[PATCH v3 1/5] hw/char/pl011: refactor FIFO depth handling code

[Evgeny Iakovlev <eiakovlev@linux.microsoft.com>]

PL011 can be in either of 2 modes depending guest config: FIFO and
single register. The last mode could be viewed as a 1-element-deep FIFO.

Current code open-codes a bunch of depth-dependent logic. Refactor FIFO
depth handling code to isolate calculating current FIFO depth.

One functional (albeit guest-invisible) side-effect of this change is
that previously we would always increment s->read_pos in UARTDR read
handler even if FIFO was disabled, now we are limiting read_pos to not
exceed FIFO depth (read_pos itself is reset to 0 if user disables FIFO).

Signed-off-by: Evgeny Iakovlev <eiakovlev@linux.microsoft.com>
---
 hw/char/pl011.c         | 30 ++++++++++++++++++------------
 include/hw/char/pl011.h |  5 ++++-
 2 files changed, 22 insertions(+), 13 deletions(-)

diff --git a/hw/char/pl011.c b/hw/char/pl011.c
index c076813423..3fa3b75d04 100644
--- a/hw/char/pl011.c
+++ b/hw/char/pl011.c
@@ -81,6 +81,17 @@ static void pl011_update(PL011State *s)
     }
 }
 
+static bool pl011_is_fifo_enabled(PL011State *s)
+{
+    return (s->lcr & 0x10) != 0;
+}
+
+static inline unsigned pl011_get_fifo_depth(PL011State *s)
+{
+    /* Note: FIFO depth is expected to be power-of-2 */
+    return pl011_is_fifo_enabled(s) ? PL011_FIFO_DEPTH : 1;
+}
+
 static uint64_t pl011_read(void *opaque, hwaddr offset,
                            unsigned size)
 {
@@ -94,8 +105,7 @@ static uint64_t pl011_read(void *opaque, hwaddr offset,
         c = s->read_fifo[s->read_pos];
         if (s->read_count > 0) {
             s->read_count--;
-            if (++s->read_pos == 16)
-                s->read_pos = 0;
+            s->read_pos = (s->read_pos + 1) & (pl011_get_fifo_depth(s) - 1);
         }
         if (s->read_count == 0) {
             s->flags |= PL011_FLAG_RXFE;
@@ -273,11 +283,7 @@ static int pl011_can_receive(void *opaque)
     PL011State *s = (PL011State *)opaque;
     int r;
 
-    if (s->lcr & 0x10) {
-        r = s->read_count < 16;
-    } else {
-        r = s->read_count < 1;
-    }
+    r = s->read_count < pl011_get_fifo_depth(s);
     trace_pl011_can_receive(s->lcr, s->read_count, r);
     return r;
 }
@@ -286,15 +292,15 @@ static void pl011_put_fifo(void *opaque, uint32_t value)
 {
     PL011State *s = (PL011State *)opaque;
     int slot;
+    unsigned pipe_depth;
 
-    slot = s->read_pos + s->read_count;
-    if (slot >= 16)
-        slot -= 16;
+    pipe_depth = pl011_get_fifo_depth(s);
+    slot = (s->read_pos + s->read_count) & (pipe_depth - 1);
     s->read_fifo[slot] = value;
     s->read_count++;
     s->flags &= ~PL011_FLAG_RXFE;
     trace_pl011_put_fifo(value, s->read_count);
-    if (!(s->lcr & 0x10) || s->read_count == 16) {
+    if (s->read_count == pipe_depth) {
         trace_pl011_put_fifo_full();
         s->flags |= PL011_FLAG_RXFF;
     }
@@ -359,7 +365,7 @@ static const VMStateDescription vmstate_pl011 = {
         VMSTATE_UINT32(dmacr, PL011State),
         VMSTATE_UINT32(int_enabled, PL011State),
         VMSTATE_UINT32(int_level, PL011State),
-        VMSTATE_UINT32_ARRAY(read_fifo, PL011State, 16),
+        VMSTATE_UINT32_ARRAY(read_fifo, PL011State, PL011_FIFO_DEPTH),
         VMSTATE_UINT32(ilpr, PL011State),
         VMSTATE_UINT32(ibrd, PL011State),
         VMSTATE_UINT32(fbrd, PL011State),
diff --git a/include/hw/char/pl011.h b/include/hw/char/pl011.h
index dc2c90eedc..926322e242 100644
--- a/include/hw/char/pl011.h
+++ b/include/hw/char/pl011.h
@@ -27,6 +27,9 @@ OBJECT_DECLARE_SIMPLE_TYPE(PL011State, PL011)
 /* This shares the same struct (and cast macro) as the base pl011 device */
 #define TYPE_PL011_LUMINARY "pl011_luminary"
 
+/* Depth of UART FIFO in bytes, when FIFO mode is enabled (else depth == 1) */
+#define PL011_FIFO_DEPTH 16
+
 struct PL011State {
     SysBusDevice parent_obj;
 
@@ -39,7 +42,7 @@ struct PL011State {
     uint32_t dmacr;
     uint32_t int_enabled;
     uint32_t int_level;
-    uint32_t read_fifo[16];
+    uint32_t read_fifo[PL011_FIFO_DEPTH];
     uint32_t ilpr;
     uint32_t ibrd;
     uint32_t fbrd;
-- 
2.34.1