Re: [Qemu-devel] [PATCH v4 1/3] util/fifo8: implement push/pop of multiple bytes

[Peter Maydell]

On 26 January 2014 21:39, Beniamino Galvani <address@hidden> wrote:
> In some circumstances it is useful to be able to push the entire
> content of a memory buffer to the fifo or to pop multiple bytes with a
> single operation.
>
> The functions fifo8_has_space() and fifo8_push_all() added by this
> patch allow to perform the first kind of operation efficiently.
>
> The function fifo8_pop_buf() can be used instead to pop multiple bytes
> from the fifo, returning a pointer to the backing buffer.


Thanks; a few nits below but mostly this looks OK.


> Signed-off-by: Beniamino Galvani <address@hidden>
> ---
>  include/qemu/fifo8.h |   43 +++++++++++++++++++++++++++++++++++++++++++
>  util/fifo8.c         |   44 ++++++++++++++++++++++++++++++++++++++++++++
>  2 files changed, 87 insertions(+)
>
> diff --git a/include/qemu/fifo8.h b/include/qemu/fifo8.h
> index d318f71..ea6e9f6 100644
> --- a/include/qemu/fifo8.h
> +++ b/include/qemu/fifo8.h
> @@ -44,6 +44,19 @@ void fifo8_destroy(Fifo8 *fifo);
>  void fifo8_push(Fifo8 *fifo, uint8_t data);
>
>  /**
> + * fifo8_push_all:
> + * @fifo: FIFO to push to
> + * @data: data to push
> + * @size: number of bytes to push
> + *
> + * Push a byte array to the FIFO. Behaviour is undefined is the FIFO is

"if the FIFO"

> + * full. Clients are responsible for checking the space left in the FIFO 
> using
> + * fifo8_has_space().
> + */
> +
> +void fifo8_push_all(Fifo8 *fifo, const uint8_t *data, uint32_t num);
> +
> +/**
>   * fifo8_pop:
>   * @fifo: fifo to pop from
>   *
> @@ -56,6 +69,24 @@ void fifo8_push(Fifo8 *fifo, uint8_t data);
>  uint8_t fifo8_pop(Fifo8 *fifo);
>
>  /**
> + * fifo8_pop_buf:
> + * @fifo: FIFO to pop from
> + * @max: maximum number of bytes to pop
> + * @num: actual number of returned bytes
> + *
> + * Pop a number of elements from the FIFO up to a maximum of max. The buffer
> + * containing the popped data is returned. This buffer points directly into
> + * the FIFO backing store and data is invalidated once any of the fifo8_* 
> APIs
> + * are called on the FIFO.
> + *
> + * May short return, the number of valid bytes returned is populated in
> + * *num. Will always return at least 1 byte.

What does "May short return" mean here? Rephrasing might help.

Like fifo8_pop, you should say that clients are responsible
for checking that the fifo is empty before calling this.

> + *
> + * Behavior is undefined if max == 0.
> + */
> +const uint8_t *fifo8_pop_buf(Fifo8 *fifo, uint32_t max, uint32_t *num);
> +
> +/**
>   * fifo8_reset:
>   * @fifo: FIFO to reset
>   *
> @@ -86,6 +117,18 @@ bool fifo8_is_empty(Fifo8 *fifo);
>
>  bool fifo8_is_full(Fifo8 *fifo);
>
> +/**
> + * fifo8_has_space:
> + * @fifo: FIFO to check
> + * @num: number of bytes
> + *
> + * Check if a given number of bytes can be pushed to a FIFO.
> + *
> + * Returns: True if the fifo can hold the given elements, false otherwise.
> + */
> +
> +bool fifo8_has_space(Fifo8 *fifo, uint32_t num);
> +
>  extern const VMStateDescription vmstate_fifo8;
>
>  #define VMSTATE_FIFO8(_field, _state) {                              \
> diff --git a/util/fifo8.c b/util/fifo8.c
> index 013e903..2808169 100644
> --- a/util/fifo8.c
> +++ b/util/fifo8.c
> @@ -15,6 +15,8 @@
>  #include "qemu-common.h"
>  #include "qemu/fifo8.h"
>
> +#define min(a, b) ((a) < (b) ? (a) : (b))

Just use MIN, the QEMU include files define it for you.

>  void fifo8_create(Fifo8 *fifo, uint32_t capacity)
>  {
>      fifo->data = g_new(uint8_t, capacity);
> @@ -37,6 +39,27 @@ void fifo8_push(Fifo8 *fifo, uint8_t data)
>      fifo->num++;
>  }
>
> +void fifo8_push_all(Fifo8 *fifo, const uint8_t *data, uint32_t num)
> +{
> +    uint32_t start, avail;
> +
> +    if (fifo->num + num > fifo->capacity) {
> +        abort();
> +    }
> +
> +    start = (fifo->head + fifo->num) % fifo->capacity;
> +
> +    if (start + num <= fifo->capacity) {
> +        memcpy(&fifo->data[start], data, num);
> +    } else {
> +        avail = fifo->capacity - start;
> +        memcpy(&fifo->data[start], data, avail);
> +        memcpy(&fifo->data[0], &data[avail], num - avail);
> +    }
> +
> +    fifo->num += num;
> +}
> +
>  uint8_t fifo8_pop(Fifo8 *fifo)
>  {
>      uint8_t ret;
> @@ -50,9 +73,25 @@ uint8_t fifo8_pop(Fifo8 *fifo)
>      return ret;
>  }
>
> +const uint8_t *fifo8_pop_buf(Fifo8 *fifo, uint32_t max, uint32_t *num)
> +{
> +    uint8_t *ret;
> +
> +    *num = min(fifo->capacity - fifo->head, max);
> +    *num = min(*num, fifo->num);
> +
> +    ret = &fifo->data[fifo->head];
> +
> +    fifo->head += *num;
> +    fifo->head %= fifo->capacity;
> +
> +    return ret;
> +}
> +
>  void fifo8_reset(Fifo8 *fifo)
>  {
>      fifo->num = 0;
> +    fifo->head = 0;

This is a bug fix, right? It should go in its own patch.

>  }
>
>  bool fifo8_is_empty(Fifo8 *fifo)
> @@ -65,6 +104,11 @@ bool fifo8_is_full(Fifo8 *fifo)
>      return (fifo->num == fifo->capacity);
>  }
>
> +bool fifo8_has_space(Fifo8 *fifo, uint32_t num)
> +{
> +    return (fifo->num + num <= fifo->capacity);
> +}
> +
>  const VMStateDescription vmstate_fifo8 = {
>      .name = "Fifo8",
>      .version_id = 1,
> --
> 1.7.10.4
>

thanks
-- PMM