Re: [PATCH v1 2/6] gpio: realtek-otto: Support reversed port layouts

[Sander Vanheule <sander@svanheule.net>]

Hi Linus,

On Thu, 2022-04-21 at 01:01 +0200, Linus Walleij wrote:
> On Sat, Apr 9, 2022 at 9:56 PM Sander Vanheule <sander@svanheule.net> wrote:
> 
> > +       if (dev_flags & GPIO_PORTS_REVERSED) {
> > +               bgpio_flags = 0;
> > +               ctrl->port_offset_u8 = realtek_gpio_port_offset_u8_rev;
> > +               ctrl->port_offset_u16 = realtek_gpio_port_offset_u16_rev;
> > +       } else {
> > +               bgpio_flags = BGPIOF_BIG_ENDIAN_BYTE_ORDER;
> > +               ctrl->port_offset_u8 = realtek_gpio_port_offset_u8;
> > +               ctrl->port_offset_u16 = realtek_gpio_port_offset_u16;
> > +       }
> 
> Just checking: is this really a different silicon block, or is this
> GPIO_PORTS_REVERSED flag passed around just a way of saying:
> 
> if (!IS_ENABLED(CONFIG_CPU_BIG_ENDIAN)) ...?

The kernel for RTL930x SoC is built with CONFIG_CPU_BIG_ENDIAN=y, just like the
older SoCs that were previously supported. The SoC's IRQ controller is also the
same across RTL930x/RTL839x/RTL838x, even though 32-bit registers are used
there.

On RTL838x/RTL839x the GPIO IRQ control registers have byte layout:
	[H1] [L1] [H2] [L2]
	[H3] [L3] [H4] [L4]

On RTL930x, the GPIO IRQ control registers are:
	[H2] [L2] [H1] [L1]
	[H4] [L4] [H3] [L3]
which is the reverse of:
	[L1] [H1] [L2] [H2]
	[L3] [H3] [L4] [H4]


Same for the GPIO registers:
	On RTL83xx: [P1] [P2] [P3] [P4] (four 8b ports)
	On RTL930x: [P4] [P3] [P2] [P1] (one BE32 port)

It looks like the RTL930x could use a little-endian interpretation of the 32b
registers, followed by a little-endian interpretation of the contained port
values. That would mean two reorderings for every 16b read or write operation,
and manual manipulation of the register values. Although I have to say that the
current offset calculation is not too pretty either.

We also discussed this with Andy with the original submission of the driver:
https://lore.kernel.org/linux-gpio/CAHp75VdrqE0kBwzK9Jk7pZGjyfFnhatfa8UY0z-3T1w1PrbAbw@mail.gmail.com/

Best,
Sander