On Wed 2018-06-06 07:33:32, H. Nikolaus Schaller wrote: > Hi, > > > Am 05.06.2018 um 22:39 schrieb Pavel Machek <pavel@xxxxxx>: > > > > On Tue 2018-06-05 18:37:21, Andy Shevchenko wrote: > >> On Wed, May 23, 2018 at 5:06 PM, Pavel Machek <pavel@xxxxxx> 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@xxxxxxxxx>. > >> > >>>> 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. > > > > Yes, I thought that caching bank_shift might be good idea. I thought > > it was constant for given chip... > > Yes, it is an f(chip), but the question that comes to my mind is if > optimization is worth any effort. This is an accessor method over It will also be less ugly. Copy&pasted complex exprepsion all over the driver is not nice. Pavel -- (english) http://www.livejournal.com/~pavelmachek (cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
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