On Wed, May 23, 2018 at 5:06 PM, Pavel Machek [off-list ref] wrote:

On Thu 2018-05-17 06:59:49, H. Nikolaus Schaller wrote:

quoted

The register constants are so far defined in a way that they fit
for the pcal9555a when shifted by the number of banks, i.e. are
multiplied by 2 in the accessor function.

Now, the pcal6524 has 3 banks which means the relative offset
is multiplied by 4 for the standard registers.

Simply applying the bit shift to the extended registers gives
a wrong result, since the base offset is already included in
the offset.

Therefore, we have to add code to the 24 bit accessor functions
that adjusts the register number for these exended registers.

The formula finally used was developed and proposed by
Andy Shevchenko [off-list ref].

quoted

      int bank_shift = fls((chip->gpio_chip.ngpio - 1) / BANK_SZ);
+     int addr = (reg & PCAL_GPIO_MASK) << bank_shift;
+     int pinctrl = (reg & PCAL_PINCTRL_MASK) << 1;

Is this reasonable to do on each register access? Compiler will not be
able to optimize out fls and shifts, right?

On modern CPUs fls() is one assembly command. OTOH, any proposal to do
this better?

What I can see is that bank_shift is invariant to the function, and
maybe cached.

-- 
With Best Regards,
Andy Shevchenko