[PATCH] i2c: mux: pca954x: fix i2c mux selection caching

[Peter Rosin]

On 2016-12-16 21:06, Russell King wrote:
> smbus functions return -ve on error, 0 on success.  However,
> __i2c_transfer() have a different return signature - -ve on error, or
> number of buffers transferred (which may be zero or greater.)
> 
> The upshot of this is that the sense of the test is reversed when using
> the mux on a bus supporting the master_xfer method: we cache the value
> and never retry if we fail to transfer any buffers, but if we succeed,
> we clear the cached value.

Ouch! Thanks for catching this.

> Fix this.

But lets fix the corner case of __i2c_transfer returning 0 instead of
the expected 1 as well (not sure if that's even possible, but lets close
the possibility just in case), so I'd prefer if you could fix
pca954x_reg_write() to return 0 iff __i2c_transfer(...) returns 1
instead, and -EREMOTEIO on other non-negative return values. Thanks!

Cheers,
peda

> Fixes: 463e8f845cbf ("i2c: mux: pca954x: retry updating the mux selection on failure")
> Signed-off-by: Russell King <rmk+kernel at armlinux.org.uk>
> ---
>  drivers/i2c/muxes/i2c-mux-pca954x.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/i2c/muxes/i2c-mux-pca954x.c b/drivers/i2c/muxes/i2c-mux-pca954x.c
> index 8bc3d36d2837..b6d62ecbd5b6 100644
> --- a/drivers/i2c/muxes/i2c-mux-pca954x.c
> +++ b/drivers/i2c/muxes/i2c-mux-pca954x.c
> @@ -179,7 +179,7 @@ static int pca954x_select_chan(struct i2c_mux_core *muxc, u32 chan)
>  	/* Only select the channel if its different from the last channel */
>  	if (data->last_chan != regval) {
>  		ret = pca954x_reg_write(muxc->parent, client, regval);
> -		data->last_chan = ret ? 0 : regval;
> +		data->last_chan = ret >= 0 ? regval : 0;
>  	}
>  
>  	return ret;
>