linux-kernel - Re: [Outreachy kernel] [PATCH 1/2] staging: rtl8723bs: hal: Remove set but unused variables

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Message-ID: <20210429120729.GA1393@agape.jhs>
Date:   Thu, 29 Apr 2021 14:07:30 +0200
From:   Fabio Aiuto <fabioaiuto83@...il.com>
To:     "Fabio M. De Francesco" <fmdefrancesco@...il.com>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        outreachy-kernel@...glegroups.com, linux-staging@...ts.linux.dev,
        linux-kernel@...r.kernel.org
Subject: Re: [Outreachy kernel] [PATCH 1/2] staging: rtl8723bs: hal: Remove
 set but unused variables

> > > Yes, but many types of hardware _REQUIRE_ reads to do something.  So
> > > "read that does nothing" is a requirement for some operations.
> > > 
> > > As an example, a write is only guaranteed to have been finished if you
> > > do a read of the same location back from it on some hardware busses.
> > > The bus can reorder things, but a write/read of the same location can
> > > not be reordered.
> > > 
> > > Sometimes you have to do reads multiple times to get things to "stick".
> > > 
> > > Other times reading from a location changes a state in the hardware
> > > (horrid but HW designers aren't the brightest at times...)
> > > 
> > > So you can NOT just remove reads without knowing that the hardware does
> > > not require this.  This is an issue where GCC "warnings" mean nothing as
> > > gcc does not actually know what hardware does, or does not, do for many
> > > things.
> > > 
> > > thanks,
> > > 
> > > greg k-h
> > 
> > thank you for explanation, my hardware knowledge is poor:(
> > Sorry for noise.
> > 
> > fabio
> >
> I suspected that removing those variables could have been a source of troubles 
> (but I was thinking of possible side effects on internal kernel's data, not of 
> hardware related idiosyncrasies), however I think that you did well to point 
> it out because:
> 
> 1) We learned something new from Greg;

yes that's been very good for me

> 2) I learned that, for the purpose of finding definitions, vim's ctrl-] is not 
> the right way to work out the problem.

3) I learned that with ctrl-] in vim one could (in some misterious conditions)
   see a function definition :-D

It seems that you know more than me about vim, I make intensive use of grep
for finding function defs and usages in code.

> 
> If you have time, I'd appreciate some comments on the topic of line (2).
> 
> Thanks,
> 
> Fabio
> 
> 
> 

thank you,

fabio