e1000: Optimize recv to avoid MMIO writes

fwsGonzo · fwsGonzo · commit b736c9179db1 · 2018-01-15T21:32:16.000+01:00
diff --git a/src/drivers/e1000.cpp b/src/drivers/e1000.cpp
@@ -216,6 +216,8 @@ void e1000::event_handler()
 
 void e1000::recv_handler()
 {
+  uint16_t old_idx = 0xffff;
+
   while (rx.desc[rx.current].status & 1)
   {
     auto& tk = rx.desc[rx.current];
@@ -229,10 +231,11 @@ void e1000::recv_handler()
     tk.addr = (uint64_t) this->new_rx_packet();
     tk.status = 0;
     // go to next index
-    uint16_t old_idx = rx.current;
+    old_idx = rx.current;
     rx.current = (rx.current + 1) % NUM_RX_DESC;
-    write_cmd(REG_RXDESCTAIL, old_idx);
   }
+  if (old_idx != 0xffff)
+    write_cmd(REG_RXDESCTAIL, old_idx);
 }
 
 void e1000::transmit(net::Packet_ptr pckt)

Original file line number	Diff line number	Diff line change
`@@ -216,6 +216,8 @@ void e1000::event_handler()`
`216`	`216`
`217`	`217`	`void e1000::recv_handler()`
`218`	`218`	`{`
	`219`	`+ uint16_t old_idx = 0xffff;`
	`220`	`+`
`219`	`221`	`while (rx.desc[rx.current].status & 1)`
`220`	`222`	`{`
`221`	`223`	`auto& tk = rx.desc[rx.current];`
`@@ -229,10 +231,11 @@ void e1000::recv_handler()`
`229`	`231`	`tk.addr = (uint64_t) this->new_rx_packet();`
`230`	`232`	`tk.status = 0;`
`231`	`233`	`// go to next index`
`232`		`- uint16_t old_idx = rx.current;`
	`234`	`+ old_idx = rx.current;`
`233`	`235`	`rx.current = (rx.current + 1) % NUM_RX_DESC;`
`234`		`- write_cmd(REG_RXDESCTAIL, old_idx);`
`235`	`236`	`}`
	`237`	`+ if (old_idx != 0xffff)`
	`238`	`+ write_cmd(REG_RXDESCTAIL, old_idx);`
`236`	`239`	`}`
`237`	`240`
`238`	`241`	`void e1000::transmit(net::Packet_ptr pckt)`