linux-kernel - Re: Buiild error in i915/xe

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Message-ID: <20250119090935.7c690f85@pumpkin>
Date: Sun, 19 Jan 2025 09:09:35 +0000
From: David Laight <david.laight.linux@...il.com>
To: Guenter Roeck <linux@...ck-us.net>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>, David Laight
 <David.Laight@...lab.com>, Arnd Bergmann <arnd@...nel.org>,
 "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, Jens Axboe
 <axboe@...nel.dk>, Matthew Wilcox <willy@...radead.org>, Christoph Hellwig
 <hch@...radead.org>, Andrew Morton <akpm@...ux-foundation.org>, Andy
 Shevchenko <andriy.shevchenko@...ux.intel.com>, Dan Carpenter
 <dan.carpenter@...aro.org>, "Jason A . Donenfeld" <Jason@...c4.com>,
 "pedro.falcato@...il.com" <pedro.falcato@...il.com>, Mateusz Guzik
 <mjguzik@...il.com>, "linux-mm@...ck.org" <linux-mm@...ck.org>, Lorenzo
 Stoakes <lorenzo.stoakes@...cle.com>, intel-xe@...ts.freedesktop.org,
 intel-gfx@...ts.freedesktop.org, David Airlie <airlied@...il.com>, Simona
 Vetter <simona@...ll.ch>, Jani Nikula <jani.nikula@...ux.intel.com>,
 Rodrigo Vivi <rodrigo.vivi@...el.com>
Subject: Re: Buiild error in i915/xe

On Sat, 18 Jan 2025 14:58:48 -0800
Guenter Roeck <linux@...ck-us.net> wrote:

> On 1/18/25 14:11, David Laight wrote:
> > On Sat, 18 Jan 2025 13:21:39 -0800
> > Linus Torvalds <torvalds@...ux-foundation.org> wrote:
> >   
> >> On Sat, 18 Jan 2025 at 09:49, Guenter Roeck <linux@...ck-us.net> wrote:  
> >>>
> >>> No idea why the compiler would know that the values are invalid.  
> >>
> >> It's not that the compiler knows tat they are invalid, but I bet what
> >> happens is in scale() (and possibly other places that do similar
> >> checks), which does this:
> >>
> >>          WARN_ON(source_min > source_max);
> >>          ...
> >>          source_val = clamp(source_val, source_min, source_max);
> >>
> >> and the compiler notices that the ordering comparison in the first
> >> WARN_ON() is the same as the one in clamp(), so it basically converts
> >> the logic to
> >>
> >>          if (source_min > source_max) {
> >>                  WARN(..);
> >>                  /* Do the clamp() knowing that source_min > source_max */
> >>                  source_val = clamp(source_val, source_min, source_max);
> >>          } else {
> >>                  /* Do the clamp knowing that source_min <= source_max */
> >>                  source_val = clamp(source_val, source_min, source_max);
> >>          }
> >>
> >> (obviously I dropped the other WARN_ON in the conversion, it wasn't
> >> relevant for this case).
> >>
> >> And now that first clamp() case is done with source_min > source_max,
> >> and it triggers that build error because that's invalid.
> >>
> >> So the condition is not statically true in the *source* code, but in
> >> the "I have moved code around to combine tests" case it now *is*
> >> statically true as far as the compiler is concerned.  
> > 
> > Well spotted :-)
> > 
> > One option would be to move the WARN_ON() below the clamp() and
> > add an OPTIMISER_HIDE_VAR(source_max) between them.
> > 
> > Or do something more sensible than the WARN().
> > Perhaps return target_min on any such errors?
> >   
> 
> This helps:
> 
> -       WARN_ON(source_min > source_max);
> -       WARN_ON(target_min > target_max);
> -
>          /* defensive */
>          source_val = clamp(source_val, source_min, source_max);
> 
> +       WARN_ON(source_min > source_max);
> +       WARN_ON(target_min > target_max);

That is a 'quick fix' ...

Much better would be to replace the WARN() with (say):
	if (target_min >= target_max)
		return target_min;
	if (source_min >= source_max)
		return target_min + (target_max - target_min)/2;
So that the return values are actually in range (in as much as one is defined).
Note that the >= cpmparisons also remove a divide by zero.

	David

> 
> Guenter
>