linux-kernel - Re: [PATCH v2 2/2] gpio: Add Realtek Otto GPIO support

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Message-ID: <e89125c1ffb56f64c7c2037192e490d56ab12492.camel@svanheule.net>
Date:   Fri, 19 Mar 2021 22:48:00 +0100
From:   Sander Vanheule <sander@...nheule.net>
To:     Andy Shevchenko <andy.shevchenko@...il.com>
Cc:     "open list:GPIO SUBSYSTEM" <linux-gpio@...r.kernel.org>,
        devicetree <devicetree@...r.kernel.org>,
        Linus Walleij <linus.walleij@...aro.org>,
        Bartosz Golaszewski <bgolaszewski@...libre.com>,
        Rob Herring <robh+dt@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Marc Zyngier <maz@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Bert Vermeulen <bert@...t.com>
Subject: Re: [PATCH v2 2/2] gpio: Add Realtek Otto GPIO support

On Fri, 2021-03-19 at 23:24 +0200, Andy Shevchenko wrote:
> On Fri, Mar 19, 2021 at 11:20 PM Sander Vanheule <
> sander@...nheule.net> wrote:
> > On Fri, 2021-03-19 at 19:57 +0200, Andy Shevchenko wrote:
> > > On Fri, Mar 19, 2021 at 5:51 PM Sander Vanheule
> > > <sander@...nheule.net> wrote:
> > > > On Wed, 2021-03-17 at 15:08 +0200, Andy Shevchenko wrote:
> > > > > On Mon, Mar 15, 2021 at 11:11 PM Sander Vanheule <
> > > > > sander@...nheule.net> wrote:
> 
> ...
> 
> > > > > > +       return swab32(readl(ctrl->base +
> > > > > > REALTEK_GPIO_REG_ISR));
> > > > > 
> > > > > Why swab?! How is this supposed to work on BE CPUs?
> > > > > Ditto for all swabXX() usage.
> > > > 
> > > > My use of swab32/swahw32 has little to do with the CPU being BE
> > > > or
> > > > LE,
> > > > but more with the register packing in the GPIO peripheral.
> > > > 
> > > > The supported SoCs have port layout A-B-C-D in the registers,
> > > > where
> > > > firmware built with Realtek's SDK always denotes A0 as the first
> > > > GPIO
> > > > line. So bit 24 in a register has the value for A0 (with the
> > > > exception
> > > > of the IMR register).
> > > > 
> > > > I wrote these wrapper functions to be able to use the BIT() macro
> > > > with
> > > > the GPIO line number, similar to how gpio-mmio uses ioread32be()
> > > > when
> > > > the BGPIOF_BIG_ENDIAN_BYTE_ORDER flag is used.
> > > > 
> > > > For the IMR register, port A again comes first, but is now 16
> > > > bits
> > > > wide
> > > > instead of 8, with A0 at bits 16:17. That's why swahw32 is used
> > > > for
> > > > this register.
> > > > 
> > > > On the currently unsupported RTL9300-series, the port layout is
> > > > reversed: D-C-B-A. GPIO line A0 is then at bit 0, so the swapping
> > > > functions won't be required. When support for this alternate port
> > > > layout is added, some code will need to be added to differentiate
> > > > between the two cases.
> > > 
> > > Yes, you have different endianess on the hardware level, why not to
> > > use the proper accessors (with or without utilization of the above
> > > mentioned BGPIOF_BIG_ENDIAN_BYTE_ORDER)?
> > 
> > The point I was trying to make, is that it isn't an endianess issue.
> > I
> > shouldn't have used a register with single byte values to try to
> > illustrate that.
> > 
> > Consider instead the interrupt masking registers. To write the IMR
> > bits
> > for port A (GPIO 0-7), a 16-bit value must be written. This value
> > (e.g.
> > u16 port_a_imr) is always BE, independent of the packing order of the
> > ports in the registers:
> > 
> >    // On RTL8380: port A is in the upper word
> >    writew(port_a_imr, base + OFFSET_IMR_AB);
> > 
> >    // On RTL9300: port A is in the lower word
> >    writew(port_a_imr, base + OFFSET_IMR_AB + 2);
> > 
> > I want the low GPIO lines to be in the lower half-word, so I can
> > manipulate GPIO lines 0-15 with simple mask and shift operations.
> > 
> > It just so happens, that all registers needed by bgpio_init contain
> > single-byte values. With BGPIO_BIG_ENDIAN_BYTE_ORDER  the port order
> > is
> > reversed as required, but it's a bit of a misnomer here.
> 
> How many registers (per GPIO / port) do you have?
> Can you list them and show endianess of the data for each of them and
> for old and new hardware (something like a 3 column table)?

Each GPIO bank, with 32 GPIO lines, consists of four 8-line ports.
There are seven registers per port, but only five are used:

       |        | Data    | RTL8380    | RTL9300 
Reg    | Offset | type    | byte order | byte order
-------+--------+---------+------------+-----------
DIR    | 0x08   | 4 * u8  | A-B-C-D    | D-C-B-A
DATA   | 0x0C   | 4 * u8  | A-B-C-D    | D-C-B-A
ISR    | 0x10   | 4 * u8  | A-B-C-D    | D-C-B-A
IMR_AB | 0x14   | 2 * u16 | A-A-B-B    | B-B-A-A
IMR_CD | 0x18   | 2 * u16 | C-C-D-D    | D-D-C-C

The unused other registers are all 4*u8.

A-B-C-D means:  (A << 24) | (B << 16) | (C << 8) | D
A-A-B-B means:  (A << 16) | B

--
Best,
Sander