From: Vladimir Oltean <vladimir.oltean@nxp.com>

This is new API which caters to the following requirements:

- Pack or unpack a large number of fields to/from a buffer with a small
  code footprint. The current alternative is to open-code a large number
  of calls to pack() and unpack(), or to use packing() to reduce that
  number to half. But packing() is not const-correct.

- Use unpacked numbers stored in variables smaller than u64. This
  reduces the rodata footprint of the stored field arrays.

- Perform error checking at compile time, rather than at runtime, and
  return void from the API functions. To that end, we introduce
  CHECK_PACKED_FIELD_*() macros to be used on the arrays of packed
  fields. Note: the C preprocessor can't generate variable-length code
  (loops),  as would be required for array-style definitions of struct
  packed_field arrays. So the sanity checks use code generation at
  compile time to $KBUILD_OUTPUT/include/generated/packing-checks.h.
  There are explicit macros for sanity-checking arrays of 1 packed
  field, 2 packed fields, 3 packed fields, ..., all the way to 50 packed
  fields. In practice, the sja1105 driver will actually need the variant
  with 40 fields. This isn't as bad as it seems: feeding a 39 entry
  sized array into the CHECK_PACKED_FIELDS_40() macro will actually
  generate a compilation error, so mistakes are very likely to be caught
  by the developer and thus are not a problem.

- Reduced rodata footprint for the storage of the packed field arrays.
  To that end, we have struct packed_field_s (small) and packed_field_m
  (medium). More can be added as needed (unlikely for now). On these
  types, the same generic pack_fields() and unpack_fields() API can be
  used, thanks to the new C11 _Generic() selection feature, which can
  call pack_fields_s() or pack_fields_m(), depending on the type of the
  "fields" array - a simplistic form of polymorphism. It is evaluated at
  compile time which function will actually be called.

Over time, packing() is expected to be completely replaced either with
pack() or with pack_fields().

Co-developed-by: Jacob Keller <jacob.e.keller@intel.com>
Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
---
 include/linux/packing.h  |  69 ++++++++++++++++++++++
 lib/gen_packing_checks.c |  31 ++++++++++
 lib/packing.c            | 149 ++++++++++++++++++++++++++++++++++++++++++++++-
 Kbuild                   |  13 ++++-
 4 files changed, 259 insertions(+), 3 deletions(-)

diff --git a/include/linux/packing.h b/include/linux/packing.h
index 5d36dcd06f60..eeb23d90e5e0 100644
--- a/include/linux/packing.h
+++ b/include/linux/packing.h

@@ -26,4 +26,73 @@ int pack(void *pbuf, u64 uval, size_t startbit, size_t endbit, size_t pbuflen,
 int unpack(const void *pbuf, u64 *uval, size_t startbit, size_t endbit,
 	   size_t pbuflen, u8 quirks);
 
+#define GEN_PACKED_FIELD_MEMBERS(__type) \
+	__type startbit; \
+	__type endbit; \
+	__type offset; \
+	__type size;
+
+/* Small packed field. Use with bit offsets < 256, buffers < 32B and
+ * unpacked structures < 256B.
+ */
+struct packed_field_s {
+	GEN_PACKED_FIELD_MEMBERS(u8);
+};
+
+/* Medium packed field. Use with bit offsets < 65536, buffers < 8KB and
+ * unpacked structures < 64KB.
+ */
+struct packed_field_m {
+	GEN_PACKED_FIELD_MEMBERS(u16);
+};
+
+#define PACKED_FIELD(start, end, struct_name, struct_field) \
+	{ \
+		(start), \
+		(end), \
+		offsetof(struct_name, struct_field), \
+		sizeof_field(struct_name, struct_field), \
+	}
+
+#define CHECK_PACKED_FIELD(field, pbuflen) \
+	({ typeof(field) __f = (field); typeof(pbuflen) __len = (pbuflen); \
+	BUILD_BUG_ON(__f.startbit < __f.endbit); \
+	BUILD_BUG_ON(__f.startbit >= BITS_PER_BYTE * __len); \
+	BUILD_BUG_ON(__f.startbit - __f.endbit >= BITS_PER_BYTE * __f.size); \
+	BUILD_BUG_ON(__f.size != 1 && __f.size != 2 && __f.size != 4 && __f.size != 8); })
+
+#define CHECK_PACKED_FIELD_OVERLAP(field1, field2) \
+	({ typeof(field1) _f1 = (field1); typeof(field2) _f2 = (field2); \
+	BUILD_BUG_ON(max(_f1.endbit, _f2.endbit) <=  min(_f1.startbit, _f2.startbit)); })
+
+#include <generated/packing-checks.h>
+
+void pack_fields_s(void *pbuf, size_t pbuflen, const void *ustruct,
+		   const struct packed_field_s *fields, size_t num_fields,
+		   u8 quirks);
+
+void pack_fields_m(void *pbuf, size_t pbuflen, const void *ustruct,
+		   const struct packed_field_m *fields, size_t num_fields,
+		   u8 quirks);
+
+void unpack_fields_s(const void *pbuf, size_t pbuflen, void *ustruct,
+		     const struct packed_field_s *fields, size_t num_fields,
+		     u8 quirks);
+
+void unpack_fields_m(const void *pbuf, size_t pbuflen, void *ustruct,
+		      const struct packed_field_m *fields, size_t num_fields,
+		      u8 quirks);
+
+#define pack_fields(pbuf, pbuflen, ustruct, fields, quirks) \
+	_Generic((fields), \
+		 const struct packed_field_s * : pack_fields_s, \
+		 const struct packed_field_m * : pack_fields_m \
+		)(pbuf, pbuflen, ustruct, fields, ARRAY_SIZE(fields), quirks)
+
+#define unpack_fields(pbuf, pbuflen, ustruct, fields, quirks) \
+	_Generic((fields), \
+		 const struct packed_field_s * : unpack_fields_s, \
+		 const struct packed_field_m * : unpack_fields_m \
+		)(pbuf, pbuflen, ustruct, fields, ARRAY_SIZE(fields), quirks)
+
 #endif

