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

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Message-ID: <20220309220516.smxhbtikbvctlkeh@soft-dev3-1.localhost>
Date:   Wed, 9 Mar 2022 23:05:16 +0100
From:   Horatiu Vultur <horatiu.vultur@...rochip.com>
To:     Andrew Lunn <andrew@...n.ch>
CC:     <netdev@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        <UNGLinuxDriver@...rochip.com>, <davem@...emloft.net>,
        <kuba@...nel.org>
Subject: Re: [PATCH net-next] net: lan966x: Improve the CPU TX bitrate.

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