Re: [PATCH v11 3/5] drivers/soc/litex: add LiteX SoC Controller driver

[Geert Uytterhoeven <geert@linux-m68k.org>]

Hi Gabriel,

On Fri, Sep 25, 2020 at 5:06 PM Gabriel L. Somlo <gsomlo@gmail.com> wrote:
> On Fri, Sep 25, 2020 at 03:16:02PM +0200, Geert Uytterhoeven wrote:
> > On Wed, Sep 23, 2020 at 12:10 PM Mateusz Holenko <mholenko@antmicro.com> wrote:
> > > + */
> > > +#define LITEX_REG_SIZE             0x4
> > > +#define LITEX_SUBREG_SIZE          0x1
> > > +#define LITEX_SUBREG_SIZE_BIT      (LITEX_SUBREG_SIZE * 8)
> > > +
> > > +static DEFINE_SPINLOCK(csr_lock);
> > > +
> > > +/*
> > > + * LiteX SoC Generator, depending on the configuration,
> > > + * can split a single logical CSR (Control & Status Register)
> > > + * into a series of consecutive physical registers.
> > > + *
> > > + * For example, in the configuration with 8-bit CSR Bus,
> > > + * 32-bit aligned (the default one for 32-bit CPUs) a 32-bit
> > > + * logical CSR will be generated as four 32-bit physical registers,
> > > + * each one containing one byte of meaningful data.
> > > + *
> > > + * For details see: https://github.com/enjoy-digital/litex/wiki/CSR-Bus
> > > + *
> > > + * The purpose of `litex_set_reg`/`litex_get_reg` is to implement
> > > + * the logic of writing to/reading from the LiteX CSR in a single
> > > + * place that can be then reused by all LiteX drivers.
> > > + */
> > > +void litex_set_reg(void __iomem *reg, unsigned long reg_size,
> > > +                   unsigned long val)
> > > +{
> > > +       unsigned long shifted_data, shift, i;
> > > +       unsigned long flags;
> > > +
> > > +       spin_lock_irqsave(&csr_lock, flags);
> > > +
> > > +       for (i = 0; i < reg_size; ++i) {
> > > +               shift = ((reg_size - i - 1) * LITEX_SUBREG_SIZE_BIT);
> > > +               shifted_data = val >> shift;
> > > +
> > > +               writel((u32 __force)cpu_to_le32(shifted_data), reg + (LITEX_REG_SIZE * i));
> > > +       }
> > > +
> > > +       spin_unlock_irqrestore(&csr_lock, flags);
> > > +}
> > > +EXPORT_SYMBOL_GPL(litex_set_reg);
> >
> > I'm still wondering about the overhead of loops and multiple accesses,
> > and the need for them (see also BenH's earlier comment).
> > If e.g. the register widths change for LiteUART (currently they're
> > hardcoded to one), would you still consider it using the same
> > programming interface, and thus compatible with "litex,liteuart"?
>
> There's been talk within the LiteX dev community to standardize on a
> LITEX_SUBREG_SIZE of 0x4 (i.e., using all 32 bits of a 32-bit
> (LITEX_REG_SIZE) aligned MMIO location). Early 32-bit (vexriscv based)
> Linux capable LiteX designs started out with only the 8 LSBits used
> within a 32-bit MMIO location, but 64-bit (Rocket chip) based LiteX SoCs
> use 4-byte aligned, fully populated MMIO registers (i.e., both
> LITEX_SUBREG_SIZE *and* LITEX_REG_SIZE are 4). There's also been talk of
> deprecating LITEX_SUBREG_SIZE == 0x1 for "linux-capable LiteX builds",
> but nothing definitive yet AFAIK.

That sounds like a good idea to me.
Having 8-bit accesses may be worthwhile on a small microcontroller, but a
full-fledge Linux system can use more and wider MMIO.

> Geert: note that LiteX has wider-than-32-bit registers spread across
> multiple 32-bit aligned, 8- or 32-bit wide "subregisters", so looping
> and shifting will still be necessary, even with LITEX_SUBREG_SIZE 0x4.

Can these be different than 64-bit (and 128-bit)?
That's not unlike accessors on other 32-bit platforms.
Still, no loop needed, just doing two (or four) 32-bit accesses in a row
is fine (but requires using inlines instead of your current single
out-of-line function).

> > > --- /dev/null
> > > +++ b/include/linux/litex.h

> > > +void litex_set_reg(void __iomem *reg, unsigned long reg_sz, unsigned long val);
> > > +
> > > +unsigned long litex_get_reg(void __iomem *reg, unsigned long reg_sz);
> >
> > Perhaps you can add static inline litex_{read,write}{8,16,32}() wrappers,
> > so drivers don't have to pass the reg_sz parameter explicitly,
> > and to make it look more like accessors of other bus types?
>
> Seconded -- perhaps simply cut'n'paste and/or adapt from
> https://github.com/litex-hub/linux/blob/litex-rocket-rebase/include/linux/litex.h#L78
> (from the 64-bit port of the LiteX linux patch set)

Yes, you definitely want the 32-bit and 64-bit ports to agree ;-)
Note that these are using the "old" "bwlq" convention (with "l"
predating 64-bit long on 64-bit platforms) instead of the more modern
explicit {8,16,32,64}, but that's a minor detail.

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds