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

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

[Finn Thain]

On Sun, 4 Jan 2015, Geert Uytterhoeven wrote:

> On Sun, Jan 4, 2015 at 8:21 AM, Finn Thain <fthain@telegraphics.com.au> 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.

The only user of generic_nvram is PPC32 (PPC64 could benefit though). 
PPC32 uses the driver by defining both CONFIG_NVRAM and 
CONFIG_GENERIC_NVRAM.

I tried to simplify this so that CONFIG_GENERIC_NVRAM would build 
drivers/char/generic_nvram, while CONFIG_NVRAM would build 
drivers/char/nvram, in order that it would become possible to have a 
multi-platform kernel with both, either, or niether.

But that approach brings new problems:

- An m68k multi-platform kernel would need to contain both modules, and 
  both modules would need MODULE_ALIAS_MISCDEV(NVRAM_MINOR). This isn't 
  going to work. (It's likely that other platforms might also want to use 
  generic_nvram and the same problem would apply to x86 and ARM.)

- The misc device code in drivers/char/nvram is duplicated in 
  generic_nvram. To avoid this duplication, the nvram module could 
  leverage the generic_nvram module instead. This doesn't work either.  
  Systems like mine (Gentoo) have "alias char-major-10-144 nvram" in 
  /etc/modprobe.d/i386.conf, which means that accessing /dev/nvram causes 
  the wrong module to load.

In the end I concluded that the only plausible "generic" driver is 
actually drivers/char/nvram itself. Otherwise it would be under an arch/ 
directory and not under drivers/.

So the nvram module should be the one with 
MODULE_ALIAS_MISCDEV(NVRAM_MAJOR), and it should work on any architecture 
that needs to use it. (Sure enough, drivers/char/generic_nvram lacks the 
MODULE_ALIAS.)

So I believe that the solution is to eliminate drivers/char/generic_nvram 
altogether, and move the architecture-specific code out of 
drivers/char/nvram, so the nvram module can be re-used more easily. I 
think that PPC32, PPC64 and m68k could readily re-use it.

Drivers themselves all test for CONFIG_NVRAM; never CONFIG_GENERIC_NVRAM. 
This is another indication that the generic_nvram driver is surplus to 
requirement.

The CONFIG_PROC_FS support (/proc/driver/nvram) in the drivers/char/nvram 
module is inherently architecture-specific. I suspect that the 
Atari-specific code should move to arch/m68k/atari/ and the x86-specific 
code should move to arch/x86/.

I find the ARM support in drivers/char/nvram to be surprising, not to say 
questionable. The /proc/driver/nvram implementation, given 
defined(__arm__), decodes the NVRAM contents in exactly the same format as 
when defined(__i386__) || defined(__x86_64__). Whereas, only MIPS and 
PowerPC defconfigs set CONFIG_RTC_DRV_CMOS at all, and without that symbol 
the driver will never be built for ARM. This raises the question, does 
/proc/driver/nvram do anything useful on any ARM platforms?

Some guidance on this problem would be appreciated; all the approaches I 
tried led to unsatisfactory compromises. I don't want to keep re-writing 
these patches without a workable plan.

-- 

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

[Russell King - ARM Linux]

On Sun, Feb 01, 2015 at 02:39:42PM +1100, Finn Thain wrote:
> I find the ARM support in drivers/char/nvram to be surprising, not to say 
> questionable. The /proc/driver/nvram implementation, given 
> defined(__arm__), decodes the NVRAM contents in exactly the same format as 
> when defined(__i386__) || defined(__x86_64__).

That's because it's used on the Netwinder and EBSA285 platforms, which
are PCI-like, complete with a southbridge which makes them look like a
PC.

> Whereas, only MIPS and 
> PowerPC defconfigs set CONFIG_RTC_DRV_CMOS at all, and without that symbol 
> the driver will never be built for ARM. This raises the question, does 
> /proc/driver/nvram do anything useful on any ARM platforms?

Yes.

