From: Geert Uytterhoeven <geert@linux-m68k.org> To: michalj@gmail.com, Rolf Eike Beer <eike-kernel@sf-tec.de>, Andrew Morton <akpm@linux-foundation.org> Cc: linux-kernel@vger.kernel.org, Linux Fbdev development list <linux-fbdev@vger.kernel.org> Subject: abs() vs. abs64() (was: Re: [PATCH] fbdev: fix nearest mode search) Date: Thu, 18 Nov 2010 07:40:19 +0100 [thread overview] Message-ID: <AANLkTimYDAJr9x00sjGGnDF2vOVXU12qmbnmfuzA5_0h@mail.gmail.com> (raw) [-- Warning: decoded text below may be mangled, UTF-8 assumed --] [-- Attachment #1: Type: text/plain; charset=UTF-8, Size: 6070 bytes --] [Don't use the obsolete linux-fbdev-devel address] On Thu, Nov 18, 2010 at 00:08, Michal Januszewski <michalj@gmail.com> wrote: > In the framebuffer subsystem the abs() macro is often used as a part of > the calculation of a Manhattan metric, which in turn is used as a measure > of similarity between video modes.  The arguments of abs() are sometimes > unsigned numbers.  This worked fine until commit a49c59c0, which changed > the definition of abs() to prevent truncation.  As a result of this > change, in the following piece of code: > >  u32 a = 0, b = 1; >  u32 c = abs(a - b); Indeed, the difference of 2 numbers is unsigned, as per C. > 'c' will end up with a value of 0xffffffff instead of the expected 0x1. This happens on 64-bit only, right? Anyway, I think commit a49c59c0 is wrong: abs() operates on signed (32-bit) numbers. For larger (64-bit signed) numbers, we have abs64(). > A problem caused by this change and visible by the end user is that > framebuffer drivers relying on functions from modedb.c will fail to > find high resolution video modes similar to that explicitly requested > by the user if an exact match cannot be found (see e.g. > https://bugs.gentoo.org/show_bug.cgi?id=296539). > > Fix this problem by casting all arguments of abs() to an int prior > to the macro evaluation in modedb.c and uvesafb.c. > > Signed-off-by: Michal Januszewski <michalj@gmail.com> > --- > diff --git a/drivers/video/modedb.c b/drivers/video/modedb.c > index 0a4dbdc..878bea1 100644 > --- a/drivers/video/modedb.c > +++ b/drivers/video/modedb.c > @@ -636,8 +636,10 @@ done: >             if (refresh_specified && db[i].refresh == refresh) { >                 return 1; >             } else { > -                if (abs(db[i].refresh - refresh) < diff) { > -                    diff = abs(db[i].refresh - refresh); > +                if (abs((int)(db[i].refresh - refresh)) < > +                  diff) { > +                    diff = abs((int)(db[i].refresh - > +                        refresh)); >                     best = i; >                 } >             } > @@ -654,8 +656,8 @@ done: >     for (i = 0; i < dbsize; i++) { >         DPRINTK("Trying %ix%i\n", db[i].xres, db[i].yres); >         if (!fb_try_mode(var, info, &db[i], bpp)) { > -            tdiff = abs(db[i].xres - xres) + > -                abs(db[i].yres - yres); > +            tdiff = abs((int)(db[i].xres - xres)) + > +                abs((int)(db[i].yres - yres)); > >             /* >             * Penalize modes with resolutions smaller > @@ -851,13 +853,13 @@ const struct fb_videomode *fb_find_nearest_mode(const struct fb_videomode *mode, >         modelist = list_entry(pos, struct fb_modelist, list); >         cmode = &modelist->mode; > > -        d = abs(cmode->xres - mode->xres) + > -            abs(cmode->yres - mode->yres); > +        d = abs((int)(cmode->xres - mode->xres)) + > +            abs((int)(cmode->yres - mode->yres)); >         if (diff > d) { >             diff = d; >             best = cmode; >         } else if (diff == d) { > -            d = abs(cmode->refresh - mode->refresh); > +            d = abs((int)(cmode->refresh - mode->refresh)); >             if (diff_refresh > d) { >                 diff_refresh = d; >                 best = cmode; > diff --git a/drivers/video/uvesafb.c b/drivers/video/uvesafb.c > index 7b8839e..6621427 100644 > --- a/drivers/video/uvesafb.c > +++ b/drivers/video/uvesafb.c > @@ -320,9 +320,9 @@ static