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

[Peter Crosthwaite]

On Tue, Jan 28, 2014 at 4:32 AM, Peter Maydell <address@hidden> wrote:
> 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.
>

No bug - where the ring buffer starts following a reset is undefined
and need not be defined. But it improves the predicatability of the
newly added pop_buf fn as you can now following a reset, guarantee
that a single pop_buf will take all contents if its the first pop
(which is how its being used in P2).

>>  }
>>
>>  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);
>> +}
>> +

This is a little specific and perhaps limited in usage. Can we just
add occupancy/vacancy getters and let the caller do what it wants:

uint32_t fifo8_num_free(Fifo *fifo) {
    return fifo->capacity - fifo->num;
}

Regards,
Peter

>>  const VMStateDescription vmstate_fifo8 = {
>>      .name = "Fifo8",
>>      .version_id = 1,
>> --
>> 1.7.10.4
>>
>
> thanks
> -- PMM
>