powerpc/pseries: Add a clear modifier to ibm,pa/pi-features parser

When a new ibm,pa/pi-features bit is introduced that is intended to
apply to existing systems and features, it may have an "inverted"
meaning (i.e., bit clear => feature available; bit set => unavailable).
Depending on the nature of the feature, this may give the best
backward compatibility result where old firmware will continue to
have that bit clear and therefore the feature available.

The 'invert' modifier presumably was introduced for this type of
feature bit. However it invert will set the feature if the bit is
clear, which prevents it being used in the situation where an old
CPU lacks a feature that a new CPU has, then a new firmware comes
out to disable that feature on the new CPU if the bit is set.
Adding an 'invert' entry for that feature would incorrectly enable
it for the old CPU.

So add a 'clear' modifier that clears the feature if the bit is set,
but it does not set the feature if the bit is clear. The feature
is expected to be set in the cpu table.

This replaces the 'invert' modifier, which is unused since commit
7d470345 ("powerpc/feature: Remove CPU_FTR_NODSISRALIGN").
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Tested-by: Vaibhav Jain <vaibhav@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20240207035220.339726-1-npiggin@gmail.com

arch/powerpc/kernel/prom.c

@@ -151,6 +151,9 @@ static void __init move_device_tree(void)
  * pa-features property is missing, or a 1/0 to indicate if the feature
  * is supported/not supported.  Note that the bit numbers are
  * big-endian to match the definition in PAPR.
+ * Note: the 'clear' flag clears the feature if the bit is set in the
+ * ibm,pa/pi-features property, it does not set the feature if the
+ * bit is clear.
  */
 struct ibm_feature {
 	unsigned long	cpu_features;	/* CPU_FTR_xxx bit */
@@ -159,7 +162,7 @@ struct ibm_feature {
 	unsigned int	cpu_user_ftrs2;	/* PPC_FEATURE2_xxx bit */
 	unsigned char	pabyte;		/* byte number in ibm,pa/pi-features */
 	unsigned char	pabit;		/* bit number (big-endian) */
-	unsigned char	invert;		/* if 1, pa bit set => clear feature */
+	unsigned char	clear;		/* if 1, pa bit set => clear feature */
 };
 
 static struct ibm_feature ibm_pa_features[] __initdata = {
@@ -220,12 +223,12 @@ static void __init scan_features(unsigned long node, const unsigned char *ftrs,
 		if (fp->pabyte >= ftrs[0])
 			continue;
 		bit = (ftrs[2 + fp->pabyte] >> (7 - fp->pabit)) & 1;
-		if (bit ^ fp->invert) {
+		if (bit && !fp->clear) {
 			cur_cpu_spec->cpu_features |= fp->cpu_features;
 			cur_cpu_spec->cpu_user_features |= fp->cpu_user_ftrs;
 			cur_cpu_spec->cpu_user_features2 |= fp->cpu_user_ftrs2;
 			cur_cpu_spec->mmu_features |= fp->mmu_features;
-		} else {
+		} else if (bit == fp->clear) {
 			cur_cpu_spec->cpu_features &= ~fp->cpu_features;
 			cur_cpu_spec->cpu_user_features &= ~fp->cpu_user_ftrs;
 			cur_cpu_spec->cpu_user_features2 &= ~fp->cpu_user_ftrs2;