static
void *i2400m_tx_fifo_push(struct i2400m *i2400m, size_t size,
size_t padding, bool try_head)
{
struct device *dev = i2400m_dev(i2400m);
size_t room, tail_room, needed_size;
void *ptr;
needed_size = size + padding;
room = I2400M_TX_BUF_SIZE - (i2400m->tx_in - i2400m->tx_out);
if (room < needed_size) {
d_printf(2, dev, "fifo push %zu/%zu: no space\n",
size, padding);
return NULL;
}
tail_room = __i2400m_tx_tail_room(i2400m);
if (!try_head && tail_room < needed_size) {
if (room - tail_room >= needed_size) {
d_printf(2, dev, "fifo push %zu/%zu: tail full\n",
size, padding);
return TAIL_FULL;
} else {
d_printf(2, dev, "fifo push %zu/%zu: no head space\n",
size, padding);
return NULL;
}
}
ptr = i2400m->tx_buf + i2400m->tx_in % I2400M_TX_BUF_SIZE;
d_printf(2, dev, "fifo push %zu/%zu: at @%zu\n", size, padding,
i2400m->tx_in % I2400M_TX_BUF_SIZE);
i2400m->tx_in += size;
return ptr;
}
static
void i2400m_tx_skip_tail(struct i2400m *i2400m)
{
struct device *dev = i2400m_dev(i2400m);
size_t tx_in = i2400m->tx_in % I2400M_TX_BUF_SIZE;
size_t tail_room = __i2400m_tx_tail_room(i2400m);
struct i2400m_msg_hdr *msg = i2400m->tx_buf + tx_in;
if (unlikely(tail_room == 0))
return;
BUG_ON(tail_room < sizeof(*msg));
msg->size = tail_room | I2400M_TX_SKIP;
d_printf(2, dev, "skip tail: skipping %zu bytes @%zu\n",
tail_room, tx_in);
i2400m->tx_in += tail_room;
}
/*
* Allocate @size bytes in the TX fifo, return a pointer to it
*
* @i2400m: device descriptor
* @size: size of the buffer we need to allocate
* @padding: ensure that there is at least this many bytes of free
* contiguous space in the fifo. This is needed because later on
* we might need to add padding.
* @try_head: specify either to allocate head room or tail room space
* in the TX FIFO. This boolean is required to avoids a system hang
* due to an infinite loop caused by i2400m_tx_fifo_push().
* The caller must always try to allocate tail room space first by
* calling this routine with try_head = 0. In case if there
* is not enough tail room space but there is enough head room space,
* (i2400m_tx_fifo_push() returns TAIL_FULL) try to allocate head
* room space, by calling this routine again with try_head = 1.
*
* Returns:
*
* Pointer to the allocated space. NULL if there is no
* space. TAIL_FULL if there is no space at the tail but there is at
* the head (Case B below).
*
* These are the two basic cases we need to keep an eye for -- it is
* much better explained in linux/kernel/kfifo.c, but this code
* basically does the same. No rocket science here.
*
* Case A Case B
* N ___________ ___________
* | tail room | | data |
* | | | |
* |<- IN ->| |<- OUT ->|
* | | | |
* | data | | room |
* | | | |
* |<- OUT ->| |<- IN ->|
* | | | |
* | head room | | data |
* 0 ----------- -----------
*
* We allocate only *contiguous* space.
*
* We can allocate only from 'room'. In Case B, it is simple; in case
* A, we only try from the tail room; if it is not enough, we just
* fail and return TAIL_FULL and let the caller figure out if we wants to
* skip the tail room and try to allocate from the head.
*
* There is a corner case, wherein i2400m_tx_new() can get into
* an infinite loop calling i2400m_tx_fifo_push().
* In certain situations, tx_in would have reached on the top of TX FIFO
* and i2400m_tx_tail_room() returns 0, as described below:
*
* N ___________ tail room is zero
* |<- IN ->|
* | |
* | |
* | |
* | data |
* |<- OUT ->|
* | |
* | |
* | head room |
* 0 -----------
* During such a time, where tail room is zero in the TX FIFO and if there
* is a request to add a payload to TX FIFO, which calls:
* i2400m_tx()
* ->calls i2400m_tx_close()
* ->calls i2400m_tx_skip_tail()
* goto try_new;
* ->calls i2400m_tx_new()
* |----> [try_head:]
* infinite loop | ->calls i2400m_tx_fifo_push()
* | if (tail_room < needed)
* | if (head_room => needed)
* | return TAIL_FULL;
* |<---- goto try_head;
*
* i2400m_tx() calls i2400m_tx_close() to close the message, since there
* is no tail room to accommodate the payload and calls
* i2400m_tx_skip_tail() to skip the tail space. Now i2400m_tx() calls
* i2400m_tx_new() to allocate space for new message header calling
* i2400m_tx_fifo_push() that returns TAIL_FULL, since there is no tail space
* to accommodate the message header, but there is enough head space.
* The i2400m_tx_new() keeps re-retrying by calling i2400m_tx_fifo_push()
* ending up in a loop causing system freeze.
*
* This corner case is avoided by using a try_head boolean,
* as an argument to i2400m_tx_fifo_push().
*
* Note:
*
* Assumes i2400m->tx_lock is taken, and we use that as a barrier
*
* The indexes keep increasing and we reset them to zero when we
* pop data off the queue
*/
static
void *i2400m_tx_fifo_push(struct i2400m *i2400m, size_t size,
size_t padding, bool try_head)
{
struct device *dev = i2400m_dev(i2400m);
size_t room, tail_room, needed_size;
void *ptr;
needed_size = size + padding;
room = I2400M_TX_BUF_SIZE - (i2400m->tx_in - i2400m->tx_out);
if (room < needed_size) { /* this takes care of Case B */
d_printf(2, dev, "fifo push %zu/%zu: no space\n",
size, padding);
return NULL;
}
/* Is there space at the tail? */
tail_room = __i2400m_tx_tail_room(i2400m);
if (!try_head && tail_room < needed_size) {
/*
* If the tail room space is not enough to push the message
* in the TX FIFO, then there are two possibilities:
* 1. There is enough head room space to accommodate
* this message in the TX FIFO.
* 2. There is not enough space in the head room and
* in tail room of the TX FIFO to accommodate the message.
* In the case (1), return TAIL_FULL so that the caller
* can figure out, if the caller wants to push the message
* into the head room space.
* In the case (2), return NULL, indicating that the TX FIFO
* cannot accommodate the message.
*/
if (room - tail_room >= needed_size) {
d_printf(2, dev, "fifo push %zu/%zu: tail full\n",
size, padding);
return TAIL_FULL; /* There might be head space */
} else {
d_printf(2, dev, "fifo push %zu/%zu: no head space\n",
size, padding);
return NULL; /* There is no space */
}
}
ptr = i2400m->tx_buf + i2400m->tx_in % I2400M_TX_BUF_SIZE;
d_printf(2, dev, "fifo push %zu/%zu: at @%zu\n", size, padding,
i2400m->tx_in % I2400M_TX_BUF_SIZE);
i2400m->tx_in += size;
return ptr;
}