linux-kernel - Re: const vs _

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Date:	Wed, 24 Oct 2007 09:23:05 +0200 (CEST)
From:	Geert Uytterhoeven <Geert.Uytterhoeven@...ycom.com>
To:	Jeff Garzik <jeff@...zik.org>
cc:	Bartlomiej Zolnierkiewicz <bzolnier@...il.com>,
	linux-kernel@...r.kernel.org,
	Randy Dunlap <randy.dunlap@...cle.com>,
	Greg KH <greg@...ah.com>
Subject: Re: const vs __{dev}initdata

On Tue, 23 Oct 2007, Jeff Garzik wrote:
> Bartlomiej Zolnierkiewicz wrote:
> > It seems that const cannot be (officialy) mixed with __{dev}initdata,
> > 
> > include/linux/init.h:
> > ...
> >  *
> >  * Also note, that this data cannot be "const".
> >  */
> > ...
> > 
> > [ BTW Greg: this doesn't seem to be mentioned in LDD3. ]
> > 
> > Any reason for this limitation (other than not having .init.rodata section
> > and __const__{dev}initdata tag)?  There are places where we really would
> > like
> > to have both, i.e. struct ide_port_info in drivers/ide.
> 
> Thank you for asking this question...  we really do need a solution for this.
> 
> As you point out, there are many valid cases for both const and initdata.

Indeed. And sometimes it works, depending on the toolchain version. Cfr.
the patch below which I couldn't submit due to this.

Patch monkeys: DO NOT APPLY ;-)

Subject: fbdev: move logo externs to header file

Move the external declarations for the various linux logo structures to
<linux/linux_logo.h>. As a consequence, I had to change scripts/pnmtologo.c to
add the appropriate `const' to the generated logo source code.

FIXME: This one is really tricky and cannot be applied!
  - gcc 3.4.6 20060404 (Red Hat 3.4.6-3) needs xxx_data[] and
    xxx_clut[] to be const too, else it complains about a section type conflict
  - ppu-gcc 4.1.0 20060304 (Red Hat 4.1.0-3) needs xxx_data[] and xxx_clut[] to
    be non-const, else it complains about a section type conflict
Anyone with a suggestion how to fix this?

Signed-off-by: Geert Uytterhoeven <Geert.Uytterhoeven@...ycom.com>
Acked-By: James Simmons <jsimmons@...radead.org>
---
 arch/powerpc/kernel/prom_init.c |    3 ---
 drivers/video/logo/logo.c       |   13 -------------
 include/linux/linux_logo.h      |   13 +++++++++++++
 scripts/pnmtologo.c             |    2 +-
 4 files changed, 14 insertions(+), 17 deletions(-)

--- a/arch/powerpc/kernel/prom_init.c
+++ b/arch/powerpc/kernel/prom_init.c
@@ -44,10 +44,7 @@
 #include <asm/sections.h>
 #include <asm/machdep.h>
 
-#ifdef CONFIG_LOGO_LINUX_CLUT224
 #include <linux/linux_logo.h>
-extern const struct linux_logo logo_linux_clut224;
-#endif
 
 /*
  * Properties whose value is longer than this get excluded from our
--- a/drivers/video/logo/logo.c
+++ b/drivers/video/logo/logo.c
@@ -21,19 +21,6 @@
 #include <asm/bootinfo.h>
 #endif
 
-extern const struct linux_logo logo_linux_mono;
-extern const struct linux_logo logo_linux_vga16;
-extern const struct linux_logo logo_linux_clut224;
-extern const struct linux_logo logo_dec_clut224;
-extern const struct linux_logo logo_mac_clut224;
-extern const struct linux_logo logo_parisc_clut224;
-extern const struct linux_logo logo_sgi_clut224;
-extern const struct linux_logo logo_sun_clut224;
-extern const struct linux_logo logo_superh_mono;
-extern const struct linux_logo logo_superh_vga16;
-extern const struct linux_logo logo_superh_clut224;
-extern const struct linux_logo logo_m32r_clut224;
-
 
 const struct linux_logo *fb_find_logo(int depth)
 {
--- a/include/linux/linux_logo.h
+++ b/include/linux/linux_logo.h
@@ -32,6 +32,19 @@ struct linux_logo {
 	const unsigned char *data;
 };
 
+extern const struct linux_logo logo_linux_mono;
+extern const struct linux_logo logo_linux_vga16;
+extern const struct linux_logo logo_linux_clut224;
+extern const struct linux_logo logo_dec_clut224;
+extern const struct linux_logo logo_mac_clut224;
+extern const struct linux_logo logo_parisc_clut224;
+extern const struct linux_logo logo_sgi_clut224;
+extern const struct linux_logo logo_sun_clut224;
+extern const struct linux_logo logo_superh_mono;
+extern const struct linux_logo logo_superh_vga16;
+extern const struct linux_logo logo_superh_clut224;
+extern const struct linux_logo logo_m32r_clut224;
+
 extern const struct linux_logo *fb_find_logo(int depth);
 
 #endif /* _LINUX_LINUX_LOGO_H */
--- a/scripts/pnmtologo.c
+++ b/scripts/pnmtologo.c
@@ -244,7 +244,7 @@ static void write_header(void)
 static void write_footer(void)
 {
     fputs("\n};\n\n", out);
-    fprintf(out, "struct linux_logo %s __initdata = {\n", logoname);
+    fprintf(out, "const struct linux_logo %s __initdata = {\n", logoname);
     fprintf(out, "    .type\t= %s,\n", logo_types[logo_type]);
     fprintf(out, "    .width\t= %d,\n", logo_width);
     fprintf(out, "    .height\t= %d,\n", logo_height);

With kind regards,
 
Geert Uytterhoeven
Software Architect

Sony Network and Software Technology Center Europe
The Corporate Village · Da Vincilaan 7-D1 · B-1935 Zaventem · Belgium
 
Phone:    +32 (0)2 700 8453	
Fax:      +32 (0)2 700 8622	
E-mail:   Geert.Uytterhoeven@...ycom.com	
Internet: http://www.sony-europe.com/
 	
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