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

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Message-Id: <74a36bce-3d6e-4124-add1-8ed44ba2c80d@www.fastmail.com>
Date:   Tue, 29 Mar 2022 21:58:43 +0200
From:   "Sven Peter" <sven@...npeter.dev>
To:     Martin Povišer <povik+lin@...ebit.org>,
        "Hector Martin" <marcan@...can.st>,
        "Michael Ellerman" <mpe@...erman.id.au>
Cc:     "Alyssa Rosenzweig" <alyssa@...enzweig.io>,
        "Benjamin Herrenschmidt" <benh@...nel.crashing.org>,
        "Paul Mackerras" <paulus@...ba.org>,
        "Jean Delvare" <khali@...ux-fr.org>,
        "Olof Johansson" <olof@...om.net>,
        linux-arm-kernel@...ts.infradead.org,
        linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>,
        linux-i2c@...r.kernel.org, linux-kernel@...r.kernel.org,
        "Janne Grunau" <j@...nau.net>
Subject: Re: [PATCH] i2c: pasemi: Wait for write xfers to finish

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@...ebit.org>
> ---

Reviewed-by: Sven Peter <sven@...npeter.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