Re: [PATCH] char: nvram: disable on ARM

On 07/02/2018 at 15:00:04 +0100, Arnd Bergmann wrote:

On Wed, Feb 7, 2018 at 1:48 PM, Alexandre Belloni
[off-list ref] wrote:

quoted

On 07/02/2018 at 11:33:55 +0100, Arnd Bergmann wrote:

quoted

On Wed, Feb 7, 2018 at 2:55 AM, Alexandre Belloni

quoted

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$ cat /proc/driver/nvram
Checksum status: valid
# floppies     : 0
Floppy 0 type  : none
Floppy 1 type  : none
HD 0 type      : none
HD 1 type      : none
HD type 48 data: 0/0/0 C/H/S, precomp 0, lz 0
HD type 49 data: 156/0/0 C/H/S, precomp 0, lz 0
DOS base memory: 635 kB
Extended memory: 65535 kB (configured), 65535 kB (tested)
Gfx adapter    : EGA, VGA, ... (with BIOS)
FPU            : not installed

I really don't think anyone is using that but I don't really know much
about x86 and the specification this may be part of.

I see the info may be used in drivers/video/fbdev/ and
drivers/platform/x86/thinkpad_acpi.c

The thinkpad_acpi driver seems to look at some other bytes
in the nvram, which have a platform specific meaning.

Yeah, I was more concerned that they need drivers/char/nvram.c for
nvram_read_byte so we can't simply remove the driver.

Ok, so the procfs interface may be obsolete, but we still need an
interface into the CMOS NVRAM data.

Actually, I just found
https://unix.stackexchange.com/questions/331419/is-dev-nvram-dangerous-to-write-to

So it seems to have real values for some people (even if they are
wrong).

That also points to https://sourceforge.net/projects/nvram-wakeup/ but I
don't think it is necessary. The RTC driver should be able to wakeup an
x86 platform.

All the other uses of /dev/nvram I could find with a simple google
search (i.e. saving and restoring CMOS settings) could just use
/sys/class/rtc/rtc0/device/nvram

I see that the x86 version of nvram_read_byte is just a wrapper
around CMOS_READ(14 + addr). We also have some drivers
that call the low-level function directly:

arch/x86/include/asm/floppy.h:  val = CMOS_READ(0x10) & 15;
arch/x86/kernel/bootflag.c:     v = CMOS_READ(sbf_port);
drivers/char/mwave/smapi.c:     usSmapiID = CMOS_READ(0x7C);
drivers/input/misc/wistron_btns.c:              qlen = CMOS_READ(cmos_address);

I suppose we could make the thinkpad driver do the same,
or provide a 'static inline' version of nvram_read_byte somewhere.

I guess we can do that, provided we take rtc_lock before using
CMOS_READ.

Thinking of it, I think this means we don't need the lock for powerpc as
nvram_read_byte doesn't take it. So I guess it is only needed on x86.

-- 
Alexandre Belloni, Bootlin (formerly Free Electrons)
Embedded Linux and Kernel engineering
http://bootlin.com