-- 
FTTC broadband for 0.8mile line: currently at 10.5Mbps down 400kbps up
according to speedtest.net.

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

[Geert Uytterhoeven]

On Sun, Feb 1, 2015 at 4:39 AM, Finn Thain <fthain@telegraphics.com.au> wrote:
> On Sun, 4 Jan 2015, Geert Uytterhoeven wrote:
>
>> On Sun, Jan 4, 2015 at 8:21 AM, Finn Thain <fthain@telegraphics.com.au> 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.

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert at linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds

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

[Finn Thain]

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);
#ifdef CONFIG_PPC
        void            (*sync)(void);
#else
        long            (*set_checksum)(void);
        long            (*initialize)(void);
#endif
};

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.

-- 

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

[Finn Thain]

On Sun, 1 Feb 2015, Russell King - ARM Linux wrote:

> On Sun, Feb 01, 2015 at 02:39:42PM +1100, Finn Thain wrote:
> > I find the ARM support in drivers/char/nvram to be surprising, not to 
> > say questionable. The /proc/driver/nvram implementation, given 
> > defined(__arm__), decodes the NVRAM contents in exactly the same 
> > format as when defined(__i386__) || defined(__x86_64__).
> 
> That's because it's used on the Netwinder and EBSA285 platforms, which 
> are PCI-like, complete with a southbridge which makes them look like a 
> PC.

Well, that explains the presence of RTC NVRAM, but the question was also 
about the format of that NVRAM.

The the code in question in drivers/char/nvram.c decodes RTC NVRAM on ARM 
as if it took the same format as x86 PC RTC NVRAM. Apparently they are not 
in the same format (for EBSA-285 at least).

I downloaded the EBSA-285 BIOS,
ftp://ftp.arm.linux.org.uk/pub/armlinux/source/boot/bios-1.11.tar.gz

This is from bios/init/cfg.c --

        img_nr = rtc_read_cmos(0);
        root_dev = rtc_read_cmos(2) | rtc_read_cmos(3) << 8;

        for (i = 0; i < 80; i++) {
                extra_args[i] = rtc_read_cmos(128+i);
                if (!extra_args[i])
                        break;
        }
        extra_args[79] = '\0';

(Like /dev/nvram, file offset 0 is actually device offset 14.)

This EBSA-285 BIOS code uses bytes 128 through 208 as "extra_args", 
whereas /dev/nvram has only 114 bytes in total. The EBSA-285 BIOS doesn't 
read or write any checksum.

The first two bytes aren't reported in /proc/driver/nvram, whereas 
EBSA-285 BIOS uses them for img_nr and root_dev. The next one is reported 
as "floppy type" on x86 PC, though the EBSA-285 BIOS uses it for 
root_dev >> 8.

This is from an x86 PC, for example:

# cat /proc/driver/nvram 
Checksum status: valid
# floppies     : 1
Floppy 0 type  : none
Floppy 1 type  : none
HD 0 type      : 01
HD 1 type      : none
HD type 48 data: 512/0/0 C/H/S, precomp 0, lz 256
HD type 49 data: 1/124/0 C/H/S, precomp 0, lz 0
DOS base memory: 640 kB
Extended memory: 64512 kB (configured), 64512 kB (tested)
Gfx adapter    : EGA, VGA, ... (with BIOS)
FPU            : installed

My original question was whether the CONFIG_PROC_FS support found in 
drivers/char/nvram.c should be moved to arch/x86 and arch/m68k.

So that question now seems to hinge on the Netwinder ROM ("nettrom") which 
I gather is proprietary (I didn't find any source code). Does anyone know 
what format the Netwinder NVRAM takes?

If NVRAM on Netwinder has a different format to x86 PC, then the 
CONFIG_PROC_FS code in drivers/char/nvram should go elsewhere.

And if the NVRAM on Netwinder had no checksum, drivers/char/nvram.c would 
become a generic /dev/nvram misc device, and drivers/char/generic_nvram.c 
could go away, along with its inherent problems.

--