[RFC 1/3] hw: m25p80: add prereading ability in transfer8

[Iris Chen]

With SPI-GPIO we don't have the input bits until
all 8 bits of the output have been shifted out,
so we have to prime the MISO with bogus values (0xFF).

Signed-off-by: Iris Chen <irischenlj@fb.com>
---
 hw/block/m25p80.c    | 29 ++++++++++++++++++++++++++++-
 hw/ssi/ssi.c         |  4 ----
 include/hw/ssi/ssi.h |  5 +++++
 3 files changed, 33 insertions(+), 5 deletions(-)

diff --git a/hw/block/m25p80.c b/hw/block/m25p80.c
index a8d2519141..9b26bb96e5 100644
--- a/hw/block/m25p80.c
+++ b/hw/block/m25p80.c
@@ -1462,7 +1462,7 @@ static int m25p80_cs(SSIPeripheral *ss, bool select)
     return 0;
 }
 
-static uint32_t m25p80_transfer8(SSIPeripheral *ss, uint32_t tx)
+static uint32_t m25p80_transfer8_ex(SSIPeripheral *ss, uint32_t tx)
 {
     Flash *s = M25P80(ss);
     uint32_t r = 0;
@@ -1548,6 +1548,33 @@ static uint32_t m25p80_transfer8(SSIPeripheral *ss, 
uint32_t tx)
     return r;
 }
 
+/*
+ * Pre-reading logic for m25p80_transfer8:
+ * The existing SPI model assumes the output byte is fully formed,
+ * can be passed to the SPI device, and the input byte can be returned,
+ * all in one operation. With SPI-GPIO, we don't have the input byte
+ * until all 8 bits of the output have been shifted out, so we have
+ * to prime the MISO with bogus values (0xFF).
+ */
+static uint32_t m25p80_transfer8(SSIPeripheral *ss, uint32_t tx)
+{
+    Flash *s = M25P80(ss);
+    SSIBus *ssibus = (SSIBus *)qdev_get_parent_bus(DEVICE(s));
+
+    uint8_t prev_state = s->state;
+    uint32_t r = m25p80_transfer8_ex(ss, tx);
+    uint8_t curr_state = s->state;
+
+    if (ssibus->preread &&
+       /* cmd state has changed into DATA reading state */
+       (!(prev_state == STATE_READ || prev_state == STATE_READING_DATA) &&
+       (curr_state == STATE_READ || curr_state == STATE_READING_DATA))) {
+        r = m25p80_transfer8_ex(ss, 0xFF);
+    }
+
+    return r;
+}
+
 static void m25p80_write_protect_pin_irq_handler(void *opaque, int n, int 
level)
 {
     Flash *s = M25P80(opaque);
diff --git a/hw/ssi/ssi.c b/hw/ssi/ssi.c
index 003931fb50..21570fe25f 100644
--- a/hw/ssi/ssi.c
+++ b/hw/ssi/ssi.c
@@ -19,10 +19,6 @@
 #include "qapi/error.h"
 #include "qom/object.h"
 
-struct SSIBus {
-    BusState parent_obj;
-};
-
 #define TYPE_SSI_BUS "SSI"
 OBJECT_DECLARE_SIMPLE_TYPE(SSIBus, SSI_BUS)
 
diff --git a/include/hw/ssi/ssi.h b/include/hw/ssi/ssi.h
index f411858ab0..e54073d822 100644
--- a/include/hw/ssi/ssi.h
+++ b/include/hw/ssi/ssi.h
@@ -30,6 +30,11 @@ enum SSICSMode {
     SSI_CS_HIGH,
 };
 
+struct SSIBus {
+    BusState parent_obj;
+    bool preread;
+};
+
 /* Peripherals.  */
 struct SSIPeripheralClass {
     DeviceClass parent_class;
-- 
2.30.2