powerpc/powernv: Document cxl dependency on special case in pnv_eeh_reset()

pnv_eeh_reset() has special handling for PEs whose primary bus is the
root bus or the bus immediately underneath the root port.

The cxl bi-modal card support added in b0b5e591 ("cxl: Add
cxl_check_and_switch_mode() API to switch bi-modal cards") relies on
this behaviour when hot-resetting the CAPI adapter following a mode
switch.  Document this in pnv_eeh_reset() so we don't accidentally break
it.
Suggested-by: Gavin Shan <gwshan@linux.vnet.ibm.com>
Signed-off-by: Andrew Donnellan <andrew.donnellan@au1.ibm.com>
Reviewed-by: Gavin Shan <gwshan@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/platforms/powernv/eeh-powernv.c

@@ -1102,6 +1102,13 @@ static int pnv_eeh_reset(struct eeh_pe *pe, int option)
 		return -EIO;
 	}
 
+	/*
+	 * If dealing with the root bus (or the bus underneath the
+	 * root port), we reset the bus underneath the root port.
+	 *
+	 * The cxl driver depends on this behaviour for bi-modal card
+	 * switching.
+	 */
 	if (pci_is_root_bus(bus) ||
 	    pci_is_root_bus(bus->parent))
 		return pnv_eeh_root_reset(hose, option);