[PATCH v1] libcamera: v4l2_videodevice: `lastUsedCounter_` need not be atomic

[Kieran Bingham]

Quoting Barnabás Pőcze (2025-03-10 17:03:26)
> The `V4L2BufferCache` type is not thread-safe. Its `lastUsedCounter_`
> member is not used in contexts where its atomicity would matter.
> So it does not need to be have an atomic type.

Any indications as to why this was using atomic ? Maybe just
cautious development when it was first put in ?

As long as we're convinced it's still safe and passes the tests, I'm
fine with it. It's certainly simpler code.

Reviewed-by: Kieran Bingham <kieran.bingham at ideasonboard.com>

> Signed-off-by: Barnabás Pőcze <barnabas.pocze at ideasonboard.com>
> ---
>  include/libcamera/internal/v4l2_videodevice.h | 2 +-
>  src/libcamera/v4l2_videodevice.cpp            | 4 ++--
>  2 files changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/include/libcamera/internal/v4l2_videodevice.h b/include/libcamera/internal/v4l2_videodevice.h
> index f021c2a01..e52b50c67 100644
> --- a/include/libcamera/internal/v4l2_videodevice.h
> +++ b/include/libcamera/internal/v4l2_videodevice.h
> @@ -158,7 +158,7 @@ private:
>                 std::vector<Plane> planes_;
>         };
>  
> -       std::atomic<uint64_t> lastUsedCounter_;
> +       uint64_t lastUsedCounter_;
>         std::vector<Entry> cache_;
>         /* \todo Expose the miss counter through an instrumentation API. */
>         unsigned int missCounter_;
> diff --git a/src/libcamera/v4l2_videodevice.cpp b/src/libcamera/v4l2_videodevice.cpp
> index e241eb47b..f5b3fa09d 100644
> --- a/src/libcamera/v4l2_videodevice.cpp
> +++ b/src/libcamera/v4l2_videodevice.cpp
> @@ -190,7 +190,7 @@ V4L2BufferCache::V4L2BufferCache(const std::vector<std::unique_ptr<FrameBuffer>>
>  {
>         for (const std::unique_ptr<FrameBuffer> &buffer : buffers)
>                 cache_.emplace_back(true,
> -                                   lastUsedCounter_.fetch_add(1, std::memory_order_acq_rel),
> +                                   lastUsedCounter_++,
>                                     *buffer.get());
>  }
>  
> @@ -258,7 +258,7 @@ int V4L2BufferCache::get(const FrameBuffer &buffer)
>                 return -ENOENT;
>  
>         cache_[use] = Entry(false,
> -                           lastUsedCounter_.fetch_add(1, std::memory_order_acq_rel),
> +                           lastUsedCounter_++,
>                             buffer);
>  
>         return use;
> -- 
> 2.48.1
>