issue: 4315534 Remove TX buffer pre-allocation in LwIP

Pre-allocation was done only for PBUF_RAM buffers. Also, buffer
allocation can trigger TX polling in corner cases.

Removing the pre-allocated buffer avoids keeping extra buffer per
socket. This can save resources in case of large buffers and big number
of connections.

Signed-off-by: Dmytro Podgornyi <[email protected]>

Author: pasis

src/core/lwip/tcp.c

@@ -972,7 +972,6 @@ void tcp_pcb_init(struct tcp_pcb *pcb, u8_t prio, void *container)
     pcb->is_in_input = 0;
     pcb->enable_ts_opt = enable_ts_option;
     pcb->seg_alloc = NULL;
-    pcb->pbuf_alloc = NULL;
 }
 
 /**
@@ -1019,55 +1018,33 @@ void tcp_pcb_recycle(struct tcp_pcb *pcb)
         tcp_tx_seg_free(pcb, pcb->seg_alloc);
         pcb->seg_alloc = NULL;
     }
-    if (pcb->pbuf_alloc) {
-        tcp_tx_pbuf_free(pcb, pcb->pbuf_alloc);
-        pcb->pbuf_alloc = NULL;
-    }
 }
 
 struct pbuf *tcp_tx_pbuf_alloc(struct tcp_pcb *pcb, u32_t length, pbuf_type type, pbuf_desc *desc,
                                struct pbuf *p_buff)
 {
     struct pbuf *p;
 
-    if (!pcb->pbuf_alloc || pcb->pbuf_alloc->type != type) {
-
-        // pbuf_alloc is not valid, we should allocate a new pbuf.
-        p = external_tcp_tx_pbuf_alloc(pcb, type, desc, p_buff);
-        if (!p) {
-            return NULL;
-        }
-
-        p->next = NULL;
-        p->type = type;
-        /* set reference count */
-        p->ref = 1;
-        /* set flags */
-        p->flags = 0;
-    } else {
-        // pbuf_alloc is valid, we dont need to allocate a new pbuf element.
-        p = pcb->pbuf_alloc;
-        pcb->pbuf_alloc = NULL;
+    p = external_tcp_tx_pbuf_alloc(pcb, type, desc, p_buff);
+    if (!p) {
+        return NULL;
     }
 
-    /* Set up internal structure of the pbuf. */
+    p->next = NULL;
+    p->type = type;
+    p->ref = 1;
+    p->flags = 0;
     p->len = p->tot_len = length;
 
     return p;
 }
 
-// Release preallocated buffers
 void tcp_tx_preallocted_buffers_free(struct tcp_pcb *pcb)
 {
     if (pcb->seg_alloc) {
         tcp_tx_seg_free(pcb, pcb->seg_alloc);
         pcb->seg_alloc = NULL;
     }
-
-    if (pcb->pbuf_alloc) {
-        tcp_tx_pbuf_free(pcb, pcb->pbuf_alloc);
-        pcb->pbuf_alloc = NULL;
-    }
 }
 
 void tcp_tx_pbuf_free(struct tcp_pcb *pcb, struct pbuf *p)

src/core/lwip/tcp.h

@@ -335,7 +335,6 @@ struct tcp_pcb {
 #endif /* TCP_QUEUE_OOSEQ */
 
     struct tcp_seg *seg_alloc; /* Available tcp_seg element for use */
-    struct pbuf *pbuf_alloc; /* Available pbuf element for use */
 
     /* Function to be called when data is acknowledged. */
     tcp_acked_fn acked_cb;

src/core/lwip/tcp_out.c

@@ -1390,11 +1390,6 @@ err_t tcp_output(struct tcp_pcb *pcb)
         pcb->seg_alloc = tcp_create_segment(pcb, NULL, 0, 0, 0);
     }
 
-    if (!pcb->pbuf_alloc) {
-        // Fetch pbuf for the next packet.
-        pcb->pbuf_alloc = tcp_tx_pbuf_alloc(pcb, 0, PBUF_RAM, NULL, NULL);
-    }
-
     return rc == ERR_WOULDBLOCK ? ERR_OK : rc;
 }