PCI: dwc: ep: Add comment explaining controller level PTM access in multi PF setup

PCIe r6.0, section 7.9.15 requires PTM capability in exactly one
function to control all PTM-capable functions. This makes PTM registers
controller level rather than per-function.

Add a comment explaining why PTM capability registers are accessed
using the standard DBI accessors instead of func_no indexed
per-function accessors.

Suggested-by: Niklas Cassel <cassel@kernel.org>
Signed-off-by: Aksh Garg <a-garg7@ti.com>
Signed-off-by: Manivannan Sadhasivam <mani@kernel.org>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Niklas Cassel <cassel@kernel.org>
Link: https://patch.msgid.link/20260130115516.515082-4-a-garg7@ti.com

Author: AkshGarg-19

drivers/pci/controller/dwc/pcie-designware-ep.c

@@ -1187,6 +1187,18 @@ int dw_pcie_ep_init_registers(struct dw_pcie_ep *ep)
 	if (ep->ops->init)
 		ep->ops->init(ep);
 
+	/*
+	 * PCIe r6.0, section 7.9.15 states that for endpoints that support
+	 * PTM, this capability structure is required in exactly one
+	 * function, which controls the PTM behavior of all PTM capable
+	 * functions. This indicates the PTM capability structure
+	 * represents controller-level registers rather than per-function
+	 * registers.
+	 *
+	 * Therefore, PTM capability registers are configured using the
+	 * standard DBI accessors, instead of func_no indexed per-function
+	 * accessors.
+	 */
 	ptm_cap_base = dw_pcie_find_ext_capability(pci, PCI_EXT_CAP_ID_PTM);
 
 	/*