int uvesafb_vbe_find_mode(struct uvesafb_par *par, >     int i, match = -1, h = 0, d = 0x7fffffff; > >     for (i = 0; i < par->vbe_modes_cnt; i++) { > -        h = abs(par->vbe_modes[i].x_res - xres) + > -          abs(par->vbe_modes[i].y_res - yres) + > -          abs(depth - par->vbe_modes[i].depth); > +        h = abs((int)(par->vbe_modes[i].x_res - xres)) + > +          abs((int)(par->vbe_modes[i].y_res - yres)) + > +          abs((int)(depth - par->vbe_modes[i].depth)); > >         /* >         * We have an exact match in terms of resolution > @@ -1375,7 +1375,7 @@ static int uvesafb_check_var(struct fb_var_screeninfo *var, >     * which is theoretically incorrect, but which we'll try to handle >     * here. >     */ > -    if (depth == 0 || abs(depth - var->bits_per_pixel) >= 8) > +    if (depth == 0 || abs((int)(depth - var->bits_per_pixel)) >= 8) >         depth = var->bits_per_pixel; > >     match = uvesafb_vbe_find_mode(par, var->xres, var->yres, depth, Gr{oetje,eeting}s,             Geert -- Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org In personal conversations with technical people, I call myself a hacker. But when I'm talking to journalists I just say "programmer" or something like that.                  -- Linus Torvalds ÿôèº{.nÇ+·®+%Ëÿ±éݶ\x17¥wÿº{.nÇ+·¥{±þG«éÿ{ayº\x1dÊÚë,j\a¢f£¢·hïêÿêçz_è®\x03(éÝ¢j"ú\x1a¶^[m§ÿÿ¾\a«þG«éÿ¢¸?¨èÚ&£ø§~á¶iOæ¬z·vØ^\x14\x04\x1a¶^[m§ÿÿÃ\fÿ¶ìÿ¢¸?I¥
WARNING: multiple messages have this Message-ID (diff)
From: Geert Uytterhoeven <geert@linux-m68k.org> To: michalj@gmail.com, Rolf Eike Beer <eike-kernel@sf-tec.de>, Andrew Morton <akpm@linux-foundation.org> Cc: linux-kernel@vger.kernel.org, Linux Fbdev development list <linux-fbdev@vger.kernel.org> Subject: abs() vs. abs64() (was: Re: [PATCH] fbdev: fix nearest mode search) Date: Thu, 18 Nov 2010 06:40:19 +0000 [thread overview] Message-ID: <AANLkTimYDAJr9x00sjGGnDF2vOVXU12qmbnmfuzA5_0h@mail.gmail.com> (raw) [-- Warning: decoded text below may be mangled, UTF-8 assumed --] [-- Attachment #1: Type: text/plain; charset="windows-1253", Size: 6031 bytes --] [Don't use the obsolete linux-fbdev-devel address] On Thu, Nov 18, 2010 at 00:08, Michal Januszewski <michalj@gmail.com> wrote: > In the framebuffer subsystem the abs() macro is often used as a part of > the calculation of a Manhattan metric, which in turn is used as a measure > of similarity between video modes.  The arguments of abs() are sometimes > unsigned numbers.  This worked fine until commit a49c59c0, which changed > the definition of abs() to prevent truncation.  As a result of this > change, in the following piece of code: > >  u32 a = 0, b = 1; >  u32 c = abs(a - b); Indeed, the difference of 2 numbers is unsigned, as per C. > 'c' will end up with a value of 0xffffffff instead of the expected 0x1. This happens on 64-bit only, right? Anyway, I think commit a49c59c0 is wrong: abs() operates on signed (32-bit) numbers. For larger (64-bit signed) numbers, we have abs64(). > A problem caused by this change and visible by the end user is that > framebuffer drivers relying on functions from modedb.c will fail to > find high resolution video modes similar to that explicitly requested > by the user if an exact match cannot be found (see e.g. > https://bugs.gentoo.org/show_bug.cgi?id)6539). > > Fix this problem by casting all arguments of abs() to an int prior > to the macro evaluation in modedb.c and uvesafb.c. > > Signed-off-by: Michal Januszewski <michalj@gmail.com> > --- > diff --git a/drivers/video/modedb.c b/drivers/video/modedb.c > index 0a4dbdc..878bea1 100644 > --- a/drivers/video/modedb.c > +++ b/drivers/video/modedb.c > @@ -636,8 +636,10 @@ done: >             if (refresh_specified && db[i].refresh = refresh) { >                 return 1; >             } else { > -                if (abs(db[i].refresh - refresh) < diff) { > -                    diff = abs(db[i].refresh - refresh); > +                if (abs((int)(db[i].refresh - refresh)) < > +                  diff) { > +                    diff = abs((int)(db[i].refresh - > +                        refresh)); >                     best = i; >                 } >             } > @@ -654,8 +656,8 @@ done: >     for (i = 0; i < dbsize; i++) { >         DPRINTK("Trying %ix%i\n", db[i].xres, db[i].yres); >         if (!fb_try_mode(var, info, &db[i], bpp)) { > -            tdiff = abs(db[i].xres - xres) + > -                abs(db[i].yres - yres); > +            tdiff = abs((int)(db[i].xres - xres)) + > +                abs((int)(db[i].yres - yres)); > >             /* >             * Penalize modes with resolutions smaller > @@ -851,13 +853,13 @@ const struct fb_videomode *fb_find_nearest_mode(const struct fb_videomode *mode, >         modelist = list_entry(pos, struct fb_modelist, list); >         cmode = &modelist->mode; > > -        d = abs(cmode->xres - mode->xres) + > -            abs(cmode->yres - mode->yres); > +        d = abs((int)(cmode->xres - mode->xres)) + > +            abs((int)(cmode->yres - mode->yres)); >         if (diff > d) { >             diff = d; >             best = cmode; >         } else if (diff = d) { > -            d = abs(cmode->refresh - mode->refresh); > +            d = abs((int)(cmode->refresh - mode->refresh)); >             if (diff_refresh > d) { >                 diff_refresh = d; >                 best = cmode; > diff --git a/drivers/video/uvesafb.c b/drivers/video/uvesafb.c > index 7b8839e..6621427 100644 > --- a/drivers/video/uvesafb.c > +++ b/drivers/video/uvesafb.c > @@ -320,9 +320,9 @@ static int uvesafb_vbe_find_mode(struct uvesafb_par *par, >     int i, match = -1, h = 0, d = 0x7fffffff; > >     for (i = 0; i < par->vbe_modes_cnt; i++) { > -        h = abs(par->vbe_modes[i].x_res - xres) + > -          abs(par->vbe_modes[i].y_res - yres) + > -          abs(depth - par->vbe_modes[i].depth); > +        h = abs((int)(par->vbe_modes[i].x_res - xres)) + > +          abs((int)(par->vbe_modes[i].y_res - yres)) + > +          abs((int)(depth - par->vbe_modes[i].depth)); > >         /* >         * We have an exact match in terms of resolution > @@ -1375,7 +1375,7 @@ static int uvesafb_check_var(struct fb_var_screeninfo *var, >     * which is theoretically incorrect, but which we'll try to handle >     * here. >     */ > -    if (depth = 0 || abs(depth - var->bits_per_pixel) >= 8) > +    if (depth = 0 || abs((int)(depth - var->bits_per_pixel)) >= 8) >         depth = var->bits_per_pixel; > >     match = uvesafb_vbe_find_mode(par, var->xres, var->yres, depth, Gr{oetje,eeting}s,             Geert -- Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org In personal conversations with technical people, I call myself a hacker. But when I'm talking to journalists I just say "programmer" or something like that.                  -- Linus Torvalds ÿôèº{.nÇ+·®+%Ëÿ±éݶ\x17¥wÿº{.nÇ+·¥{±ýöÝzÿâØ^nr¡ö¦zË\x1aëh¨èÚ&£ûàz¿äz¹Þú+Ê+zf£¢·h§~Ûiÿÿïêÿêçz_è®\x0fæj:+v¨þ)ߣøm
next reply other threads:[~2010-11-18 6:40 UTC|newest] Thread overview: 12+ messages / expand[flat|nested] mbox.gz Atom feed top 2010-11-18 6:40 Geert Uytterhoeven [this message] 2010-11-18 6:40 ` abs() vs. abs64() (was: Re: [PATCH] fbdev: fix nearest mode search) Geert Uytterhoeven 2010-11-19 22:07 ` Andrew Morton 2010-11-19 22:07 ` abs() vs. abs64() (was: Re: [PATCH] fbdev: fix nearest mode Andrew Morton 2010-11-19 22:28 ` abs() vs. abs64() (was: Re: [PATCH] fbdev: fix nearest mode search) Michal Januszewski 2010-11-19 22:28 ` abs() vs. abs64() (was: Re: [PATCH] fbdev: fix nearest mode Michal Januszewski 2010-11-19 23:04 ` abs() vs. abs64() (was: Re: [PATCH] fbdev: fix nearest mode search) Andrew Morton 2010-11-19 23:04 ` abs() vs. abs64() (was: Re: [PATCH] fbdev: fix nearest mode Andrew Morton 2010-11-19 23:19 ` abs() vs. abs64() (was: Re: [PATCH] fbdev: fix nearest mode search) Andrew Morton 2010-11-19 23:19 ` abs() vs. abs64() (was: Re: [PATCH] fbdev: fix nearest mode Andrew Morton 2010-11-20 8:56 ` abs() vs. abs64() (was: Re: [PATCH] fbdev: fix nearest mode search) Geert Uytterhoeven 2010-11-20 8:56 ` Geert Uytterhoeven
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