[PATCH 3/4] iio: ad7949: fix SPI xfer delays

[Andrea Merello <andrea.merello@gmail.com>]

The driver calls udelay(2) after each SPI xfer. However, according to
the specifications, the SPI timing should be as follows:

1- The end of SPI xfer (CNV/CS rising edge) causes the device to initiate
   the conversion phase, which takes up to 2.2uS.

2- At the end of the conversion phase, the device starts the acquisition
   phase for the next conversion automatically (regardless to the state of
   CNV pin); the conversion phase should last at least 1.8 uS

The whole cycle timing is thus 4uS long. The SPI data is read during the
acquisition phase (RAC mode, no need to worry about "Tdata").

In order to be compliant wrt these timing specifications we should wait
4uS after each SPI xfer (that is conservative, because there is also the
SPI xfer duration itself - which at the maximum supported clock should be
about 320nS).

This patch enlarges the delay up to 4uS and it also removes the explicit
calls to udelay(), relying on spi_transfer->delay_usecs.

Signed-off-by: Andrea Merello <andrea.merello@gmail.com>
---
 drivers/iio/adc/ad7949.c | 13 ++-----------
 1 file changed, 2 insertions(+), 11 deletions(-)

diff --git a/drivers/iio/adc/ad7949.c b/drivers/iio/adc/ad7949.c
index 5c2b3446fa4a..25d1e1b24257 100644
--- a/drivers/iio/adc/ad7949.c
+++ b/drivers/iio/adc/ad7949.c
@@ -69,6 +69,7 @@ static int ad7949_spi_write_cfg(struct ad7949_adc_chip *ad7949_adc, u16 val,
 			.tx_buf = &ad7949_adc->buffer,
 			.len = 2,
 			.bits_per_word = bits_per_word,
+			.delay_usecs = 4,
 		},
 	};
 
@@ -77,11 +78,6 @@ static int ad7949_spi_write_cfg(struct ad7949_adc_chip *ad7949_adc, u16 val,
 	spi_message_init_with_transfers(&msg, tx, 1);
 	ret = spi_sync(ad7949_adc->spi, &msg);
 
-	/*
-	 * This delay is to avoid a new request before the required time to
-	 * send a new command to the device
-	 */
-	udelay(2);
 	return ret;
 }
 
@@ -97,6 +93,7 @@ static int ad7949_spi_read_channel(struct ad7949_adc_chip *ad7949_adc, int *val,
 			.rx_buf = &ad7949_adc->buffer,
 			.len = 2,
 			.bits_per_word = bits_per_word,
+			.delay_usecs = 4,
 		},
 	};
 
@@ -112,12 +109,6 @@ static int ad7949_spi_read_channel(struct ad7949_adc_chip *ad7949_adc, int *val,
 	if (ret)
 		return ret;
 
-	/*
-	 * This delay is to avoid a new request before the required time to
-	 * send a new command to the device
-	 */
-	udelay(2);
-
 	ad7949_adc->current_channel = channel;
 
 	*val = ad7949_adc->buffer & mask;
-- 
2.17.1