[RFC PATCH] i2c: imx: avoid rescheduling when waiting for bus not busy

[Stefan Eichenberger]

From: Stefan Eichenberger <stefan.eichenberger at toradex.com>

On our i.MX8M Mini based module we have an ADS1015 I2C ADC connected to
the I2C bus. The ADS1015 I2C ADC will timeout after 25ms when the I2C
bus is idle. The imx i2c driver will call schedule when waiting for the
bus to become idle after switching to master mode. When the i2c
controller switches to master mode it pulls SCL and SDA low, if the
ADS1015 I2C ADC sees this for more than 25 ms without seeing SCL
clocking, it will timeout and ignore all signals until the next start
condition occurs (SCL and SDA low). This can occur when the system load
is high and schedule returns after more than 25 ms.

This rfc tries to solve the problem by using a udelay for the first 10
ms before calling schedule. This reduces the chance that we will
reschedule. However, it is still theoretically possible for the problem
to occur. To properly solve the problem, we would also need to disable
interrupts during the transfer.

After some internal discussion, we see three possible solutions:
1. Use udelay as shown in this rfc and also disable the interrupts
   during the transfer. This would solve the problem but disable the
   interrupts. Also, we would have to re-enable the interrupts if the
   timeout is longer than 1ms (TBD).
2. We use a retry mechanism in the ti-ads1015 driver. When we see a
   timeout, we try again.
3. We use the suggested solution and accept that there is an edge case
   where the timeout can happen.

There may be a better way to do this, which is why this is an RFC.

Signed-off-by: Stefan Eichenberger <stefan.eichenberger at toradex.com>
---
 drivers/i2c/busses/i2c-imx.c | 14 +++++++++++---
 1 file changed, 11 insertions(+), 3 deletions(-)

diff --git a/drivers/i2c/busses/i2c-imx.c b/drivers/i2c/busses/i2c-imx.c
index 3842e527116b7..179f8367490a5 100644
--- a/drivers/i2c/busses/i2c-imx.c
+++ b/drivers/i2c/busses/i2c-imx.c
@@ -503,10 +503,18 @@ static int i2c_imx_bus_busy(struct imx_i2c_struct *i2c_imx, int for_busy, bool a
 				"<%s> I2C bus is busy\n", __func__);
 			return -ETIMEDOUT;
 		}
-		if (atomic)
+		if (atomic) {
 			udelay(100);
-		else
-			schedule();
+		} else {
+			/*
+			 * Avoid rescheduling in the first 10 ms to avoid
+			 * timeouts for SMBus like devices
+			 */
+			if (time_before(jiffies, orig_jiffies + msecs_to_jiffies(10)))
+				udelay(10);
+			else
+				schedule();
+		}
 	}
 
 	return 0;
-- 
2.40.1