Re: [PATCH] i2c: pasemi: Wait for write xfers to finish

Sven Peter
2022-03-29T19:59:11Z

On Tue, Mar 29, 2022, at 20:38, Martin Povišer wrote:
> Wait for completion of write transfers before returning from the driver.
> At first sight it may seem advantageous to leave write transfers queued
> for the controller to carry out on its own time, but there's a couple of
> issues with it:
>
> * Driver doesn't check for FIFO space.

Maybe we should also check that in a follow-up patch :-)

>
> * The queued writes can complete while the driver is in its I2C read
> transfer path which means it will get confused by the raising of
> XEN (the 'transaction ended' signal). This can cause a spurious
> ENODATA error due to premature reading of the MRXFIFO register.
>
> Adding the wait fixes some unreliability issues with the driver. There's
> some efficiency cost to it (especially with pasemi_smb_waitready doing
> its polling), but that will be alleviated once the driver receives
> interrupt support.
>
> Fixes: beb58aa39e6e ("i2c: PA Semi SMBus driver")
> Signed-off-by: Martin Povišer <povik+lin@cutebit.org>
> ---

Reviewed-by: Sven Peter <sven@svenpeter.dev>

>
> Tested on Apple's t8103 chip. To my knowledge the PA Semi controller
> in its pre-Apple occurences behaves the same as far as this patch is
> concerned.
>
> drivers/i2c/busses/i2c-pasemi-core.c | 6 ++++++
> 1 file changed, 6 insertions(+)
>
> diff --git a/drivers/i2c/busses/i2c-pasemi-core.c
> b/drivers/i2c/busses/i2c-pasemi-core.c
> index 7728c8460dc0..9028ffb58cc0 100644
> --- a/drivers/i2c/busses/i2c-pasemi-core.c
> +++ b/drivers/i2c/busses/i2c-pasemi-core.c
> @@ -137,6 +137,12 @@ static int pasemi_i2c_xfer_msg(struct i2c_adapter
> *adapter,
>
> TXFIFO_WR(smbus, msg->buf[msg->len-1] |
> (stop ? MTXFIFO_STOP : 0));
> +
> + if (stop) {
> + err = pasemi_smb_waitready(smbus);
> + if (err)
> + goto reset_out;
> + }

Looks like pasemi_smb_xfer doesn't suffer from the same issue.
I wonder if every device connected to the bus on the original PA Semi boards
only used that path and that's why no one noticed it.

Sven