[PATCH] lib/fonts: Avoid unncessary 64-bit math in font code

[PATCH] lib/fonts: Avoid unncessary 64-bit math in font code

[Helge Deller]

The text display code used in the Risc PC kernel image decompression
code uses arch/arm/boot/compressed/font.c, which includes
lib/fonts/font_acorn_8x8.c, which further includes <linux/font.h>.

Since commit 97df8960240a ("lib/fonts: Provide helpers for calculating
glyph pitch and size") <linux/font.h> contains inline functions that
require __do_div64, which is not linked into the ARM kernel
decompressor. This makes Risc PC zImages fail to build.

There is no need to use 64-bit division code here, so resolve this issue
by using plain standard addition and shift maths.

Fixes: 97df8960240a ("lib/fonts: Provide helpers for calculating glyph pitch and size")
Reported-by: Ethan Nelson-Moore <enelsonmoore@gmail.com>
Signed-off-by: Helge Deller <deller@gmx.de>
---
 include/linux/font.h | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/include/linux/font.h b/include/linux/font.h
index 6845f02d739a..67d32268989d 100644
--- a/include/linux/font.h
+++ b/include/linux/font.h
@@ -11,7 +11,6 @@
 #ifndef _VIDEO_FONT_H
 #define _VIDEO_FONT_H
 
-#include <linux/math.h>
 #include <linux/types.h>
 
 struct console_font;
@@ -35,7 +34,7 @@ struct console_font;
  */
 static inline unsigned int font_glyph_pitch(unsigned int width)
 {
-	return DIV_ROUND_UP(width, 8);
+	return (width + 7) >> 3;
 }
 
 /**
-- 
2.54.0

Re: [PATCH] lib/fonts: Avoid unncessary 64-bit math in font code

[Ethan Nelson-Moore]

On Sun, Jun 7, 2026 at 2:02 PM Helge Deller <deller@gmx.de> wrote:
> The text display code used in the Risc PC kernel image decompression
> code uses arch/arm/boot/compressed/font.c, which includes
> lib/fonts/font_acorn_8x8.c, which further includes <linux/font.h>.
>
> Since commit 97df8960240a ("lib/fonts: Provide helpers for calculating
> glyph pitch and size") <linux/font.h> contains inline functions that
> require __do_div64, which is not linked into the ARM kernel
> decompressor. This makes Risc PC zImages fail to build.
>
> There is no need to use 64-bit division code here, so resolve this issue
> by using plain standard addition and shift maths.

Hi, Helge,

I can confirm that this patch fixes the issue. Thank you for your
thorough investigation.

Ethan

Re: [PATCH] lib/fonts: Avoid unncessary 64-bit math in font code

[Thomas Zimmermann]

Hi

Am 07.06.26 um 23:02 schrieb Helge Deller:
> The text display code used in the Risc PC kernel image decompression
> code uses arch/arm/boot/compressed/font.c, which includes
> lib/fonts/font_acorn_8x8.c, which further includes <linux/font.h>.
>
> Since commit 97df8960240a ("lib/fonts: Provide helpers for calculating
> glyph pitch and size") <linux/font.h> contains inline functions that
> require __do_div64, which is not linked into the ARM kernel
> decompressor. This makes Risc PC zImages fail to build.
>
> There is no need to use 64-bit division code here, so resolve this issue
> by using plain standard addition and shift maths.

Why is there a 64-bit division at all?

>
> Fixes: 97df8960240a ("lib/fonts: Provide helpers for calculating glyph pitch and size")
> Reported-by: Ethan Nelson-Moore <enelsonmoore@gmail.com>
> Signed-off-by: Helge Deller <deller@gmx.de>
> ---
>   include/linux/font.h | 3 +--
>   1 file changed, 1 insertion(+), 2 deletions(-)
>
> diff --git a/include/linux/font.h b/include/linux/font.h
> index 6845f02d739a..67d32268989d 100644
> --- a/include/linux/font.h
> +++ b/include/linux/font.h
> @@ -11,7 +11,6 @@
>   #ifndef _VIDEO_FONT_H
>   #define _VIDEO_FONT_H
>   
> -#include <linux/math.h>
>   #include <linux/types.h>
>   
>   struct console_font;
> @@ -35,7 +34,7 @@ struct console_font;
>    */
>   static inline unsigned int font_glyph_pitch(unsigned int width)
>   {
> -	return DIV_ROUND_UP(width, 8);
> +	return (width + 7) >> 3;

Ok by me, if that's what's necessary. But can we try to keep a 
documented macro for the division to make the code explain itself? Does 
it work with DIV_ROUND_UP_POW2() ?

Best regards
Thomas

>   }
>   
>   /**

-- 
--
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Frankenstr. 146, 90461 Nürnberg, Germany, www.suse.com
GF: Jochen Jaser, Andrew McDonald, Werner Knoblich, (HRB 36809, AG Nürnberg)

Re: [PATCH] lib/fonts: Avoid unncessary 64-bit math in font code

[Helge Deller]

On 6/8/26 13:25, Thomas Zimmermann wrote:
> Hi
> 
> Am 07.06.26 um 23:02 schrieb Helge Deller:
>> The text display code used in the Risc PC kernel image decompression
>> code uses arch/arm/boot/compressed/font.c, which includes
>> lib/fonts/font_acorn_8x8.c, which further includes <linux/font.h>.
>>
>> Since commit 97df8960240a ("lib/fonts: Provide helpers for calculating
>> glyph pitch and size") <linux/font.h> contains inline functions that
>> require __do_div64, which is not linked into the ARM kernel
>> decompressor. This makes Risc PC zImages fail to build.
>>
>> There is no need to use 64-bit division code here, so resolve this issue
>> by using plain standard addition and shift maths.
> 
> Why is there a 64-bit division at all?

