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Date:   Sat, 9 Jun 2018 22:21:06 +1000 (AEST)
From:   Finn Thain <fthain@telegraphics.com.au>
To:     Michael Schmitz <schmitzmic@gmail.com>
cc:     Andreas Schwab <schwab@linux-m68k.org>,
        Benjamin Herrenschmidt <benh@kernel.crashing.org>,
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        Geert Uytterhoeven <geert@linux-m68k.org>
Subject: Re: [PATCH v2 08/12] macintosh/via-pmu68k: Don't load driver on
 unsupported hardware
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> > > Is this enum used by any user space code? If so, perhaps rather 
> > > leave the PMU_68K_V1 in there to avoid upsetting that?
> > 
> > It also changes the value of PMU_68K_V2, which is an ABI break.
> 
> Yes, that's what I worry about - but do we know of any users of that 
> particular interface?

There is no ABI issue AFAIK. The value of pmu_kind is visible to userland 
only on powerpc. /dev/pmu and /proc/pmu/* do not exist on m68k. This patch 
series will make these UAPIs available on m68k, and for that reason I've 
chosen the value PMU_UNKNOWN for pmu_kind.

New pmu_kind values can be defined as and when the need arises. But that 
would imply a useful classification scheme for pre-PCI powerbooks, and I 
don't know what that scheme will look like because at this stage there is 
neither userland nor kernel code to support backlight, buttons and battery 
for pre-PCI powerbooks.

In anycase, the "v1" and "v2" scheme is obviously inadequate when you 
consider the range of m68k powerbook models. Also, consider the 
out-of-tree adaptation of via-pmu by the Nubus-PMac project, which has 
this ABI break:

diff --git a/include/linux/pmu.h b/include/linux/pmu.h
index cafe98d9694..9882a185a52 100644
--- a/include/linux/pmu.h
+++ b/include/linux/pmu.h
@@ -90,6 +90,7 @@ enum {
         PMU_HEATHROW_BASED,     /* PowerBook G3 series */
         PMU_PADDINGTON_BASED,   /* 1999 PowerBook G3 */
         PMU_KEYLARGO_BASED,     /* Core99 motherboard (PMU99) */
+        PMU_NUBUS_BASED,        /* 1400, 2300, 5300 */
         PMU_68K_V1,             /* 68K PMU, version 1 */
         PMU_68K_V2,             /* 68K PMU, version 2 */
 };

(BTW, these powerbooks are not "nubus based", they are "pre-PCI", so I 
wouldn't want this to go upstream in this form. It could be that 
PMU_NUBUS_BASED should be PMU_UNKNOWN too.)

--