hw/char/pl011: refactor FIFO depth handling code

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>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-id: 20230123162304.26254-2-eiakovlev@linux.microsoft.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>

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;