[PATCH v2 4/4] i2c: imx: prevent rescheduling in non dma mode

[Oleksij Rempel]

On Mon, Aug 19, 2024 at 09:19:10AM +0200, Stefan Eichenberger wrote:
> From: Stefan Eichenberger <stefan.eichenberger at toradex.com>
> 
> We are experiencing a problem with the i.MX I2C controller when
> communicating with SMBus devices. We are seeing devices time-out because
> the time between sending/receiving two bytes is too long, and the SMBus
> device returns to the idle state. This happens because the i.MX I2C
> controller sends and receives byte by byte. When a byte is sent or
> received, we get an interrupt and can send or receive the next byte.
> 
> The current implementation sends a byte and then waits for an event
> generated by the interrupt subroutine. After the event is received, the
> next byte is sent and we wait again. This waiting allows the scheduler
> to reschedule other tasks, with the disadvantage that we may not send
> the next byte for a long time because the send task is not immediately
> scheduled. For example, if the rescheduling takes more than 25ms, this
> can cause SMBus devices to timeout and communication to fail.
> 
> This patch changes the behavior so that we do not reschedule the
> send/receive task, but instead send or receive the next byte in the
> interrupt subroutine. This prevents rescheduling and drastically reduces
> the time between sending/receiving bytes. The cost in the interrupt
> subroutine is relatively small, we check what state we are in and then
> send/receive the next byte. Before we had to call wake_up, which is even
> less expensive. However, we also had to do some scheduling, which
> increased the overall cost compared to the new solution. The wake_up
> function to wake up the send/receive task is now only called when an
> error occurs or when the transfer is complete.
> 
> Signed-off-by: Stefan Eichenberger <stefan.eichenberger at toradex.com>

Acked-by: Oleksij Rempel <o.rempel at pengutronix.de>

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