Not sure. Might be platform specific.
Maybe, because you add two integers and divide by an integer, that the
compiler then chooses to use 64-bit integer division by 32-bit integer.
  
>> Fixes: 97df8960240a ("lib/fonts: Provide helpers for calculating glyph pitch and size")
>> Reported-by: Ethan Nelson-Moore <enelsonmoore@gmail.com>
>> Signed-off-by: Helge Deller <deller@gmx.de>
>> ---
>>   include/linux/font.h | 3 +--
>>   1 file changed, 1 insertion(+), 2 deletions(-)
>>
>> diff --git a/include/linux/font.h b/include/linux/font.h
>> index 6845f02d739a..67d32268989d 100644
>> --- a/include/linux/font.h
>> +++ b/include/linux/font.h
>> @@ -11,7 +11,6 @@
>>   #ifndef _VIDEO_FONT_H
>>   #define _VIDEO_FONT_H
>> -#include <linux/math.h>
>>   #include <linux/types.h>
>>   struct console_font;
>> @@ -35,7 +34,7 @@ struct console_font;
>>    */
>>   static inline unsigned int font_glyph_pitch(unsigned int width)
>>   {
>> -    return DIV_ROUND_UP(width, 8);
>> +    return (width + 7) >> 3;
> 
> Ok by me, if that's what's necessary.
> But can we try to keep a documented macro for the division to make the code explain itself?

I'd expect everyone who messes with this kind of low-level graphics and bitmaps
to understand this math addition and bit shift, and as such I think it should
be self-explained.

> Does it work with DIV_ROUND_UP_POW2() ?
IMHO that's even worse than DIV_ROUND_UP().

Heleg

Re: [PATCH] lib/fonts: Avoid unncessary 64-bit math in font code

[Ethan Nelson-Moore]

Hi, Helge and Thomas,

On Mon, Jun 8, 2026 at 12:58 PM Helge Deller <deller@gmx.de> wrote:
>
> On 6/8/26 13:25, Thomas Zimmermann wrote:
> > Why is there a 64-bit division at all?
>
> Not sure. Might be platform specific.
> Maybe, because you add two integers and divide by an integer, that the
> compiler then chooses to use 64-bit integer division by 32-bit integer.

Actually, I think the real issue is that
arch/arm/boot/compressed/Makefile defines "static" to nothing when
compiling its copy of lib/fonts/font_acorn_8x8.c (via font.c), so that
the font array is available outside of the object file. I assume that
this causes various unused static inline functions in the headers it
includes (such as <linux/math.h>) to be included in the object file
because they become normal inline functions.

Ethan

Re: [PATCH] lib/fonts: Avoid unncessary 64-bit math in font code

[Helge Deller]

* Ethan Nelson-Moore <enelsonmoore@gmail.com>:
> Hi, Helge and Thomas,
> 
> On Mon, Jun 8, 2026 at 12:58 PM Helge Deller <deller@gmx.de> wrote:
> >
> > On 6/8/26 13:25, Thomas Zimmermann wrote:
> > > Why is there a 64-bit division at all?
> >
> > Not sure. Might be platform specific.
> > Maybe, because you add two integers and divide by an integer, that the
> > compiler then chooses to use 64-bit integer division by 32-bit integer.
> 
> Actually, I think the real issue is that
> arch/arm/boot/compressed/Makefile defines "static" to nothing when
> compiling its copy of lib/fonts/font_acorn_8x8.c (via font.c), so that
> the font array is available outside of the object file. I assume that
> this causes various unused static inline functions in the headers it
> includes (such as <linux/math.h>) to be included in the object file
> because they become normal inline functions.

Does this patch fix the issue then?

diff --git a/arch/arm/boot/compressed/Makefile b/arch/arm/boot/compressed/Makefile
index a159120d1e42..e3f550d62857 100644
--- a/arch/arm/boot/compressed/Makefile
+++ b/arch/arm/boot/compressed/Makefile
@@ -157,4 +157,4 @@ $(obj)/piggy_data: $(obj)/../Image FORCE
 
 $(obj)/piggy.o: $(obj)/piggy_data
 
-CFLAGS_font.o := -Dstatic=
+CFLAGS_font.o := -DBOOTLOADER
diff --git a/lib/fonts/font_acorn_8x8.c b/lib/fonts/font_acorn_8x8.c
index 36c51016769d..3327aa6d161d 100644
--- a/lib/fonts/font_acorn_8x8.c
+++ b/lib/fonts/font_acorn_8x8.c
@@ -5,7 +5,11 @@
 
 #define FONTDATAMAX 2048
 
+#ifndef BOOTLOADER
 static const struct font_data acorndata_8x8 = {
+#else
+const struct font_data acorndata_8x8 = {
+#endif
 { 0, 0, FONTDATAMAX, 0 }, {
 /* 00 */  0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, /* ^@ */
 /* 01 */  0x7e, 0x81, 0xa5, 0x81, 0xbd, 0x99, 0x81, 0x7e, /* ^A */

Re: [PATCH] lib/fonts: Avoid unncessary 64-bit math in font code

[Ethan Nelson-Moore]

Hi, Helge,

On Mon, Jun 8, 2026 at 1:26 PM Helge Deller <deller@kernel.org> wrote:
> Does this patch fix the issue then?

Yes, it does. I remember you suggested this solution a while ago. Thank you.

Ethan