Re: [PATCH v3 1/3] iio: dac: add support for ltc2688

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On Mon, 21 Feb 2022 19:04:38 +0200
Andy Shevchenko <andriy.shevchenko@xxxxxxxxx> wrote:

> On Mon, Feb 21, 2022 at 01:48:12PM +0100, Nuno Sá wrote:
> > On Sun, 2022-02-20 at 13:32 +0200, Andy Shevchenko wrote:  
> > > On Fri, Feb 18, 2022 at 02:51:28PM +0100, Nuno Sá wrote:  
> > > > On Mon, 2022-02-14 at 15:49 +0200, Andy Shevchenko wrote:  
> > > > > On Mon, Feb 07, 2022 at 09:19:46PM +0100, Nuno Sá wrote:  
> > > > > > On Mon, 2022-02-07 at 13:09 +0200, Andy Shevchenko wrote:  
> > > > > > > On Sun, Feb 06, 2022 at 01:19:59PM +0000, Sa, Nuno wrote:  
> > > > > > > > > From: Andy Shevchenko <andriy.shevchenko@xxxxxxxxx>
> > > > > > > > > Sent: Saturday, February 5, 2022 6:30 PM
> > > > > > > > > On Fri, Jan 21, 2022 at 03:24:59PM +0100, Nuno Sá wrote:  
> 
> ...
> 
> > > > > > > > > > +       ret = kstrtou16(buf, 10, &val);  
> > > > > > > > > 
> > > > > > > > > In other function you have long, here u16. I would expect that
> > > > > > > > > the types are of the same class, e.g. if here you have u16,
> > > > > > > > > then there something like s32 / s64.  Or here something like
> > > > > > > > > unsigned short.
> > > > > > > > > 
> > > > > > > > > A bit of elaboration why u16 is chosen here?  
> > > > > > > > 
> > > > > > > > Well, I never really saw any enforcement here to be honest
> > > > > > > > (rather than using stdint types...). So I pretty much just use
> > > > > > > > these in unsigned types because I'm lazy and u16 is faster to
> > > > > > > > type than unsigned short...  In this case, unless Jonathan really
> > > > > > > > asks for it, I prefer not to go all over the driver and change
> > > > > > > > this...  
> > > > > > > 
> > > > > > > This is about consistency. It may work as is, but it feels not good
> > > > > > > when for int (or unsigned int) one uses fixed-width types. Also
> > > > > > > it's non- written advice to use fixed-width variables when it's
> > > > > > > about programming registers or so, for the rest, use POD types.  
> > > > 
> > > > Ok, going a bit back in the discussion, you argued that in one place I
> > > > was using long while here u16. Well, in the place I'm using long, that
> > > > was on purpose because that value is to be compared against an array of
> > > > longs (which has to be long because it depends on CCF rates). I guess I
> > > > can als0 use s64, but there is also a reason why long was used.
> > > > 
> > > > In the u16 case, we really want to have 2 bytes because I'm going to use
> > > > that value to write the dac code which is 2 bytes.  
> > > 
> > > Okay, that's what I want to hear. If it's indeed goes to be a value to the
> > > register, then it's fine.
> > > 
> > > Perhaps a comment?  
> > 
> > I guess you mean to have a comment to state that here we have fixed
> > size type (as opposed to long, used in another place), because we
> > directly use the value on a register write?
> > 
> > Asking it because I'm not planning to add comments in all the places
> > where I have fixed size types for register read/writes...  
> 
> Thinking more about it and now I'm convinced that using the value that goes to
> the register in ABI is bad idea (means that user space must not care about the
> size or contents of the hardware register and should be abstract representation
> of the HW).
> 
> OTOH this seems to be "raw" value of something. So, I maybe missed the convention
> in IIO about this kind of values WRT the variable types used on ABI side.
> 
> That said, I leave it to Jonathan since I'm not convinced that u16 is a proper
> choice here.


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