Re: [PATCH v2] i2c: designware: Fix corrupted memory seen in the ISR

[Serge Semin]

Hi Jan

On Wed, Sep 13, 2023 at 04:29:38PM -0700, Jan Bottorff wrote:
> Errors were happening in the ISR that looked like corrupted
> memory. This was because memory writes from the core enabling
> interrupts were not yet visible to the core running the ISR. The
> kernel log would get the message "i2c_designware APMC0D0F:00:
> controller timed out" during in-band IPMI SSIF stress tests.
> 
> Add a write barrier before enabling interrupts to assure data written
> by the current core is visible to all cores before the interrupt fires.
> 
> The ARM Barrier Litmus Tests and Cookbook has an example under
> Sending Interrupts and Barriers that matches the usage in this
> driver. That document says a DSB barrier is required.
> 
> Signed-off-by: Jan Bottorff <janb@xxxxxxxxxxxxxxxxxxxxxx>
> Reviewed-by: Yann Sionneau <ysionneau@xxxxxxxxxxxxx>
> Tested-by: Yann Sionneau <ysionneau@xxxxxxxxxxxxx>
> ---
>  drivers/i2c/busses/i2c-designware-master.c | 8 ++++++++
>  1 file changed, 8 insertions(+)
> 
> diff --git a/drivers/i2c/busses/i2c-designware-master.c b/drivers/i2c/busses/i2c-designware-master.c
> index ca1035e010c7..1694ac6bb592 100644
> --- a/drivers/i2c/busses/i2c-designware-master.c
> +++ b/drivers/i2c/busses/i2c-designware-master.c
> @@ -248,6 +248,14 @@ static void i2c_dw_xfer_init(struct dw_i2c_dev *dev)
>  	/* Dummy read to avoid the register getting stuck on Bay Trail */
>  	regmap_read(dev->map, DW_IC_ENABLE_STATUS, &dummy);
>  
> +	/*
> +	 * To guarantee data written by the current core is visible to
> +	 * all cores, a write barrier is required. This needs to be
> +	 * before an interrupt causes execution on another core.
> +	 * For ARM processors, this needs to be a DSB barrier.
> +	 */

> +	wmb();

Based on the patch log and the comment, smp_wmb() seems to be more
suitable here since the problem looks like SMP-specific. Most
importantly the smp_wmb() will get to be just the compiler barrier on
the UP system, so no cache and pipeline flushes in that case.
Meanwhile 

I am not ARM expert, but based on the problem and the DMB/DSB barriers
descriptions using DMB should be enough in your case since you only
need memory syncs.

-Serge(y)

> +
>  	/* Clear and enable interrupts */
>  	regmap_read(dev->map, DW_IC_CLR_INTR, &dummy);
>  	regmap_write(dev->map, DW_IC_INTR_MASK, DW_IC_INTR_MASTER_MASK);
> -- 
> 2.41.0
>