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

[Laurent Pinchart]

Hi Kieran,

On Thu, Mar 05, 2020 at 04:26:03PM +0000, Kieran Bingham wrote:
> 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 ...

The idea is to offer a safe way to access memory in the byte stream
buffer through a typed pointer, without casting explicitly in the caller
as that would be unsafe.

> 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"

I've used the second option.

> > + * \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?

'nullptr' is a value, like NULL
(https://en.cppreference.com/w/cpp/language/nullptr), so I think this is
correct.

> 
> > + *
> > + * \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 ...)

It's supported by both gcc and clang.

> > +		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,

Laurent Pinchart