PCI: dwc: Simplify unrolled iATU detection

The unrolled version of the internal ATU has been available since the DWC
PCIe v4.80a IP core, but it may not be enabled.  Per [1], if unrolled ATU
is enabled, the PCIE_ATU_VIEWPORT does not exist and reads as 0xffffffff;
while if unrolled ATU is disabled, PCIE_ATU_VIEWPORT will contain some
zeros.

Simplify dw_pcie_iatu_unroll_enabled() by checking the value of
PCIE_ATU_VIEWPORT.

[1] DesignWare Cores, PCI Express Controller, Register Desciptions,
v.4.90a, December 2016, p.855

[bhelgaas: commit log]
Link: https://lore.kernel.org/r/20220624143428.8334-10-Sergey.Semin@baikalelectronics.ruTested-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Signed-off-by: Serge Semin <Sergey.Semin@baikalelectronics.ru>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Reviewed-by: Rob Herring <robh@kernel.org>

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

@@ -600,15 +600,15 @@ static void dw_pcie_link_set_max_speed(struct dw_pcie *pci, u32 link_gen)
 
 }
 
-static u8 dw_pcie_iatu_unroll_enabled(struct dw_pcie *pci)
+static bool dw_pcie_iatu_unroll_enabled(struct dw_pcie *pci)
 {
 	u32 val;
 
 	val = dw_pcie_readl_dbi(pci, PCIE_ATU_VIEWPORT);
 	if (val == 0xffffffff)
-		return 1;
+		return true;
 
-	return 0;
+	return false;
 }
 
 static void dw_pcie_iatu_detect_regions_unroll(struct dw_pcie *pci)
@@ -680,9 +680,8 @@ void dw_pcie_iatu_detect(struct dw_pcie *pci)
 	struct device *dev = pci->dev;
 	struct platform_device *pdev = to_platform_device(dev);
 
-	if (pci->version >= 0x480A || (!pci->version &&
-				       dw_pcie_iatu_unroll_enabled(pci))) {
-		pci->iatu_unroll_enabled = true;
+	pci->iatu_unroll_enabled = dw_pcie_iatu_unroll_enabled(pci);
+	if (pci->iatu_unroll_enabled) {
 		if (!pci->atu_base) {
 			struct resource *res =
 				platform_get_resource_byname(pdev, IORESOURCE_MEM, "atu");