[U-Boot] [PATCH u-boot-marvell v2 15/15] i2c: mvtwsi: fix reading status register after interrupt

[Heiko Schocher]

Hello Marek,

Am 30.04.2019 um 03:48 schrieb Marek Behún:
> The twsi_wait function reads the control register for interrupt flag,
> and if interrupt flag is present, it immediately reads status register.
> 
> On our device this sometimes causes bad value being read from status
> register, as if the value was not yet updated.
> 
> My theory is that the controller does approximately this:
>    1. sets interrupt flag in control register,
>    2. sets the value of status register,
>    3. causes an interrupt
> 
> In U-Boot we do not use interrupts, so I think that it is possible that
> sometimes the status register in the twsi_wait function is read between
> points 1 and 2.
> 
> The bug does not appear if I add a small delay before reading status
> register.

Strange.

> Wait 100ns (which in U-Boot currently means 1 us, because ndelay(i)
> function calls udelay(DIV_ROUND_UP(i, 1000))) before reading the status
> register.
> 
> Signed-off-by: Marek Behún <marek.behun at nic.cz>
> Cc: Mario Six <mario.six at gdsys.cc>
> Cc: Stefan Roese <sr at denx.de>
> Cc: Baruch Siach <baruch at tkos.co.il>
> Cc: Heiko Schocher <hs at denx.de>
> ---
>   drivers/i2c/mvtwsi.c | 1 +
>   1 file changed, 1 insertion(+)
> 
> diff --git a/drivers/i2c/mvtwsi.c b/drivers/i2c/mvtwsi.c
> index 74ac0a4aa7..483a71ba12 100644
> --- a/drivers/i2c/mvtwsi.c
> +++ b/drivers/i2c/mvtwsi.c
> @@ -271,6 +271,7 @@ static int twsi_wait(struct mvtwsi_registers *twsi, int expected_status,
>   	do {
>   		control = readl(&twsi->control);
>   		if (control & MVTWSI_CONTROL_IFLG) {
> +			ndelay(100);
>   			status = readl(&twsi->status);
>   			if (status == expected_status)
>   				return 0;
> 

Can you please add a comment in the code too?

Beside of that:

Acked-by: Heiko Schocher <hs at denx.de>

bye,
Heiko
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