PCI: endpoint: epf-mhi: Avoid NULL dereference if DT lacks 'mmio'

If platform_get_resource_byname() fails and returns NULL because DT lacks
an 'mmio' property for the MHI endpoint, dereferencing res->start will
cause a NULL pointer access. Add a check to prevent it.

Fixes: 1bf5f25 ("PCI: endpoint: Add PCI Endpoint function driver for MHI bus")
Link: https://lore.kernel.org/r/20241105120735.1240728-1-quic_zhonhan@quicinc.com
Signed-off-by: Zhongqiu Han <quic_zhonhan@quicinc.com>
[kwilczynski: error message update per the review feedback]
Signed-off-by: Krzysztof Wilczyński <kwilczynski@kernel.org>
[bhelgaas: commit log]
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Niklas Cassel <cassel@kernel.org>

Author: Zhongqiu Han

drivers/pci/endpoint/functions/pci-epf-mhi.c

@@ -867,12 +867,18 @@ static int pci_epf_mhi_bind(struct pci_epf *epf)
 {
 	struct pci_epf_mhi *epf_mhi = epf_get_drvdata(epf);
 	struct pci_epc *epc = epf->epc;
+	struct device *dev = &epf->dev;
 	struct platform_device *pdev = to_platform_device(epc->dev.parent);
 	struct resource *res;
 	int ret;
 
 	/* Get MMIO base address from Endpoint controller */
 	res = platform_get_resource_byname(pdev, IORESOURCE_MEM, "mmio");
+	if (!res) {
+		dev_err(dev, "Failed to get \"mmio\" resource\n");
+		return -ENODEV;
+	}
+
 	epf_mhi->mmio_phys = res->start;
 	epf_mhi->mmio_size = resource_size(res);