Re: [PATCH v1 5/5] gpiolib: cdev: Utilize more bitmap APIs

[Kent Gibson <warthog618@xxxxxxxxx>]

On Wed, Sep 27, 2023 at 04:59:34PM +0300, Andy Shevchenko wrote:
> On Wed, Sep 27, 2023 at 09:49:35PM +0800, Kent Gibson wrote:
> > On Wed, Sep 27, 2023 at 03:17:06PM +0300, Andy Shevchenko wrote:
> > > On Wed, Sep 27, 2023 at 09:32:11AM +0800, Kent Gibson wrote:
> > > > On Tue, Sep 26, 2023 at 08:20:07AM +0300, Andy Shevchenko wrote:
> > > > > Currently we have a few bitmap calls that are open coded in the library
> > > > > module. Let's convert them to use generic bitmap APIs instead.
> > > > 
> > > > Firstly, I didn't consider using the bitmap module here as, in my mind at
> > > > least, that is intended for bitmaps wider than 64 bits, or with variable
> > > > width. In this case the bitmap is fixed at 64 bits, so bitops seemed more
> > > > appropriate.
> > > > 
> > > > And I would argue that they aren't "open coded" - they are parallelized
> > > > to reduce the number of passes over the bitmap.
> > > > This change serialises them, e.g. the get used to require 2 passes over
> > > > the bitmap, it now requires 3 or 4.  The set used to require 1 and now
> > > > requires 2.
> > > > And there are additional copies that the original doesn't require.
> > > > So your change looks less efficient to me - unless there is direct
> > > > hardware support for bitmap ops??
> > > > 
> > > > Wrt the argument that the serialized form is clearer and more
> > > > maintainable, optimised code is frequently more cryptic - as noted in
> > > > bitmap.c itself, and this code has remained unchanged since it was merged
> > > > 3 years ago, so the only maintenance it has required is to be more
> > > > maintainable??  Ok then.
> > > > 
> > > > Your patch is functionally equivalent and pass my uAPI tests, so 
> > > > 
> > > > Tested-by: Kent Gibson <warthog618@xxxxxxxxx>
> > > 
> > > Thanks for testing!
> > 
> > Not a problem - that is what test suites are for.
> > 
> > > > but my preference is to leave it as is.
> > > 
> > > As Yury mentioned we need to look at bitmap APIs and make them possible to have
> > > a compile-time optimizations. With that in mind, I would prefer bitmap APIs
> > > over open-coded stuff which is hardly to be understood (yes, I still point
> > > out that it takes a few hours to me, maybe because I'm stupid enough, to
> > > get what's the heck is going one there, esp. for the == 1 case).
> > 
> > Really?  With all the bits out in the open it seems pretty clear to me.
> > Clearer than scatter/gather in fact.
> 
> Yes, you are biased. :-) Ask some stranger about this code and I am pretty sure
> there will be double-figures percentage of people who can tell that the current
> code is a bit voodoo.
> 

It is the same as yours - just inside out.  i.e. it performs the ops per
selected line, not each op on the whole bitmap of lines.

> > Sure, if there is suitable hardware support then bitmaps COULD be faster
> > than bitops.  But without that, and that is the general case, it will be
> > slower.  Do you have ANY cases where your implementation is currently
> > faster?  Then you would have a stronger case.
> 
> Why do we care here about performance? But if we do, I would check this on
> the 32-bit platform where 64-bit operations somewhat problematic / slow.
> 

Yet you argue that bitmaps could be more performant??  Pick a side!

> If Yury gives an idea about performance tests I can consider to add this
> piece to compare with and we might see the difference.
> 
> > And if you find the existing implementation unclear then the appropriate
> > solution is to better document it, as bitmaps itself does, not replace it
> > with something simpler and slower.
> 
> Documentation will be needed either way. In general statistics it will be 50/50
> who (mis)understands this or new code. Pity that the original author of the code
> hadn't though about documenting this...
> 

And who was the original author?  I forget.

What you mean to say is it is a pity the reviewers at the time were
satisfied with the code as it stands, right?
Cos there is a process here.
As I recall reviewers were more often than not complaining about
pointless comments, not the lack of comments, so the natural bias as the
author is towards under-documenting...

> > > Yet, it opens a way to scale this in case we might have v3 ABI that let's say
> > > allows to work with 512 GPIOs at a time. With your code it will be much harder
> > > to achieve and see what you wrote about maintenance (in that case).
> > 
> > v3 ABI?? libgpiod v2 is barely out the door!
> > Do you have any cases where 64 lines per request is limiting?
> 
> IIRC it was SO question where the OP asks exactly about breaking the 64 lines
> limitation in the current ABI.
> 
> > If that sort of speculation isn't premature optimisation then I don't know
> > what is.
> 
> No, based on the real question / discussion, just have no link at hand.
> But it's quite a niche, I can agree.
> 

Let me know if you find a ref to that discussion - I'm curious.

Cheers,
Kent.