RE: [PATCH net] net: dsa: microchip: fix KSZ9477 set_ageing_time function

[<Tristram.Ha@microchip.com>]

> Subject: Re: [PATCH net] net: dsa: microchip: fix KSZ9477 set_ageing_time function
> 
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> On Tue, 2024-05-28 at 14:36 -0700, Tristram.Ha@microchip.com wrote:
> > From: Tristram Ha <tristram.ha@microchip.com>
> >
> > The aging count is not a simple 11-bit value but comprises a 3-bit
> > multiplier and an 8-bit second count.  The code tries to find a set of
> > values with result close to the specifying value.
> >
> > Note LAN937X has similar operation but provides an option to use
> > millisecond instead of second so there will be a separate fix in the
> > future.
> >
> > Fixes: 2c119d9982b1 ("net: dsa: microchip: add the support for set_ageing_time")
> > Signed-off-by: Tristram Ha <tristram.ha@microchip.com>
> > ---
> >  drivers/net/dsa/microchip/ksz9477.c     | 64 +++++++++++++++++++++----
> >  drivers/net/dsa/microchip/ksz9477_reg.h |  1 -
> >  2 files changed, 54 insertions(+), 11 deletions(-)
> >
> > diff --git a/drivers/net/dsa/microchip/ksz9477.c
> b/drivers/net/dsa/microchip/ksz9477.c
> > index f8ad7833f5d9..1af11aee3119 100644
> > --- a/drivers/net/dsa/microchip/ksz9477.c
> > +++ b/drivers/net/dsa/microchip/ksz9477.c
> > @@ -1099,26 +1099,70 @@ void ksz9477_get_caps(struct ksz_device *dev, int
> port,
> >  int ksz9477_set_ageing_time(struct ksz_device *dev, unsigned int msecs)
> >  {
> >       u32 secs = msecs / 1000;
> > +     u8 first, last, mult, i;
> > +     int min, ret;
> > +     int diff[8];
> >       u8 value;
> >       u8 data;
> > -     int ret;
> >
> > -     value = FIELD_GET(SW_AGE_PERIOD_7_0_M, secs);
> > +     /* The aging timer comprises a 3-bit multiplier and an 8-bit second
> > +      * value.  Either of them cannot be zero.  The maximum timer is then
> > +      * 7 * 255 = 1785.
> > +      */
> > +     if (!secs)
> > +             secs = 1;
> >
> > -     ret = ksz_write8(dev, REG_SW_LUE_CTRL_3, value);
> > +     ret = ksz_read8(dev, REG_SW_LUE_CTRL_0, &value);
> >       if (ret < 0)
> >               return ret;
> >
> > -     data = FIELD_GET(SW_AGE_PERIOD_10_8_M, secs);
> > +     /* Check whether there is need to update the multiplier. */
> > +     mult = FIELD_GET(SW_AGE_CNT_M, value);
> > +     if (mult > 0) {
> > +             /* Try to use the same multiplier already in the register. */
> > +             min = secs / mult;
> > +             if (min <= 0xff && min * mult == secs)
> > +                     return ksz_write8(dev, REG_SW_LUE_CTRL_3, min);
> > +     }
> >
> > -     ret = ksz_read8(dev, REG_SW_LUE_CTRL_0, &value);
> > -     if (ret < 0)
> > -             return ret;
> > +     /* Return error if too large. */
> > +     if (secs > 7 * 0xff)
> > +             return -EINVAL;
> > +
> > +     /* Find out which combination of multiplier * value results in a timer
> > +      * value close to the specified timer value.
> > +      */
> > +     first = (secs + 0xfe) / 0xff;
> > +     for (i = first; i <= 7; i++) {
> > +             min = secs / i;
> > +             diff[i] = secs - i * min;
> > +             if (!diff[i]) {
> > +                     i++;
> > +                     break;
> > +             }
> > +     }
> > +
> > +     last = i;
> > +     min = 0xff;
> > +     for (i = last - 1; i >= first; i--) {
> > +             if (diff[i] < min) {
> > +                     data = i;
> > +                     min = diff[i];
> > +             }
> > +             if (!min)
> > +                     break;
> > +     }
> 
> Is the additional accuracy worthy the added complexity WRT:
> 
>         mult = DIV_ROUND_UP(secs, 0xff);
> 
> ?

I do not know much accuracy is expected of this function.  I do not know
whether users can easily specify the amount, but the default is 3 seconds
which is the same as the hardware default where the multiplier is 4 and
the count is 75.  So most of the time the rest of the code will not be
executed.