Re: [PATCH v3 09/16] reset: starfive-jh7100: Add StarFive JH7100 reset driver

[Emil Renner Berthing <kernel@xxxxxxxx>]

On Tue, 2 Nov 2021 at 20:43, Andy Shevchenko <andy.shevchenko@xxxxxxxxx> wrote:
> +Cc: Yury (bitmap expert)
>
> On Tue, Nov 2, 2021 at 6:50 PM Emil Renner Berthing <kernel@xxxxxxxx> wrote:
> >
> > Add a driver for the StarFive JH7100 reset controller.
>
> ...
>
> > +#define BIT_MASK32(x) BIT((x) % 32)
>
> Possible namespace collision.
>
> ...
>
> > +/*
> > + * the registers work like a 32bit bitmap, so writing a 1 to the m'th bit of
> > + * the n'th ASSERT register asserts line 32n + m, and writing a 0 deasserts the
> > + * same line.
> > + * most reset lines have their status inverted so a 0 in the STATUS register
> > + * means the line is asserted and a 1 means it's deasserted. a few lines don't
> > + * though, so store the expected value of the status registers when all lines
> > + * are asserted.
> > + */
>
> Besides missing capitalization,

I'm confused. it was you who wanted all comments to capitalized the same..
64bi
if it sounds like bitmap, use bitmap.
> I have checked DT definitions and it seems you don't even need the
> BIT_MASK() macro,
>
> > +static const u32 jh7100_reset_asserted[4] = {
> > +       /* STATUS0 register */
> > +       BIT_MASK32(JH7100_RST_U74) |
> > +       BIT_MASK32(JH7100_RST_VP6_DRESET) |
> > +       BIT_MASK32(JH7100_RST_VP6_BRESET),
> > +       /* STATUS1 register */
> > +       BIT_MASK32(JH7100_RST_HIFI4_DRESET) |
> > +       BIT_MASK32(JH7100_RST_HIFI4_BRESET),
> > +       /* STATUS2 register */
> > +       BIT_MASK32(JH7100_RST_E24),
> > +       /* STATUS3 register */
> > +       0,
> > +};
>
> Yury, do we have any clever (clean) way to initialize a bitmap with
> particular bits so that it will be a constant from the beginning? If
> no, any suggestion what we can provide to such users?

The problem is, that even if we could initialize this without the
monstrosity in our last conversation a 64bit bitmap would still
produce worse code. As it is now it's simply a 32bit load and mask
with index and mask already calculated for the registers. In the
status callback the mask can even be folded into the register read
mask. With a 64bit bitmap you'd need to calculate new 64bit index and
masks, and then conditionally shift the bits into position.

If this reflection of the 32bit registers bothers you that much we
could alternatively do

static bool jh7100_reset_inverted(unsigned int idx)
{
  switch (idx) {
  case JH7100_RST_U74:
  case JH7100_RST_VP6_DRESET:
  ..
    return false;
  }
  return true;
}

It'd still produce worse code, but at least it would be readable.

/Emil

> ...
>
> > +       dev_dbg(rcdev->dev, "reset(%lu)\n", id);
>
> These debug messages are useless since one should use ftrace facility instead,
>
> --
> With Best Regards,
> Andy Shevchenko