Re: [PATCH 1/2] ravb: Fix descriptor counters' conditions

[Ulrich Hecht <uli@fpond.eu>]

> On 07/27/2021 10:21 AM Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com> wrote:
> 
>  
> The descriptor counters ({cur,dirty}_[rt]x) acts as free counters
> so that conditions are possible to be incorrect when a left value
> was overflowed.
> 
> So, for example, ravb_tx_free() could not free any descriptors
> because the following condition was checked as a signed value,
> and then "NETDEV WATCHDOG" happened:
> 
>     for (; priv->cur_tx[q] - priv->dirty_tx[q] > 0; priv->dirty_tx[q]++) {
> 
> To fix the issue, add get_num_desc() to calculate numbers of
> remaining descriptors.
> 
> Fixes: c156633f1353 ("Renesas Ethernet AVB driver proper")
> Signed-off-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
> ---
>  drivers/net/ethernet/renesas/ravb_main.c | 22 +++++++++++++++-------
>  1 file changed, 15 insertions(+), 7 deletions(-)
> 
> diff --git a/drivers/net/ethernet/renesas/ravb_main.c b/drivers/net/ethernet/renesas/ravb_main.c
> index 805397088850..70fbac572036 100644
> --- a/drivers/net/ethernet/renesas/ravb_main.c
> +++ b/drivers/net/ethernet/renesas/ravb_main.c
> @@ -172,6 +172,14 @@ static const struct mdiobb_ops bb_ops = {
>  	.get_mdio_data = ravb_get_mdio_data,
>  };
>  
> +static u32 get_num_desc(u32 from, u32 subtract)
> +{
> +	if (from >= subtract)
> +		return from - subtract;
> +
> +	return U32_MAX - subtract + 1 + from;
> +}

This is a very roundabout way to implement an unsigned subtraction. :)
I think it would make more sense to simply return 0 if "subtract" is larger than "from".
(Likewise for sh_eth).

CU
Uli