drivers/perf: fsl_imx8_ddr: Correct the CLEAR bit definition

When disabling a counter from ddr_perf_event_stop(), the counter value
is reset to 0 at the same time.

Preserve the counter value by performing a read-modify-write of the
PMU register and clearing only the enable bit.
Signed-off-by: Joakim Zhang <qiangqing.zhang@nxp.com>
Signed-off-by: Will Deacon <will@kernel.org>

drivers/perf/fsl_imx8_ddr_perf.c

@@ -388,9 +388,10 @@ static void ddr_perf_counter_enable(struct ddr_pmu *pmu, int config,
 
 	if (enable) {
 		/*
-		 * must disable first, then enable again
-		 * otherwise, cycle counter will not work
-		 * if previous state is enabled.
+		 * cycle counter is special which should firstly write 0 then
+		 * write 1 into CLEAR bit to clear it. Other counters only
+		 * need write 0 into CLEAR bit and it turns out to be 1 by
+		 * hardware. Below enable flow is harmless for all counters.
 		 */
 		writel(0, pmu->base + reg);
 		val = CNTL_EN | CNTL_CLEAR;
@@ -398,7 +399,8 @@ static void ddr_perf_counter_enable(struct ddr_pmu *pmu, int config,
 		writel(val, pmu->base + reg);
 	} else {
 		/* Disable counter */
-		writel(0, pmu->base + reg);
+		val = readl_relaxed(pmu->base + reg) & CNTL_EN_MASK;
+		writel(val, pmu->base + reg);
 	}
 }