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SubjectRe: [PATCH v7 3/3] gpio: pca953x: fix address calculation for pcal6524
On Wed, May 23, 2018 at 5:06 PM, Pavel Machek <pavel@ucw.cz> wrote:
> On Thu 2018-05-17 06:59:49, H. Nikolaus Schaller wrote:
>> 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 <andy.shevchenko@gmail.com>.

>> 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

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