media: cxd2841er: avoid too many status inquires

I2C ops are expensive, as the I2C bus typical speed is 100kbps.

Also, stats reading take some time, as it requires to retrieve a
certain number of packets to complete.

While we don't know the minimal for CXD2841er, trying to do it
too quickly is still a very bad idea.

So, add some sanity logic there, preventing to retrieve stats
faster than one second.

This shouldn't cause any issues with well behavior apps, as they
usually take stats on a polling rate slower than 1 second.

Signed-off-by: Mauro Carvalho Chehab <[e-mail]>
Signed-off-by: Sean Young <[e-mail]>

drivers/media/dvb-frontends/cxd2841er.c

···
 	enum cxd2841er_xtal		xtal;
 	enum fe_caps caps;
 	u32				flags;
+	unsigned long			stats_time;
 };
 
 static const struct cxd2841er_cnr_data s_cn_data[] = {
···
 		p->strength.stat[0].scale = FE_SCALE_NOT_AVAILABLE;
 
 	if (status & FE_HAS_LOCK) {
+		if (priv->stats_time &&
+		    (!time_after(jiffies, priv->stats_time)))
+			return 0;
+
+		/* Prevent retrieving stats faster than once per second */
+		priv->stats_time = jiffies + msecs_to_jiffies(1000);
+
 		cxd2841er_read_snr(fe);
 		cxd2841er_read_ucblocks(fe);
-
 		cxd2841er_read_ber(fe);
 	} else {
 		p->cnr.stat[0].scale = FE_SCALE_NOT_AVAILABLE;
···
 	p->block_error.stat[0].scale = FE_SCALE_NOT_AVAILABLE;
 	p->post_bit_error.stat[0].scale = FE_SCALE_NOT_AVAILABLE;
 	p->post_bit_count.stat[0].scale = FE_SCALE_NOT_AVAILABLE;
+
+	/* Reset the wait for jiffies logic */
+	priv->stats_time = 0;
 
 	return ret;
 }