[PATCH] i2c: mediatek: fix potential incorrect use of I2C_MASTER_WRRD

[Andi Shyti]

Hi Leilk,

On Sat, Aug 16, 2025 at 03:53:54PM +0800, Leilk Liu wrote:
> From: "Leilk.Liu" <leilk.liu at mediatek.com>
> 
> The old IC does not support the I2C_MASTER_WRRD (write-then-read)
> function, but the current code’s handling of i2c->auto_restart may
> potentially lead to entering the I2C_MASTER_WRRD software flow,
> resulting in unexpected bugs.

do you mean unexpected behaviour?

> Instead of repurposing the auto_restart flag, add a separate flag
> to signal I2C_MASTER_WRRD operations.
> 
> Also fix handling of msgs. If the operation (i2c->op) is
> I2C_MASTER_WRRD, then the msgs pointer is incremented by 2.
> For all other operations, msgs is simply incremented by 1.
> 
> Fixes: 173b77e8d8fe ("i2c: mediatek: add i2c first write then read optimization")
> 

No need for blank line here. BTW, this doesn't look the commit
that is introducing the issue.

> Signed-off-by: Leilk.Liu <leilk.liu at mediatek.com>
> ---
>  drivers/i2c/busses/i2c-mt65xx.c | 17 ++++++++++-------
>  1 file changed, 10 insertions(+), 7 deletions(-)
> 
> diff --git a/drivers/i2c/busses/i2c-mt65xx.c b/drivers/i2c/busses/i2c-mt65xx.c
> index ab456c3717db..dee40704825c 100644
> --- a/drivers/i2c/busses/i2c-mt65xx.c
> +++ b/drivers/i2c/busses/i2c-mt65xx.c
> @@ -1243,6 +1243,7 @@ static int mtk_i2c_transfer(struct i2c_adapter *adap,
>  {
>  	int ret;
>  	int left_num = num;
> +	bool write_then_read_en = false;
>  	struct mtk_i2c *i2c = i2c_get_adapdata(adap);
>  
>  	ret = clk_bulk_enable(I2C_MT65XX_CLK_MAX, i2c->clocks);
> @@ -1256,6 +1257,7 @@ static int mtk_i2c_transfer(struct i2c_adapter *adap,
>  		if (!(msgs[0].flags & I2C_M_RD) && (msgs[1].flags & I2C_M_RD) &&
>  		    msgs[0].addr == msgs[1].addr) {
>  			i2c->auto_restart = 0;
> +			write_then_read_en = true;

why don't we set "i2c->op = I2C_MASTER_WRRD" here and avoid an
extra flag?

Thanks,
Andi

>  		}
>  	}
>