Re: [PATCH v3 1/3] gpio: pca953x: Rewrite ->get_multiple() function

[Uwe Kleine-König <u.kleine-koenig@xxxxxxxxxxxxxx>]

Hello Andy,

first of all thanks for picking up my patches, very appreciated.

On Tue, Apr 21, 2020 at 03:55:53PM +0300, Andy Shevchenko wrote:
> On Mon, Apr 20, 2020 at 11:23:57PM -0400, Paul Thomas wrote:
> > On Mon, Apr 20, 2020 at 1:27 PM Andy Shevchenko
> > <andriy.shevchenko@xxxxxxxxxxxxxxx> wrote:
> > >
> > > The commit 96d7c7b3e654 ("gpio: gpio-pca953x, Add get_multiple function")
> > > basically did everything wrong from style and code reuse perspective, i.e.
> > Hi Andy,
> > 
> > Well your version is certainly elegant and simple, and does better
> > with code reuse. However there are a couple of other goals I had in
> > mind.
> > First, the "lazy" approach of 96d7c7b3e654 is actually faster when
> > user space sets up a 8-bit linehandle[1]146us (single regmap_read())
> > vs 172us (pca953x_read_regs()) which incidentally is what we do in our
> > application. In lazily reading 1 byte at a time it is the fastest
> > access for that, if user space is always setting up the linehandle for
> > the whole chip pca953x_read_regs() would be faster. Seeing as
> > get_multiple has been unimplemented for this chip until now perhaps
> > our use case deserves some consideration?
> 
> I understand completely your goal, but
> - for I²C expanders timings is the last thing to look for (they are quite slow
>   by nature), so, I really don't care about 16% speed up for one call; don't
>   forget that we are in multi-task OS, where this can be easily interrupted and
>   user will see the result quite after expected quick result

I didn't do any timing analysis and while I understand your
argumentation, I'm not sure to agree. I noticed while debugging the
problem that then resulted in my fix that gpioctl uses the .set_multiple
callback. I told my customer to use gpioctl instead of /sys/class/gpio
because it performs better just to notice that when using gpioctl to set
a single GPIO this transfers five bytes instead of only two.

Having said that I think the sane approach is to optimize
.{g,s}et_multiple to reduce the read/write size to the smallest bulk
size possible that covers all bits that have their corresponding bit set
in mask.

> - the code maintenance has a priority over micro-optimization (this driver
>   suffered many times of breakages because of some optimizations done)

ack here. Some parts of the driver were harder to grasp than necessary.

> - it breaks Uwe's approach to fix AI chips, after my patch Uwe's ones are
>   applied cleanly

I didn't check, is 96d7c7b3e654 broken for some chips?

I will add my suggested optimisation to my todo list for evaluation. If
I think it is still nice and maintainable I'll send a patch. Until I
have looked into this (or someone else did) I'm in favour of applying
Andy's patch.

Best regards
Uwe

-- 
Pengutronix e.K.                           | Uwe Kleine-König            |
Industrial Linux Solutions                 | https://www.pengutronix.de/ |