hw/ssi/pnv_spi: Put a limit to RDR match failures

There is a possibility that SPI controller can get into loop due to indefinite
RDR match failures. Hence put a limit to failures and stop the sequencer.

Signed-off-by: Chalapathi V <chalapathi.v@linux.ibm.com>
Reviewed-by: Nicholas Piggin <npiggin@gmail.com>
Message-ID: <20250303141328.23991-5-chalapathi.v@linux.ibm.com>
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>

hw/ssi/pnv_spi.c

@@ -20,6 +20,7 @@
 #define PNV_SPI_OPCODE_LO_NIBBLE(x) (x & 0x0F)
 #define PNV_SPI_MASKED_OPCODE(x) (x & 0xF0)
 #define PNV_SPI_FIFO_SIZE 16
+#define RDR_MATCH_FAILURE_LIMIT 16
 
 /*
  * Macro from include/hw/ppc/fdt.h
@@ -872,18 +873,27 @@ static void operation_sequencer(PnvSpi *s)
                 rdr_matched = does_rdr_match(s);
                 if (rdr_matched) {
                     trace_pnv_spi_RDR_match("success");
+                    s->fail_count = 0;
                     /* A match occurred, increment the sequencer index. */
                     seq_index++;
                     s->status = SETFIELD(SPI_STS_SEQ_FSM, s->status,
                                     SEQ_STATE_INDEX_INCREMENT);
                 } else {
                     trace_pnv_spi_RDR_match("failed");
+                    s->fail_count++;
                     /*
                      * Branch the sequencer to the index coded into the op
                      * code.
                      */
                     seq_index = PNV_SPI_OPCODE_LO_NIBBLE(opcode);
                 }
+                if (s->fail_count >= RDR_MATCH_FAILURE_LIMIT) {
+                    qemu_log_mask(LOG_GUEST_ERROR, "pnv_spi: RDR match failure"
+                                  " limit crossed %d times hence requesting "
+                                  "sequencer to stop.\n",
+                                  RDR_MATCH_FAILURE_LIMIT);
+                    stop = true;
+                }
                 /*
                  * Regardless of where the branch ended up we want the
                  * sequencer to continue shifting so we have to clear

include/hw/ssi/pnv_spi.h

@@ -40,6 +40,7 @@ typedef struct PnvSpi {
     MemoryRegion    xscom_spic_regs;
     Fifo8 tx_fifo;
     Fifo8 rx_fifo;
+    uint8_t fail_count; /* RDR Match failure counter */
     /* SPI object number */
     uint32_t        spic_num;
     uint32_t        chip_id;