Re: [PATCH net-next] net: lan966x: Improve the CPU TX bitrate.

Horatiu Vultur
2022-03-09T22:02:37Z

The 03/09/2022 14:11, Andrew Lunn wrote:
>
> On Tue, Mar 08, 2022 at 11:30:00PM +0100, Horatiu Vultur wrote:
> > The 03/08/2022 22:36, Andrew Lunn wrote:
> > >
> > > > static int lan966x_port_inj_ready(struct lan966x *lan966x, u8 grp)
> > > > {
> > > > - u32 val;
> > > > + unsigned long time = jiffies + usecs_to_jiffies(READL_TIMEOUT_US);
> > > > + int ret = 0;
> > > >
> > > > - return readx_poll_timeout_atomic(lan966x_port_inj_status, lan966x, val,
> > > > - QS_INJ_STATUS_FIFO_RDY_GET(val) & BIT(grp),
> > > > - READL_SLEEP_US, READL_TIMEOUT_US);
> > > > + while (!(lan_rd(lan966x, QS_INJ_STATUS) &
> > > > + QS_INJ_STATUS_FIFO_RDY_SET(BIT(grp)))) {
> > > > + if (time_after(jiffies, time)) {
> > > > + ret = -ETIMEDOUT;
> > > > + break;
> > > > + }
> > >
> > > Did you try setting READL_SLEEP_US to 0? readx_poll_timeout_atomic()
> > > explicitly supports that.
> >
> > I have tried but it didn't improve. It was the same as before.
>
> The reason i recommend ipoll.h is that your implementation has the
> usual bug, which iopoll does not have. Since you are using _atomic()
> it is less of an issue, but it still exists.
>
> while (!(lan_rd(lan966x, QS_INJ_STATUS) &
> QS_INJ_STATUS_FIFO_RDY_SET(BIT(grp)))) {
>
> Say you take an interrupt here
>
> if (time_after(jiffies, time)) {
> ret = -ETIMEDOUT;
> break;
> }
>
>
> The interrupt takes a while, so that by the time you get to
> time_after(), you have reached your timeout. So -ETIMEDOUT is
> returned. But in fact, the hardware has done its thing, and if you
> where to read the status the bit would be set, and you should actually
> return O.K, not an error.

That is a good catch and really nice explanation!
Then if I add also the check at the end, then it should be also OK.

>
> iopoll does another check of the status before deciding to return
> -ETIMEDOUT or O.K.
>
> If you decide to simply check the status directly after the write, i
> suggest you then use readx_poll_timeout_atomic() if you do need to
> poll.
>
> Andrew

--
/Horatiu