Re: [PATCH 2/4] pinmux: Add TB10x pinmux driver

[Christian Ruppert <christian.ruppert@abilis.com>]

On Wed, Jul 10, 2013 at 01:27:52PM -0600, Stephen Warren wrote:
> On 07/08/2013 07:02 AM, Christian Ruppert wrote:
> ...
> > OK, a small drawing of our hardware should make this clear, let's take
> > an imaginary example of one port with 10 pins, one i2c interface, one
> > spi interface and one GPIO bank:
> > 
> >               | mux N-1|
> >               +........+
> >               |        |  2
> >               |        +--/-- i2c
> >               |        |
> >            10 |        |  4
> >    Pins  --/--+ mux N  +--/-- spi
> >               |        |
> >               |        |  10
> >               |        +--/-- GPIO
> >               |        |
> >               +........+
> >               | mux N+1|
> >
> > This example shows the mux N inside the pin controller. It controls
> > all pins associated to port N through a single register value M. Let's
> > assume the pins are configured as follows in function of the register
> > value:
> > 
> >  pin      M=0       M=1     M=2      M=3
> >   0      GPIO0   SPI_MISO  GPIO0   SPI_MISO
> >   1      GPIO1   SPI_MOSI  GPIO1   SPI_MOSI
> >   2      GPIO2    SPI_CK   GPIO2    SPI_CK
> >   3      GPIO3    SPI_CS   GPIO3    SPI_CS
> >   4      GPIO4    GPIO4    GPIO4    GPIO4
> >   5      GPIO5    GPIO5    GPIO5    GPIO5
> >   6      GPIO6    GPIO6    GPIO6    GPIO6
> >   7      GPIO7    GPIO7    GPIO7    GPIO7
> >   8      GPIO8    GPIO8   I2C_SDA  I2C_SDA
> >   9      GPIO9    GPIO9   I2C_SCL  I2C_SCL
> 
> 
> In that scenario, in the language of Linux's pinctrl subsystem, what you
> have is:
> 
> 10 pins, named 0..9
> 1 pin group, named perhaps "mux N".
> 4 different functions; values M==0, 1, 2, 3.
> 
> > We now have three pin groups defined, corresponding to the chip-side
> > ports of the pin controller:
> > GPIO = {0, 1, 2, 3, 4, 5, 6, 7, 8, 9}
> > SPI  = {0, 1, 2, 3}
> > I2C  = {8, 9}
> 
> You would usually only define pin groups for the pin/ball/package side
> of the pinmux HW. IIRC, you're also wanting to define pin groups for the
> intra-chip side of the pinmux HW. However, you're not muxing functions
> onto those pingroups; they're just there to help with naming the
> GPIO<->pinmux mapping. You only mux functions onto the pin/ball/package
> side pins/pingroups.

Well, the GPIO<->pinmux mapping is not the only reason for defining
these groups wrt. the chip-side of the pin controller. The other reasons
are:
  - Make different interfaces on the same MUX orthogonal wrt. each
    other, i.e. make it possible to request one independently of the
    other. In the example above, SPI and I2C can be requested completely
    independently and the pin controller driver decides which mode to
    use.
  - Make pin allocation more fine-grained (in the example above, only
    pins 0-4 are "allocated" in case SPI is requested). This makes
    GPIO<->interface pin conflict management more natural.
    BTW: When managing conflicts between hardware interfaces and GPIOs,
    this scheme actually leads to _less_ pin groups than if we defined
    one group at the pin side and four different modes:
    chip-side groups: GPIO, SPI, I2C
                      => 3 groups.
    pin-side groups:  MUX_N, GPIOS_M0, GPIOS_M1, GPIOS_M2, GPIOS_M3
                      in our special case MUX_N and GPIOS_M0 are the
                      same.
                      => 4 groups (or 5 in the general case).
    It is true that GPIO_Mx don't need to be named but they need to be
    defined inside the driver nevertheless.

> > abilis,pingrp now specifies one of the three pin groups. Note that I2C
> >        and SPI can be requested independently in a completely orthogonal
> >        manner: The information if I2C is reqired or not is confined to
> >        the I2C request and does not leak into the SPI request as would
> >        be the case if we configured the entire port at the same time.
> 
> The pingrp should represent the pin/ball/package side pins/groups. In
> this case, it should specify "N".
> 
> > abilis,ioport specifies N.
> 
> That is replaced be pingrp.
> 
> > abilis,ioconf specifies M.
> 
> That'd be better named "function" or something like that, in order to
> indicate that it specifies which function is mux'd onto the specified
> pin(s)/pingroup(s).

-- 
  Christian Ruppert              ,          <christian.ruppert@abilis.com>
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