bios/sdram: improve/simplify read window selection

Compute a score for each window and select the best

diff --git a/litex/soc/software/bios/sdram.c b/litex/soc/software/bios/sdram.c
index fbe3a27e086babbc5fd83e22269f98b0b288b0ef..76c5f13e7ae6091f34a8e822b6a6b386159e2414 100644 (file)
--- a/litex/soc/software/bios/sdram.c
+++ b/litex/soc/software/bios/sdram.c
@@ -377,9 +377,7 @@ static int read_level_scan(int silent)
        unsigned int prv;
        unsigned char prs[DFII_NPHASES*DFII_PIX_DATA_SIZE];
        int p, i, j;
-       int working;
-       int working_delays;
-       int optimal;
+       int score;
 
        if (!silent)
                printf("Read delays scan:\n");
@@ -408,32 +406,26 @@ static int read_level_scan(int silent)
        /* Calibrate each DQ in turn */
        sdram_dfii_pird_address_write(0);
        sdram_dfii_pird_baddress_write(0);
-       working = 0;
-       working_delays = 0;
-       optimal = 1;
+       score = 0;
        for(i=DFII_PIX_DATA_SIZE/2-1;i>=0;i--) {
                if (!silent)
                        printf("m%d: ", (DFII_PIX_DATA_SIZE/2-i-1));
                ddrphy_dly_sel_write(1 << (DFII_PIX_DATA_SIZE/2-i-1));
                ddrphy_rdly_dq_rst_write(1);
                for(j=0; j<ERR_DDRPHY_DELAY;j++) {
-                       int working_delay;
+                       int working;
                        command_prd(DFII_COMMAND_CAS|DFII_COMMAND_CS|DFII_COMMAND_RDDATA);
                        cdelay(15);
-                       working_delay = 1;
+                       working = 1;
                        for(p=0;p<DFII_NPHASES;p++) {
                                if(MMPTR(sdram_dfii_pix_rddata_addr[p]+4*i) != prs[DFII_PIX_DATA_SIZE*p+i])
-                                       working_delay = 0;
+                                       working = 0;
                                if(MMPTR(sdram_dfii_pix_rddata_addr[p]+4*(i+DFII_PIX_DATA_SIZE/2)) != prs[DFII_PIX_DATA_SIZE*p+i+DFII_PIX_DATA_SIZE/2])
-                                       working_delay = 0;
+                                       working = 0;
                        }
-                       working |= working_delay;
-                       working_delays += working_delay;
-                       if ((j == 0) || (j == (ERR_DDRPHY_DELAY-1)))
-                               /* to have an optimal scan, first tap and last tap should not be working */
-                               optimal &= (working_delay == 0);
                        if (!silent)
-                               printf("%d", working_delay);
+                               printf("%d", working);
+                       score += working;
                        ddrphy_rdly_dq_inc_write(1);
                }
                if (!silent)
@@ -446,8 +438,7 @@ static int read_level_scan(int silent)
        command_p0(DFII_COMMAND_RAS|DFII_COMMAND_WE|DFII_COMMAND_CS);
        cdelay(15);
 
-       /* Successful if working and optimal or number of working delays > 3/4 of the taps */
-       return working & (optimal | working_delays > 3*(DFII_PIX_DATA_SIZE/2)*ERR_DDRPHY_DELAY/4);
+       return score;
 }
 
 static void read_level(void)
@@ -732,6 +723,9 @@ int sdrlevel(void)
        int high_skew[DFII_PIX_DATA_SIZE/2];
        int i, j;
        int bitslip;
+       int score;
+       int best_score;
+       int best_bitslip;
 
        for(i=0; i<DFII_PIX_DATA_SIZE/2; i++) {
                ddrphy_dly_sel_write(1<<i);
@@ -749,29 +743,35 @@ int sdrlevel(void)
        if(!write_level(delay, high_skew))
                return 0;
 #endif
-       /* check possible read windows (based on bitslip) */
+       /* scan possible read windows */
+       best_score = 0;
+       best_bitslip = 0;
        for(bitslip=0; bitslip<ERR_DDRPHY_BITSLIP; bitslip++) {
-               /* scan */
-               if (read_level_scan(1))
-                       break;
+               /* compute score */
+               score = read_level_scan(1);
+               if (score > best_score) {
+                       best_bitslip = bitslip;
+                       best_score = score;
+               }
+               /* exit */
                if (bitslip == ERR_DDRPHY_BITSLIP-1)
-                       return 0;
+                       break;
                /* increment bitslip */
                for(i=0; i<DFII_PIX_DATA_SIZE/2; i++)
                        read_bitslip_inc(i);
        }
 
-       /* select read window (bitslip)*/
-       printf("Read bitslip: %d\n", bitslip);
+       /* select best read window */
+       printf("Read bitslip: %d\n", best_bitslip);
        for(i=0; i<DFII_PIX_DATA_SIZE/2; i++) {
                ddrphy_dly_sel_write(1<<i);
                ddrphy_rdly_dq_bitslip_rst_write(1);
-               for (j=0; j<bitslip; j++)
+               for (j=0; j<best_bitslip; j++)
                        read_bitslip_inc(i);
        }
 
-       /* scan selected read window */
-       read_level_scan(0);
+       /* show scan and do leveling */
+    read_level_scan(0);
        read_level();
 
        return 1;