soc: qcom: rmtfs: Support discarding guard pages

In some configurations, the exact placement of the rmtfs shared memory
region isn't so strict. The DeviceTree author can then choose to use the
"size" property and rely on the OS for placement (in combination with
"alloc-ranges", if desired).

But on some platforms the rmtfs memory region may not be allocated
adjacent to regions allocated by other clients. Add support for
discarding the first and last 4k block in the region, if
qcom,use-guard-pages is specified in DeviceTree.
Signed-off-by: Bjorn Andersson <quic_bjorande@quicinc.com>
Reviewed-by: Stephan Gerhold <stephan@gerhold.net>
Link: https://lore.kernel.org/r/20230920-rmtfs-mem-guard-pages-v3-2-305b37219b78@quicinc.comSigned-off-by: Bjorn Andersson <andersson@kernel.org>

drivers/soc/qcom/rmtfs_mem.c

@@ -200,6 +200,15 @@ static int qcom_rmtfs_mem_probe(struct platform_device *pdev)
 	rmtfs_mem->client_id = client_id;
 	rmtfs_mem->size = rmem->size;
 
+	/*
+	 * If requested, discard the first and last 4k block in order to ensure
+	 * that the rmtfs region isn't adjacent to other protected regions.
+	 */
+	if (of_property_read_bool(node, "qcom,use-guard-pages")) {
+		rmtfs_mem->addr += SZ_4K;
+		rmtfs_mem->size -= 2 * SZ_4K;
+	}
+
 	device_initialize(&rmtfs_mem->dev);
 	rmtfs_mem->dev.parent = &pdev->dev;
 	rmtfs_mem->dev.groups = qcom_rmtfs_mem_groups;