phy: mediatek: tphy: Cleanup and document slew calibration

While it's true that, generally, the T-PHY V3 does not support the
slew calibration process, some minor versions of it actually do,
moreover, some SoCs may not support this even though the version
of the PHY IP does.

The reference clock and rate coefficient parameters are used only
for slew calibration: move those to platform data, then document
and change the checks in hs_slew_rate_calibrate() to perform the
calibration only if:
- EYE value was not supplied (pre-calculated calibration); and
- Slew reference clock value is present (not zero); and
- Slew coefficient is present (not zero).

Moreover, change the probe function to always check if both the
slew reference clock and coefficient properties are present and,
if not, assign the value from platform data (which, as reminder,
if not added means that it's zero!), instead of checking the PHY
IP version.

Signed-off-by: AngeloGioacchino Del Regno <[e-mail]>
Reviewed-by: Neil Armstrong <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Vinod Koul <[e-mail]>

drivers/phy/mediatek/phy-mtk-tphy.c

···
 #define P2F_USB_FM_VALID	BIT(0)
 #define P2F_RG_FRCK_EN		BIT(8)
 
-#define U3P_REF_CLK		26	/* MHZ */
-#define U3P_SLEW_RATE_COEF	28
 #define U3P_SR_COEF_DIVISOR	1000
 #define U3P_FM_DET_CYCLE_CNT	1024
 
···
  * @sw_efuse_supported:       Switches off eFuse auto-load from PHY and applies values
  *                            read from different nvmem (usually different eFuse array)
  *                            that is pointed at in the device tree node for this PHY
+ * @slew_ref_clk_mhz:         Default reference clock (in MHz) for slew rate calibration
+ * @slew_rate_coefficient:    Coefficient for slew rate calibration
  * @version:                  PHY IP Version
  */
 struct mtk_phy_pdata {
 	bool avoid_rx_sen_degradation;
 	bool sw_pll_48m_to_26m;
 	bool sw_efuse_supported;
+	u8 slew_ref_clock_mhz;
+	u8 slew_rate_coefficient;
 	enum mtk_phy_version version;
 };
 
···
 	int fm_out;
 	u32 tmp;
 
-	/* HW V3 doesn't support slew rate cal anymore */
-	if (tphy->pdata->version == MTK_PHY_V3)
-		return;
-
-	/* use force value */
-	if (instance->eye_src)
+	/*
+	 * If a fixed HS slew rate (EYE) value was supplied, don't run the
+	 * calibration sequence and prefer using that value instead; also,
+	 * if there is no reference clock for slew calibration or there is
+	 * no slew coefficient, this means that the slew rate calibration
+	 * sequence is not supported.
+	 */
+	if (instance->eye_src || !tphy->src_ref_clk || !tphy->src_coef)
 		return;
 
 	/* enable USB ring oscillator */
···
 
 static const struct mtk_phy_pdata tphy_v1_pdata = {
 	.avoid_rx_sen_degradation = false,
+	.slew_ref_clock_mhz = 26,
+	.slew_rate_coefficient = 28,
 	.version = MTK_PHY_V1,
 };
 
 static const struct mtk_phy_pdata tphy_v2_pdata = {
 	.avoid_rx_sen_degradation = false,
 	.sw_efuse_supported = true,
+	.slew_ref_clock_mhz = 26,
+	.slew_rate_coefficient = 28,
 	.version = MTK_PHY_V2,
 };
 
···
 
 static const struct mtk_phy_pdata mt8173_pdata = {
 	.avoid_rx_sen_degradation = true,
+	.slew_ref_clock_mhz = 26,
+	.slew_rate_coefficient = 28,
 	.version = MTK_PHY_V1,
 };
 
···
 	struct resource *sif_res;
 	struct mtk_tphy *tphy;
 	struct resource res;
-	int port;
+	int port, ret;
 
 	tphy = devm_kzalloc(dev, sizeof(*tphy), GFP_KERNEL);
 	if (!tphy)
···
 		}
 	}
 
-	if (tphy->pdata->version < MTK_PHY_V3) {
-		tphy->src_ref_clk = U3P_REF_CLK;
-		tphy->src_coef = U3P_SLEW_RATE_COEF;
-		/* update parameters of slew rate calibrate if exist */
-		device_property_read_u32(dev, "mediatek,src-ref-clk-mhz",
-					 &tphy->src_ref_clk);
-		device_property_read_u32(dev, "mediatek,src-coef",
-					 &tphy->src_coef);
-	}
+	/* Optional properties for slew calibration variation */
+	ret = device_property_read_u32(dev, "mediatek,src-ref-clk-mhz", &tphy->src_ref_clk);
+	if (ret)
+		tphy->src_ref_clk = tphy->pdata->slew_ref_clock_mhz;
+
+	ret = device_property_read_u32(dev, "mediatek,src-coef", &tphy->src_coef);
+	if (ret)
+		tphy->src_coef = tphy->pdata->slew_rate_coefficient;
 
 	port = 0;
 	for_each_child_of_node_scoped(np, child_np) {