Re: [PATCH v4 00/13] Introduce parity_odd() and refactor redundant parity code

[Yury Norov <yury.norov@gmail.com>]

On Wed, Apr 09, 2025 at 11:43:43PM +0800, Kuan-Wei Chiu wrote:
> Several parts of the kernel contain open-coded and redundant
> implementations of parity calculation. This patch series introduces
> a unified helper, parity_odd(), to simplify and standardize these
> cases.
> 
> The first patch renames parity8() to parity_odd(), changes its argument

Alright, if it's an extension of the area of applicability, it should be
renamed to just parity(). I already shared a table that summarized the
drivers authors' view on that, and they clearly prefer not to add the
suffix - 13 vs 2. The __builtin_parity() doesn't care of suffix as well. 

https://lore.kernel.org/all/Z9GtcNJie8TRKywZ@thinkpad/

Yes, the argument that boolean function should explain itself sounds
correct, but in this case, comment on top of the function looks enough
to me.

The existing codebase doesn't care about the suffix as well. If no
strong preference, let's just pick a short and sweet name?

> type from u8 to u64 for broader applicability, and updates its return
> type from int to bool to make its usage and return semantics more
> intuitive-returning true for odd parity and false for even parity. It
> also adds __attribute_const__ to enable compiler optimizations.

That's correct and nice, but can you support it with a bloat-o-meter's
before/after and/or asm snippets? I also think it worth to be a separate
patch, preferably the last patch in the series.

> While more efficient implementations may exist, further optimization is
> postponed until a use case in performance-critical paths arises.
> 
> Subsequent patches refactor various kernel components to replace
> open-coded parity logic with the new helper, reducing code duplication
> and improving consistency.
> 
> Co-developed-by: Yu-Chun Lin <eleanor15x@gmail.com>
> Signed-off-by: Yu-Chun Lin <eleanor15x@gmail.com>
> Signed-off-by: Kuan-Wei Chiu <visitorckw@gmail.com>
> ---
> 
> To H. Peter:
> I understand your preference for a parity8/16/32/64() style interface,
> and I agree that such a design would better accommodate potential
> arch-specific implementations. However, I suspect there are very few,
> if any, users who care about the performance of parity calculations
> enough to warrant such optimizations. So my inclination is to defer any
> arch-specific or optimized implementations until we see parity_odd()
> being used in hot paths.
> 
> Changes in v4:
> - Rename parity8() to parity_odd().
> - Change the argument type from u8 to u64.
> - Use a single parity_odd() function.
> 
> Changes in v3:
> - Avoid using __builtin_parity.
> - Change return type to bool.
> - Drop parity() macro.
> - Change parityXX() << y to !!parityXX() << y.
> 
> Changes in v2:
> - Provide fallback functions for __builtin_parity() when the compiler
>   decides not to inline it
> - Use __builtin_parity() when no architecture-specific implementation
>   is available
> - Optimize for constant folding when val is a compile-time constant
> - Add a generic parity() macro
> - Drop the x86 bootflag conversion patch since it has been merged into
>   the tip tree
> 
> v3: https://lore.kernel.org/lkml/20250306162541.2633025-1-visitorckw@gmail.com/
> v1: https://lore.kernel.org/lkml/20250223164217.2139331-1-visitorckw@gmail.com/
> v2: https://lore.kernel.org/lkml/20250301142409.2513835-1-visitorckw@gmail.com/
> 
> Kuan-Wei Chiu (13):
>   bitops: Change parity8() to parity_odd() with u64 input and bool
>     return type
>   media: media/test_drivers: Replace open-coded parity calculation with
>     parity_odd()
>   media: pci: cx18-av-vbi: Replace open-coded parity calculation with
>     parity_odd()
>   media: saa7115: Replace open-coded parity calculation with
>     parity_odd()
>   serial: max3100: Replace open-coded parity calculation with
>     parity_odd()
>   lib/bch: Replace open-coded parity calculation with parity_odd()
>   Input: joystick - Replace open-coded parity calculation with
>     parity_odd()
>   net: ethernet: oa_tc6: Replace open-coded parity calculation with
>     parity_odd()
>   wifi: brcm80211: Replace open-coded parity calculation with
>     parity_odd()
>   drm/bridge: dw-hdmi: Replace open-coded parity calculation with
>     parity_odd()
>   mtd: ssfdc: Replace open-coded parity calculation with parity_odd()
>   fsi: i2cr: Replace open-coded parity calculation with parity_odd()
>   nfp: bpf: Replace open-coded parity calculation with parity_odd()
> 
>  arch/x86/kernel/bootflag.c                    |  4 +--
>  drivers/fsi/fsi-master-i2cr.c                 | 20 +++------------
>  .../drm/bridge/synopsys/dw-hdmi-ahb-audio.c   |  8 ++----
>  drivers/hwmon/spd5118.c                       |  2 +-
>  drivers/i3c/master/dw-i3c-master.c            |  2 +-
>  drivers/i3c/master/i3c-master-cdns.c          |  2 +-
>  drivers/i3c/master/mipi-i3c-hci/dat_v1.c      |  2 +-
>  drivers/input/joystick/grip_mp.c              | 17 ++-----------
>  drivers/input/joystick/sidewinder.c           | 25 ++++---------------
>  drivers/media/i2c/saa7115.c                   | 12 ++-------
>  drivers/media/pci/cx18/cx18-av-vbi.c          | 12 ++-------
>  .../media/test-drivers/vivid/vivid-vbi-gen.c  |  8 ++----
>  drivers/mtd/ssfdc.c                           | 20 +++------------
>  drivers/net/ethernet/netronome/nfp/nfp_asm.c  |  7 +-----
>  drivers/net/ethernet/oa_tc6.c                 | 19 +++-----------
>  .../broadcom/brcm80211/brcmsmac/dma.c         | 18 ++-----------
>  drivers/tty/serial/max3100.c                  |  3 ++-
>  include/linux/bitops.h                        | 19 ++++++++------
>  lib/bch.c                                     | 14 +----------
>  19 files changed, 49 insertions(+), 165 deletions(-)

OK, now it looks like a nice consolidation and simplification of code
base. Thanks for the work.

Thanks,
Yury