Re: [PATCH v4 2/2] phy: qualcomm: Add Synopsys High-Speed USB PHY driver

On Fri, Nov 23, 2018 at 03:25:33PM +0530, Kishon Vijay Abraham I wrote:

quoted

+static int qcom_snps_hsphy_por_reset(struct hsphy_priv *priv)
+{
+	int ret;
+
+	ret = reset_control_assert(priv->por_reset);
+	if (ret)
+		return ret;
+
+	/*
+	 * The Femto PHY is POR reset in the following scenarios.
+	 *
+	 * 1. After overriding the parameter registers.
+	 * 2. Low power mode exit from PHY retention.
+	 *
+	 * Ensure that SIDDQ is cleared before bringing the PHY
+	 * out of reset.
+	 */
+	qcom_snps_hsphy_exit_retention(priv);
+
+	/*
+	 * As per databook, 10 usec delay is required between
+	 * PHY POR assert and de-assert.
+	 */
+	usleep_range(10, 20);
+	ret = reset_control_deassert(priv->por_reset);
+	if (ret)
+		return ret;
+
+	/*
+	 * As per databook, it takes 75 usec for PHY to stabilize
+	 * after the reset.
+	 */
+	usleep_range(80, 100);
+
+	/* Ensure that RESET operation is completed. */
+	mb();

How will you ensure the reset operation is complete with a memory barrier? mb
usage here looks incorrect to me.

I agree with you, Kishon.  I think memory barrier only ensures the
register write happens rather than that RESET operation completes.
Since the IO accessors already handle memory barrier, I will just drop
the mb() from here and function qcom_snps_hsphy_init_sequence() as well.

@Sriharsha, let us know if you have a story or reason for these memory
barriers.

Shawn