From a44648b057f5331fe6c0e863dc693ed335490e7a Mon Sep 17 00:00:00 2001 From: Anton Vorontsov Date: Fri, 10 Aug 2007 13:01:02 -0700 Subject: spi_mpc83xx: fix prescale modulus calculation Long ago I've noticed (but didn't pay much attention) that spi_mpc83xx using PM calculations that differs from what specs describe. I.e. u8 pm = mpc83xx_spi->spibrg / (spi->max_speed_hz * 4); While specs says: "The SPI baud rate generator clock source (either system clock or system clock divided by 16, depending on DIV16 bit) is divided by 4 * ([PM] + 1), a range from 4 to 64.". Thus " - 1" is missing in the spi_mpc83xx's formula. Why nobody noticed that bug? Probably because sysclk usually less then user expects, e.g. you expect 200 MHz, but real clock is 198 MHz, and integer rounding helps when this formula is used. Suppose it's SPI in QE, SYSCLK at 198 MHz, thus SPIBRG at 99MHz, 25 MHz requested. PM = (99MHz / ( 25 MHz * 4 )), PM == 0, output SPICLK will be 24.75 MHz At lower frequencies this bug is more noticeable, though. And this bug shows itself in all its beauty if SYSCLK is equal or a bit more than you expect (200 MHz SYSCLK, 100 MHz SPIBRG): PM = (100MHz / ( 25 MHz * 4 )), PM == 1, output SPICLK will be 12.625 MHz! Signed-off-by: Anton Vorontsov Signed-off-by: David Brownell Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- drivers/spi/spi_mpc83xx.c | 8 ++++++-- 1 file changed, 6 insertions(+), 2 deletions(-) (limited to 'drivers/spi') diff --git a/drivers/spi/spi_mpc83xx.c b/drivers/spi/spi_mpc83xx.c index fe69e94043e..2adf856e44c 100644 --- a/drivers/spi/spi_mpc83xx.c +++ b/drivers/spi/spi_mpc83xx.c @@ -148,6 +148,8 @@ static void mpc83xx_spi_chipselect(struct spi_device *spi, int value) if (value == BITBANG_CS_ACTIVE) { u32 regval = mpc83xx_spi_read_reg(&mpc83xx_spi->base->mode); u32 len = spi->bits_per_word; + u8 pm; + if (len == 32) len = 0; else @@ -170,7 +172,7 @@ static void mpc83xx_spi_chipselect(struct spi_device *spi, int value) regval |= SPMODE_LEN(len); if ((mpc83xx_spi->spibrg / spi->max_speed_hz) >= 64) { - u8 pm = mpc83xx_spi->spibrg / (spi->max_speed_hz * 64); + pm = mpc83xx_spi->spibrg / (spi->max_speed_hz * 64) - 1; if (pm > 0x0f) { dev_err(&spi->dev, "Requested speed is too " "low: %d Hz. Will use %d Hz instead.\n", @@ -180,7 +182,9 @@ static void mpc83xx_spi_chipselect(struct spi_device *spi, int value) } regval |= SPMODE_PM(pm) | SPMODE_DIV16; } else { - u8 pm = mpc83xx_spi->spibrg / (spi->max_speed_hz * 4); + pm = mpc83xx_spi->spibrg / (spi->max_speed_hz * 4); + if (pm) + pm--; regval |= SPMODE_PM(pm); } -- cgit v1.2.3-70-g09d2