_enable(struct spi_nor *nor)
 	return 0;
 }
 
-static int micron_quad_enable(struct spi_nor *nor)
-{
-	int ret;
-	u8 val;
-
-	ret = nor->read_reg(nor, SPINOR_OP_RD_EVCR, &val, 1);
-	if (ret < 0) {
-		dev_err(nor->dev, "error %d reading EVCR\n", ret);
-		return ret;
-	}
-
-	write_enable(nor);
-
-	/* set EVCR, enable quad I/O */
-	nor->cmd_buf[0] = val & ~EVCR_QUAD_EN_MICRON;
-	ret = nor->write_reg(nor, SPINOR_OP_WD_EVCR, nor->cmd_buf, 1);
-	if (ret < 0) {
-		dev_err(nor->dev, "error while writing EVCR register\n");
-		return ret;
-	}
-
-	ret = spi_nor_wait_till_ready(nor);
-	if (ret)
-		return ret;
-
-	/* read EVCR and check it */
-	ret = nor->read_reg(nor, SPINOR_OP_RD_EVCR, &val, 1);
-	if (ret < 0) {
-		dev_err(nor->dev, "error %d reading EVCR\n", ret);
-		return ret;
-	}
-	if (val & EVCR_QUAD_EN_MICRON) {
-		dev_err(nor->dev, "Micron EVCR Quad bit not clear\n");
-		return -EINVAL;
-	}
-
-	return 0;
-}
-
 static int set_quad_mode(struct spi_nor *nor, const struct flash_info *info)
 {
 	int status;
@@ -1153,12 +1114,7 @@ static int set_quad_mode(struct spi_nor *nor, const struct flash_info *info)
 		}
 		return status;
 	case SNOR_MFR_MICRON:
-		status = micron_quad_enable(nor);
-		if (status) {
-			dev_err(nor->dev, "Micron quad-read not enabled\n");
-			return -EINVAL;
-		}
-		return status;
+		return 0;
 	default:
 		status = spansion_quad_enable(nor);
 		if (status) {