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SubjectRe: nvram and generic_nvram modules are problematic, was Re: [PATCH] arch: m68k: mac: misc.c: Remove some unused functions

On Sun, 1 Feb 2015, Geert Uytterhoeven wrote:

> On Sun, Feb 1, 2015 at 4:39 AM, Finn Thain wrote:
> > On Sun, 4 Jan 2015, Geert Uytterhoeven wrote:
> > > On Sun, Jan 4, 2015 at 8:21 AM, Finn Thain wrote:
> > > > On Thu, 1 Jan 2015, Rickard Strandqvist wrote:
> > > > > Removes some functions that are not used anywhere:
> > > > > mac_pram_write() mac_pram_read()
> > > >
> > > > ... I'd rather not remove all of this code. Better to finish the
> > > > implementation.
> > >
> > > Indeed.
> > >
> > > > Would it be acceptable to utilize drivers/char/generic_nvram.c and
> > > > CONFIG_GENERIC_NVRAM? This is the PowerMac PRAM driver but looks
> > > > generic enough that it may not need any modification for 68k Macs.
> > >
> > > Yes, that would be great.
> > >
> >
> > Unfortunately, it seems to be unworkable.
> An alternative could be to just provide an nvram attribute file in
> sysfs, like many RTC drivers do.

Are attribute files seekable? Even if userspace could use "/dev/nvram" and
"/sys/whatever/nvram" interchangably, wouldn't it be better if PPC Macs
and 68k Macs offered a consistent API to userspace?

And your suggestion doesn't solve the problem, that is, to be able to
build a multi-platform kernel binary in which drivers can access NVRAM.

The __nvram_read_byte(), nvram_read_byte() etc functions defined in
drivers/char/nvram.c, if allowed to proliferate because random
architectures might like to use a generic /dev/nvram API, would further
uglify that file.

If the m68k Mac kernel doesn't define the nvram_read_byte() routine then
valkyriefb can't use it. (fbdev drivers are apparently the reason why
powerpc defines them.)

drivers/char/nvram.c has two sets of these routines for PC RTC NVRAM; one
for m68k (Atari) and one for arm/x86. We don't want to introduce more code
into drivers/char/nvram.c to support all four configurations:
1) arm/x86
2) atari
3) atari + mac
4) mac

So we'd end up having to move m68k-specific code out of
drivers/char/nvram.c, to make it generic. And that then begs all of the
questions in my previous message.

BTW, my experimental patches replaced all of those __nvram_* and nvram_*
functions with an ops struct. E.g.

$ cat include/linux/nvram.h
#ifndef _LINUX_NVRAM_H
#define _LINUX_NVRAM_H

#include <uapi/linux/nvram.h>

struct nvram_ops {
ssize_t (*read)(char *, size_t, loff_t *);
ssize_t (*write)(char *, size_t, loff_t *);
unsigned char (*read_byte)(int);
void (*write_byte)(unsigned char, int);
ssize_t (*get_size)(void);
void (*sync)(void);
long (*set_checksum)(void);
long (*initialize)(void);

extern const struct nvram_ops arch_nvram_ops;

extern const struct nvram_ops rtc_nvram_ops;

#endif /* _LINUX_NVRAM_H */

This experiment has m68k implement arch_nvram_ops that dispatch to Atari
or Mac methods (at compile-time for a single-platform kernel, or at
run-time for a multi-platform kernel binary).

But this implies modifications to fbdev drivers, PPC32 and PPC64, nvram
and generic_nvram modules. And any work at all done on generic_nvram seems
to be misguided, unless it is removal.


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