lib: packing: add pack_fields() and unpack_fields()

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 runtime, and return
  void from the API functions. Because the C preprocessor can't generate
  variable length code (loops), this is a bit tricky to do with macros.

  To handle this, implement macros which sanity check the packed field
  definitions based on their size. Finally, a single macro with a chain of
  __builtin_choose_expr() is used to select the appropriate macros. We
  enforce the use of ascending or descending order to avoid O(N^2) scaling
  when checking for overlap. Note that the macros are written with care to
  ensure that the compilers can correctly evaluate the resulting code at
  compile time. In particular, care was taken with avoiding too many nested
  statement expressions. Nested statement expressions trip up some
  compilers, especially when passing down variables created in previous
  statement expressions.

  There are two key design choices intended to keep the overall macro code
  size small. First, the definition of each CHECK_PACKED_FIELDS_N macro is
  implemented recursively, by calling the N-1 macro. This avoids needing
  the code to repeat multiple times.

  Second, the CHECK_PACKED_FIELD macro enforces that the fields in the
  array are sorted in order. This allows checking for overlap only with
  neighboring fields, rather than the general overlap case where each field
  would need to be checked against other fields.

  The overlap checks use the first two fields to determine the order of the
  remaining fields, thus allowing either ascending or descending order.
  This enables drivers the flexibility to keep the fields ordered in which
  ever order most naturally fits their hardware design and its associated
  documentation.

  The CHECK_PACKED_FIELDS macro is directly called from within pack_fields
  and unpack_fields, ensuring that all drivers using the API receive the
  benefits of the compile-time checks. Users do not need to directly call
  any of the macros directly.

  The CHECK_PACKED_FIELDS and its helper macros CHECK_PACKED_FIELDS_(0..50)
  are generated using a simple C program in scripts/gen_packed_field_checks.c
  This program can be compiled on demand and executed to generate the
  macro code in include/linux/packing.h. This will aid in the event that a
  driver needs more than 50 fields. The generator can be updated with a new
  size, and used to update the packing.h header file. In practice, the ice
  driver will need to support 27 fields, and the sja1105 driver will need
  to support 0 fields. This on-demand generation avoids the need to modify
  Kbuild. We do not anticipate the maximum number of fields to grow very
  often.

- Reduced rodata footprint for the storage of the packed field arrays.
  To that end, we have struct packed_field_u8 and packed_field_u16, which
  define the fields with the associated type. 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_u8() or pack_fields_16(),
  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().

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Co-developed-by: Jacob Keller <jacob.e.keller@intel.com>
Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>
Reviewed-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Link: https://patch.msgid.link/20241210-packing-pack-fields-and-ice-implementation-v10-3-ee56a47479ac@intel.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: vladimiroltean

MAINTAINERS

@@ -17644,6 +17644,7 @@ F:	Documentation/core-api/packing.rst
 F:	include/linux/packing.h
 F:	lib/packing.c
 F:	lib/packing_test.c
+F:	scripts/gen_packed_field_checks.c
 
 PADATA PARALLEL EXECUTION MECHANISM
 M:	Steffen Klassert <steffen.klassert@secunet.com>

Makefile

@@ -1367,6 +1367,10 @@ PHONY += scripts_unifdef
 scripts_unifdef: scripts_basic
 	$(Q)$(MAKE) $(build)=scripts scripts/unifdef
 
+PHONY += scripts_gen_packed_field_checks
+scripts_gen_packed_field_checks: scripts_basic
+	$(Q)$(MAKE) $(build)=scripts scripts/gen_packed_field_checks
+
 # ---------------------------------------------------------------------------
 # Install