diff --git a/lib/gen_packing_checks.c b/lib/gen_packing_checks.c
new file mode 100644
index 000000000000..3213c858c2fe
--- /dev/null
+++ b/lib/gen_packing_checks.c

@@ -0,0 +1,31 @@
+// SPDX-License-Identifier: GPL-2.0
+#include <stdio.h>
+
+int main(int argc, char **argv)
+{
+	printf("/* Automatically generated - do not edit */\n\n");
+	printf("#ifndef GENERATED_PACKING_CHECKS_H\n");
+	printf("#define GENERATED_PACKING_CHECKS_H\n\n");
+
+	for (int i = 1; i <= 50; i++) {
+		printf("#define CHECK_PACKED_FIELDS_%d(fields, pbuflen) \\\n", i);
+		printf("\t({ typeof(&(fields)[0]) _f = (fields); typeof(pbuflen) _len = (pbuflen); \\\n");
+		printf("\tBUILD_BUG_ON(ARRAY_SIZE(fields) != %d); \\\n", i);
+		for (int j = 0; j < i; j++) {
+			int final = (i == 1);
+
+			printf("\tCHECK_PACKED_FIELD(_f[%d], _len);%s\n",
+			       j, final ? " })\n" : " \\");
+		}
+		for (int j = 1; j < i; j++) {
+			for (int k = 0; k < j; k++) {
+				int final = (j == i - 1) && (k == j - 1);
+
+				printf("\tCHECK_PACKED_FIELD_OVERLAP(_f[%d], _f[%d]);%s\n",
+				       k, j, final ? " })\n" : " \\");
+			}
+		}
+	}
+
+	printf("#endif /* GENERATED_PACKING_CHECKS_H */\n");
+}

diff --git a/Kbuild b/Kbuild
index 464b34a08f51..35a8b78b72d9 100644
--- a/Kbuild
+++ b/Kbuild

@@ -34,6 +34,17 @@ arch/$(SRCARCH)/kernel/asm-offsets.s: $(timeconst-file) $(bounds-file)
 $(offsets-file): arch/$(SRCARCH)/kernel/asm-offsets.s FORCE
 	$(call filechk,offsets,__ASM_OFFSETS_H__)
 
+# Generate packing-checks.h
+
+hostprogs += lib/gen_packing_checks
+
+packing-checks := include/generated/packing-checks.h
+
+filechk_gen_packing_checks = lib/gen_packing_checks
+
+$(packing-checks): lib/gen_packing_checks FORCE
+	$(call filechk,gen_packing_checks)
+
 # Check for missing system calls
 
 quiet_cmd_syscalls = CALL    $<

@@ -70,7 +81,7 @@ $(atomic-checks): $(obj)/.checked-%: include/linux/atomic/%  FORCE
 # A phony target that depends on all the preparation targets
 
 PHONY += prepare
-prepare: $(offsets-file) missing-syscalls $(atomic-checks)
+prepare: $(offsets-file) missing-syscalls $(atomic-checks) $(packing-checks)
 	@:
 
 # Ordinary directory descending