Use of floating point in the kernel

Hi

There are a few instances of use of floating point in 2.6,

@@ -2673,9 +2673,9 @@
 	buf[1] = div & 0xff;
 	buf[2] = 0x8e;
 
-	if (freq < (u32) 16*168.25 )
+	if (freq < 2692 ) /* 16*168.25 */
 		config = 0xa0;
-	else if (freq < (u32) 16*447.25)
+	else if (freq < 7156) /* 16*447.25 */
 		config = 0x90;
 	else
 		config = 0x30;

(If I'm not mistaken a similar patch for this has already been sent to Linus, just making sure the DVB people get this in their trees as well) (u32) (16*168.25) would work too, I suppose.

The other case is a bit more widespread, patch below is mostly rhetoric :-)
http://www.winischhofer.net/linuxsisvga.shtml#download contains the latest version of the sisfb that doesn't use floating point at all, so that is certainly the better option.

@@ -616,6 +616,7 @@
 		var->left_margin + var->xres + var->right_margin +
 		var->hsync_len;
 	unsigned int vtotal = 0; 
+#error Use of floating point in the kernel is NOT safe!
 	double drate = 0, hrate = 0;
 	int found_mode = 0;
 	int old_mode;

@@ -704,7 +704,7 @@
 
 config FB_SIS
 	tristate "SIS acceleration"
-	depends on FB && PCI
+	depends on FB && PCI && BROKEN
 	help
 	  This is the frame buffer device driver for the SiS 630 and 640 Super
 	  Socket 7 UMA cards.  Specs available at <http://www.sis.com.tw/>.

Pekka Pietikainen

Has anyone considered asking the gcc people to add an -fno-fpu (or -mno-fpu) option, throwing an error if any FP instructions are used?

-hpa

building with -msoft-float gets you this.

Dave

We really should, but there really are some rare cases where it is actually ok.

In particular, you _can_ do math, if you just do the proper "kernel_fpu_begin()"/"kernel_fpu_end()" around it, and you have reason to believe that you can assume a math processor exists.

Is it needed? I dunno. I'd frown on it in general, but I don't see it being fundamentally wrong under the rigth circumstances.

Linus

Sure; however, perhaps those can be marked separately in the Makefile.

-hpa