linux-kernel - Re: [PATCH 1/2] i2c: mux: pca9541: Change to correct bus control commands

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Date:   Tue, 2 Jun 2020 17:05:31 +0200
From:   Peter Rosin <peda@...ntia.se>
To:     Quentin Strydom <quentin.strydom@...wireless.com>,
        Guenter Roeck <linux@...ck-us.net>,
        Wolfram Sang <wsa@...-dreams.de>, linux-i2c@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] i2c: mux: pca9541: Change to correct bus control
 commands

On 2020-06-02 14:12, Quentin Strydom wrote:
> Change current bus commands to match the pca9541a datasheet
> (see table 12 on page 14 of
> https://www.nxp.com/docs/en/data-sheet/PCA9541A.pdf). Also
> where entries are marked as no change the current control
> command is repeated as the current master reading the
> control register has control of the bus and bus is on.
> 
> Signed-off-by: Quentin Strydom <quentin.strydom@...wireless.com>
> ---
>  drivers/i2c/muxes/i2c-mux-pca9541.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/i2c/muxes/i2c-mux-pca9541.c b/drivers/i2c/muxes/i2c-mux-pca9541.c
> index 6a39ada..50808fa 100644
> --- a/drivers/i2c/muxes/i2c-mux-pca9541.c
> +++ b/drivers/i2c/muxes/i2c-mux-pca9541.c
> @@ -211,7 +211,7 @@ static void pca9541_release_bus(struct i2c_client *client)
>  
>  /* Control commands per PCA9541 datasheet */
>  static const u8 pca9541_control[16] = {
> -	4, 0, 1, 5, 4, 4, 5, 5, 0, 0, 1, 1, 0, 4, 5, 1
> +	4, 4, 5, 5, 4, 4, 5, 7, 8, 0, 1, 11, 0, 0, 1, 1
>  };
>  
>  /*

I found all your mails from git send-email in my spam folder. They probably
lack some headers that have become increasingly important... [Don't ask me
for further details.]

I do not have the HW to test this. I'm only going by the datasheet.

But yes, pca9541_control[1] and [2] indeed seem exchanged with [13] and [14].

However, pca9541_control[5], [7], [8], and [11] are never used AFAICT.
Trying to write 7, 8 and 11 also attempts to write various read-only bits
and makes no sense. So, I'd skip those changes.

All that said, I'm a bit skeptic as to why this has worked at all if this
is incorrect. I would like to see a more detailed failure description that
could explain why this change is indeed "it".

Cheers,
Peter