cxl/port: Enable the HDM decoder capability for switch ports

Derick noticed, when testing hot plug, that hot-add behaves nominally
after a removal. However, if the hot-add is done without a prior
removal, CXL.mem accesses fail. It turns out that the original
implementation of the port driver and region programming wrongly assumed
that platform-firmware always enables the host-bridge HDM decoder
capability. Add support turning on switch-level HDM decoders in the case
where platform-firmware has not.

The implementation is careful to only arrange for the enable to be
undone if the current instance of the driver was the one that did the
enable. This is to interoperate with platform-firmware that may expect
CXL.mem to remain active after the driver is shutdown. This comes at the
cost of potentially not shutting down the enable on kexec flows, but it
is mitigated by the fact that the related HDM decoders still need to be
enabled on an individual basis.

Cc: <[e-mail]>
Reported-by: Derick Marks <[e-mail]>
Fixes: 54cdbf845cf7 ("cxl/port: Add a driver for 'struct cxl_port' objects")
Reviewed-by: Ira Weiny <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Dan Williams <[e-mail]>

drivers/cxl/core/pci.c

···
 	       hdm + CXL_HDM_DECODER_CTRL_OFFSET);
 }
 
-static int devm_cxl_enable_hdm(struct device *host, struct cxl_hdm *cxlhdm)
+int devm_cxl_enable_hdm(struct cxl_port *port, struct cxl_hdm *cxlhdm)
 {
-	void __iomem *hdm = cxlhdm->regs.hdm_decoder;
+	void __iomem *hdm;
 	u32 global_ctrl;
 
+	/*
+	 * If the hdm capability was not mapped there is nothing to enable and
+	 * the caller is responsible for what happens next.  For example,
+	 * emulate a passthrough decoder.
+	 */
+	if (IS_ERR(cxlhdm))
+		return 0;
+
+	hdm = cxlhdm->regs.hdm_decoder;
 	global_ctrl = readl(hdm + CXL_HDM_DECODER_CTRL_OFFSET);
+
+	/*
+	 * If the HDM decoder capability was enabled on entry, skip
+	 * registering disable_hdm() since this decode capability may be
+	 * owned by platform firmware.
+	 */
+	if (global_ctrl & CXL_HDM_DECODER_ENABLE)
+		return 0;
+
 	writel(global_ctrl | CXL_HDM_DECODER_ENABLE,
 	       hdm + CXL_HDM_DECODER_CTRL_OFFSET);
 
-	return devm_add_action_or_reset(host, disable_hdm, cxlhdm);
+	return devm_add_action_or_reset(&port->dev, disable_hdm, cxlhdm);
 }
+EXPORT_SYMBOL_NS_GPL(devm_cxl_enable_hdm, CXL);
 
 int cxl_dvsec_rr_decode(struct device *dev, int d,
 			struct cxl_endpoint_dvsec_info *info)
···
 	if (info->mem_enabled)
 		return 0;
 
-	rc = devm_cxl_enable_hdm(&port->dev, cxlhdm);
+	rc = devm_cxl_enable_hdm(port, cxlhdm);
 	if (rc)
 		return rc;
 

drivers/cxl/cxl.h

···
 struct cxl_hdm;
 struct cxl_hdm *devm_cxl_setup_hdm(struct cxl_port *port,
 				   struct cxl_endpoint_dvsec_info *info);
+int devm_cxl_enable_hdm(struct cxl_port *port, struct cxl_hdm *cxlhdm);
 int devm_cxl_enumerate_decoders(struct cxl_hdm *cxlhdm,
 				struct cxl_endpoint_dvsec_info *info);
 int devm_cxl_add_passthrough_decoder(struct cxl_port *port);

drivers/cxl/port.c

···
 static int cxl_switch_port_probe(struct cxl_port *port)
 {
 	struct cxl_hdm *cxlhdm;
-	int rc;
+	int rc, nr_dports;
 
-	rc = devm_cxl_port_enumerate_dports(port);
-	if (rc < 0)
-		return rc;
+	nr_dports = devm_cxl_port_enumerate_dports(port);
+	if (nr_dports < 0)
+		return nr_dports;
 
 	cxlhdm = devm_cxl_setup_hdm(port, NULL);
+	rc = devm_cxl_enable_hdm(port, cxlhdm);
+	if (rc)
+		return rc;
+
 	if (!IS_ERR(cxlhdm))
 		return devm_cxl_enumerate_decoders(cxlhdm, NULL);
 
···
 		return PTR_ERR(cxlhdm);
 	}
 
-	if (rc == 1) {
+	if (nr_dports == 1) {
 		dev_dbg(&port->dev, "Fallback to passthrough decoder\n");
 		return devm_cxl_add_passthrough_decoder(port);
 	}

tools/testing/cxl/Kbuild

···
 ldflags-y += --wrap=nvdimm_bus_register
 ldflags-y += --wrap=devm_cxl_port_enumerate_dports
 ldflags-y += --wrap=devm_cxl_setup_hdm
+ldflags-y += --wrap=devm_cxl_enable_hdm
 ldflags-y += --wrap=devm_cxl_add_passthrough_decoder
 ldflags-y += --wrap=devm_cxl_enumerate_decoders
 ldflags-y += --wrap=cxl_await_media_ready

tools/testing/cxl/test/mock.c

···
 }
 EXPORT_SYMBOL_NS_GPL(__wrap_devm_cxl_setup_hdm, CXL);
 
+int __wrap_devm_cxl_enable_hdm(struct cxl_port *port, struct cxl_hdm *cxlhdm)
+{
+	int index, rc;
+	struct cxl_mock_ops *ops = get_cxl_mock_ops(&index);
+
+	if (ops && ops->is_mock_port(port->uport))
+		rc = 0;
+	else
+		rc = devm_cxl_enable_hdm(port, cxlhdm);
+	put_cxl_mock_ops(index);
+
+	return rc;
+}
+EXPORT_SYMBOL_NS_GPL(__wrap_devm_cxl_enable_hdm, CXL);
+
 int __wrap_devm_cxl_add_passthrough_decoder(struct cxl_port *port)
 {
 	int rc, index;