Re: [PATCH 1/8] pinctrl: aspeed: "Not enabled" is a significant mux state

[Linus Walleij]

On Tue, Sep 27, 2016 at 4:50 PM, Andrew Jeffery <andrew@xxxxxxxx> wrote:

> Consider a scenario with one pin P that has two signals A and B, where A
> is defined to be higher priority than B: That is, if the mux IP is in a
> state that would consider both A and B to be active on P, then A will be
> the active signal.
>
> To instead configure B as the active signal we must configure the mux so
> that A is inactive. The mux state for signals can be described by
> logical operations on one or more bits from one or more registers (a
> "signal expression"), which in some cases leads to aliased mux states for
> a particular signal. Further, signals described by multi-bit bitfields
> often do not only need to record the states that would make them active
> (the "enable" expressions), but also the states that makes them inactive
> (the "disable" expressions). All of this combined leads to four possible
> states for a signal:
>
>          1. A signal is active with respect to an "enable" expression
>          2. A signal is not active with respect to an "enable" expression
>          3. A signal is inactive with respect to a "disable" expression
>          4. A signal is not inactive with respect to a "disable" expression
>
> In the case of P, if we are looking to activate B without explicitly
> having configured A it's enough to consider A inactive if all of A's
> "enable" signal expressions evaluate to "not active". If any evaluate to
> "active" then the corresponding "disable" states must be applied so it
> becomes inactive.
>
> For example, on the AST2400 the pins composing GPIO bank H provide
> signals ROMD8 through ROMD15 (high priority) and those for UART6 (low
> priority). The mux states for ROMD8 through ROMD15 are aliased, i.e.
> there are two mux states that result in the respective signals being
> configured:
>
>          A. SCU90[6]=1
>          B. Strap[4,1:0]=100
>
> Further, the second mux state is a 3-bit bitfield that explicitly
> defines the enabled state but the disabled state is implicit, i.e. if
> Strap[4,1:0] is not exactly "100" then ROMD8 through ROMD15 are not
> considered active. This requires the mux function evaluation logic to
> use approach 2. above, however the existing code was using approach 3.
> The problem was brought to light on the Palmetto machines where the
> strap register value is 0x120ce416, and prevented GPIO requests in bank
> H from succeeding despite the hardware being in a position to allow
> them.
>
> Fixes: 318398c09a8d ("pinctrl: Add core pinctrl support for Aspeed SoCs")
> Signed-off-by: Andrew Jeffery <andrew@xxxxxxxx>

Patch applied for fixes.

Yours,
Linus Walleij