Re: [PATCH 2.6.17 sparc64] 32-bit compat for Mach64 framebuffer

Mikael Pettersson wrote:

To: davem@davemloft.net
Subject: [PATCH 2.6.17 sparc64] 32-bit compat for Mach64 framebuffer
Cc: sparclinux@vger.kernel.org, linux-kernel@vger.kernel.org, linux-fbdev-devel@lists.sourceforge.net

In recent sparc64 kernels, starting a 32-bit mode X server on
a machine with a Mach64 framebuffer (CONFIG_FB_ATY_CT=y) like
an Ultra5, results in the kernel complaining:

ioctl32(X:1977): Unknown cmd fd(6) cmd(40584606){00} arg(ef8dd6d8) on /dev/fb0
ioctl32(X:1977): Unknown cmd fd(6) cmd(40184600){00} arg(ef8dd6e0) on /dev/fb0

That's FBIOGTYPE and FBIOGATTR. These errors occur because
kernel 2.6.15-rc2 changed the way sparc64 handles SPARC-specific
framebuffer ioctls from 32-bit processes: before 2.6.15-rc2
arch/sparc64/kernel/ioctl32.c handled them for all devices,
but 2.6.15-rc2 dropped that support and changed SPARC-only
framebuffer drivers like ffb.c to set up ->compat_ioctl methods
pointing to sbusfb_compat_ioctl in drivers/video/sbuslib.c.
However, drivers for framebuffers like the Mach64 that can exist
on both SPARCs and non-SPARCs were not adjusted, so in sparc64
kernels SPARC-specific framebuffer ioctls on Mach64 devices are
no longer accepted from 32-bit mode processes. Hence the errors.

The fix is to make atyfb_base.c set up a ->compat_ioctl pointing
to sbusfb_compat_ioctl when running in a sparc64 kernel with
compatibility for sparc32 user-space, and to compile and link
sbuslib.o with the frambuffer driver.

A complication is that sbuslib.c doesn't compile on non-SPARC
machines, so we must be careful to only enable it in the case
described above. That's why the patch puts an ugly "if" statement
in the Makefile.

Why not something like this?

1. In Kconfig

config FB_SBUSLIB
tristate
default n

Then all the sbus drivers will have this:

select FB_SBUSLIB

and atyfb will have this

select FB_SBUSLIB if SPARC64 && COMPAT

2. In Makefile
obj-$(CONFIG_FB_SBUSLIB)  += sbuslib.o

3. In sbuslib.h

#ifdef CONFIG_COMPAT
int sbusfb_compat_ioctl(...);
#else
#define sbusfb_compat_ioctl NULL;
#endif

This way, we can also eliminate all the #ifdef CONFIG_COMPAT in all the
cg* drivers and atyfb.

Tony