[libcamera-devel] [PATCH 24/31] libcamera: byte_stream_buffer: Add zero-copy read() variant

[Kieran Bingham]

On 29/02/2020 16:42, Laurent Pinchart wrote:
> Add a read() function to ByteStreamBuffer that returns a pointer to the
> data instead of copying it. Overflow check is still handled by the
> class, but the caller must check the returned pointer explicitly.
> 

I'm wondering why this doesn't return a span, but perhaps I'm about to
find out in a later patch if this is to implement some feature for
handling span ...

But still - this byte_stream_buffer.h/cpp knows how to deal with spans
already...

> Signed-off-by: Laurent Pinchart <laurent.pinchart at ideasonboard.com>

Minors aside,

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

> ---
>  src/libcamera/byte_stream_buffer.cpp       | 39 ++++++++++++++++++++++
>  src/libcamera/include/byte_stream_buffer.h |  9 +++++
>  2 files changed, 48 insertions(+)
> 
> diff --git a/src/libcamera/byte_stream_buffer.cpp b/src/libcamera/byte_stream_buffer.cpp
> index 40380bf0434a..576f556f54a7 100644
> --- a/src/libcamera/byte_stream_buffer.cpp
> +++ b/src/libcamera/byte_stream_buffer.cpp
> @@ -241,6 +241,19 @@ int ByteStreamBuffer::skip(size_t size)
>   * \retval -ENOSPC no more space is available in the managed memory buffer
>   */
>  
> +/**
> + * \fn template<typename T> const T *ByteStreamBuffer::read(size_t count)
> + * \brief Read data from the managed memory buffer without copy

without a copy? or "without performing a copy"

> + * \param[in] count Number of data items to read
> + *
> + * This function read \a count elements of type \a T from the buffer. Unlike


s/read/reads/

> + * the other read variants, it doesn't copy the data but returns a pointer to
> + * the first element. If data can't be read for any reason (usually due to
> + * reading more data than available), the function returns nullptr.

returns 'a' nullptr?

> + *
> + * \return A pointer to the data on success, or nullptr otherwise
> + */
> +
>  /**
>   * \fn template<typename T> int ByteStreamBuffer::write(const T *t)
>   * \brief Write \a t to the managed memory buffer
> @@ -259,6 +272,32 @@ int ByteStreamBuffer::skip(size_t size)
>   * \retval -ENOSPC no more space is available in the managed memory buffer
>   */
>  
> +const uint8_t *ByteStreamBuffer::read(size_t size, size_t count)
> +{
> +	if (!read_)
> +		return nullptr;
> +
> +	if (overflow_)
> +		return nullptr;
> +
> +	size_t bytes;
> +	if (__builtin_mul_overflow(size, count, &bytes)) {

Is this a gcc builtin? or supported in clang too? (I assume it is ...)

> +		setOverflow();
> +		return nullptr;
> +	}
> +
> +	if (read_ + bytes > base_ + size_) {
> +		LOG(Serialization, Error)
> +			<< "Unable to read " << bytes << " bytes: out of bounds";
> +		setOverflow();
> +		return nullptr;
> +	}
> +
> +	const uint8_t *data = read_;
> +	read_ += bytes;
> +	return data;
> +}
> +
>  int ByteStreamBuffer::read(uint8_t *data, size_t size)
>  {
>  	if (!read_)
> diff --git a/src/libcamera/include/byte_stream_buffer.h b/src/libcamera/include/byte_stream_buffer.h
> index 17cb0146061e..b3aaa8b9fb28 100644
> --- a/src/libcamera/include/byte_stream_buffer.h
> +++ b/src/libcamera/include/byte_stream_buffer.h
> @@ -9,6 +9,7 @@
>  
>  #include <stddef.h>
>  #include <stdint.h>
> +#include <type_traits>
>  
>  #include <libcamera/span.h>
>  
> @@ -43,6 +44,13 @@ public:
>  			    data.size_bytes());
>  	}
>  
> +	template<typename T>
> +	const std::remove_reference_t<T> *read(size_t count = 1)
> +	{
> +		using return_type = const std::remove_reference_t<T> *;
> +		return reinterpret_cast<return_type>(read(sizeof(T), count));
> +	}
> +
>  	template<typename T>
>  	int write(const T *t)
>  	{
> @@ -63,6 +71,7 @@ private:
>  	void setOverflow();
>  
>  	int read(uint8_t *data, size_t size);
> +	const uint8_t *read(size_t size, size_t count);
>  	int write(const uint8_t *data, size_t size);
>  
>  	ByteStreamBuffer *parent_;
> 

-- 
Regards
--
Kieran