Hello Thomas,
On 04/21/2015 11:56 AM, Thomas Gleixner wrote:
On Fri, 17 Apr 2015, Peter Griffin wrote:
+/* Low Power Timer */
+#define LPC_LPT_LSB_OFF 0x400
+#define LPC_LPT_MSB_OFF 0x404
+#define LPC_LPT_START_OFF 0x408
+
+struct st_lpc {
+ struct clk *clk;
+ void __iomem *iomem_cs;
+};
+
+static struct st_lpc *st_lpc;
+
+static u64 notrace st_lpc_counter_read(void)
+{
+ u64 counter;
+ u32 lower;
+ u32 upper, old_upper;
+
+ upper = readl_relaxed(st_lpc->iomem_cs + LPC_LPT_MSB_OFF);
+ do {
+ old_upper = upper;
+ lower = readl_relaxed(st_lpc->iomem_cs + LPC_LPT_LSB_OFF);
+ upper = readl_relaxed(st_lpc->iomem_cs + LPC_LPT_MSB_OFF);
+ } while (upper != old_upper);
+
+ counter = upper;
+ counter <<= 32;
+ counter |= lower;
+ return counter;
What's the point of this exercise? The kernel can handle 32bit
clocksources nicely. So why do you want to artificially expand them to
64bit by adding useless loops and hoops to a hotpath?
I agree we should use only the lower 32 bits.
This timer is moreover clocked at a slow rate (30MHz), so we won't have
too much interrupts.
Peter, doing that, you could use something like this directly:
clocksource_mmio_init(st_lpc->iomem_cs + LPC_LPT_LSB_OFF,
"st_lpc_timer", rate,
200, 24, clocksource_mmio_readl_down);
Thanks,
Maxime
Thanks,
tglx
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