aspeed/smc: handle dummy bytes when doing fast reads in command mode

When doing fast read, a certain amount of dummy bytes should be sent
before the read. This number is configurable in the controler CE0
Control Register and needs to be modeled using fake transfers to the
flash module.

This only supports command mode. User mode requires more work and a
possible extension of the m25p80 device model.
Signed-off-by: NCédric Le Goater <clg@kaod.org>
Acked-by: NMarcin Krzemiński <mar.krzeminski@gmail.com>
Message-id: 1484751701-2646-1-git-send-email-clg@kaod.org
Signed-off-by: NPeter Maydell <peter.maydell@linaro.org>

hw/ssi/aspeed_smc.c

@@ -69,7 +69,9 @@
 #define R_CTRL0           (0x10 / 4)
 #define   CTRL_CMD_SHIFT           16
 #define   CTRL_CMD_MASK            0xff
+#define   CTRL_DUMMY_HIGH_SHIFT    14
 #define   CTRL_AST2400_SPI_4BYTE   (1 << 13)
+#define   CTRL_DUMMY_LOW_SHIFT     6 /* 2 bits [7:6] */
 #define   CTRL_CE_STOP_ACTIVE      (1 << 2)
 #define   CTRL_CMD_MODE_MASK       0x3
 #define     CTRL_READMODE          0x0
@@ -485,6 +487,16 @@ static uint32_t aspeed_smc_check_segment_addr(const AspeedSMCFlash *fl,
     return addr;
 }
 
+static int aspeed_smc_flash_dummies(const AspeedSMCFlash *fl)
+{
+    const AspeedSMCState *s = fl->controller;
+    uint32_t r_ctrl0 = s->regs[s->r_ctrl0 + fl->id];
+    uint32_t dummy_high = (r_ctrl0 >> CTRL_DUMMY_HIGH_SHIFT) & 0x1;
+    uint32_t dummy_low = (r_ctrl0 >> CTRL_DUMMY_LOW_SHIFT) & 0x3;
+
+    return ((dummy_high << 2) | dummy_low) * 8;
+}
+
 static void aspeed_smc_flash_send_addr(AspeedSMCFlash *fl, uint32_t addr)
 {
     const AspeedSMCState *s = fl->controller;
@@ -521,6 +533,15 @@ static uint64_t aspeed_smc_flash_read(void *opaque, hwaddr addr, unsigned size)
         aspeed_smc_flash_select(fl);
         aspeed_smc_flash_send_addr(fl, addr);
 
+        /*
+         * Use fake transfers to model dummy bytes. The value should
+         * be configured to some non-zero value in fast read mode and
+         * zero in read mode.
+         */
+        for (i = 0; i < aspeed_smc_flash_dummies(fl); i++) {
+            ssi_transfer(fl->controller->spi, 0xFF);
+        }
+
         for (i = 0; i < size; i++) {
             ret |= ssi_transfer(s->spi, 0x0) << (8 * i);
         }