FROMLIST: PCI: qcom: Add support for host_stop_link() & host_start_link()

Add support for host_stop_link() and host_start_link() for switches like
TC956x, which require configuration before the PCIe link is established.

Assert PERST# and disable LTSSM bit to prevent the PCIe controller from
participating in link training during host_stop_link(). De-assert PERST#
and enable LTSSM bit during host_start_link().

Introduce ltssm_disable function op to stop link training.
For the switches like TC956x, which needs to configure it before
the PCIe link is established.

Link: https://lore.kernel.org/lkml/20250412-qps615_v4_1-v5-0-5b6a06132fec@oss.qualcomm.com/
Signed-off-by: Krishna Chaitanya Chundru <krishna.chundru@oss.qualcomm.com>

Author: krishnachaitanya-linux

drivers/pci/controller/dwc/pcie-qcom.c

@@ -247,6 +247,7 @@ struct qcom_pcie_ops {
 	void (*host_post_init)(struct qcom_pcie *pcie);
 	void (*deinit)(struct qcom_pcie *pcie);
 	void (*ltssm_enable)(struct qcom_pcie *pcie);
+	void (*ltssm_disable)(struct qcom_pcie *pcie);
 	int (*config_sid)(struct qcom_pcie *pcie);
 };
 
@@ -618,6 +619,37 @@ static int qcom_pcie_post_init_1_0_0(struct qcom_pcie *pcie)
 	return 0;
 }
 
+static int qcom_pcie_host_start_link(struct dw_pcie *pci)
+{
+	struct qcom_pcie *pcie = to_qcom_pcie(pci);
+
+	qcom_ep_reset_deassert(pcie);
+
+	if (pcie->cfg->ops->ltssm_enable)
+		pcie->cfg->ops->ltssm_enable(pcie);
+
+	return 0;
+}
+
+static void qcom_pcie_host_stop_link(struct dw_pcie *pci)
+{
+	struct qcom_pcie *pcie = to_qcom_pcie(pci);
+
+	qcom_ep_reset_assert(pcie);
+
+	if (pcie->cfg->ops->ltssm_disable)
+		pcie->cfg->ops->ltssm_disable(pcie);
+}
+
+static void qcom_pcie_2_3_2_ltssm_disable(struct qcom_pcie *pcie)
+{
+	u32 val;
+
+	val = readl(pcie->parf + PARF_LTSSM);
+	val &= ~LTSSM_EN;
+	writel(val, pcie->parf + PARF_LTSSM);
+}
+
 static void qcom_pcie_2_3_2_ltssm_enable(struct qcom_pcie *pcie)
 {
 	u32 val;
@@ -1362,6 +1394,7 @@ static const struct qcom_pcie_ops ops_1_9_0 = {
 	.host_post_init = qcom_pcie_host_post_init_2_7_0,
 	.deinit = qcom_pcie_deinit_2_7_0,
 	.ltssm_enable = qcom_pcie_2_3_2_ltssm_enable,
+	.ltssm_disable = qcom_pcie_2_3_2_ltssm_disable,
 	.config_sid = qcom_pcie_config_sid_1_9_0,
 };
 
@@ -1429,6 +1462,8 @@ static const struct qcom_pcie_cfg cfg_sc8280xp = {
 static const struct dw_pcie_ops dw_pcie_ops = {
 	.link_up = qcom_pcie_link_up,
 	.start_link = qcom_pcie_start_link,
+	.host_start_link = qcom_pcie_host_start_link,
+	.host_stop_link = qcom_pcie_host_stop_link,
 };
 
 static int qcom_pcie_icc_init(struct qcom_pcie *pcie)