[PATCH 4/8] cxl/pci: Fix NULL pointer dereference in reset detection

[Guixin Liu <kanie@linux.alibaba.com>]

__cxl_endpoint_decoder_reset_detected() reads the HDM decoder control
register without checking that the endpoint has any:

	cxlhdm = dev_get_drvdata(&port->dev);
	hdm = cxlhdm->regs.hdm_decoder;
	ctrl = readl(hdm + CXL_HDM_DECODER0_CTRL_OFFSET(cxld->id));

For a device that has no component registers and describes its HDM ranges
through the CXL DVSEC range registers instead, devm_cxl_setup_hdm() returns
early with cxlhdm->regs.hdm_decoder left NULL and the decoder count taken
from the DVSEC ranges. Those emulated decoders are published with
CXL_DECODER_F_ENABLE set by cxl_setup_hdm_decoder_from_dvsec(), so
cxl_reset_done() walking the endpoint's decoders after an FLR or SBR
reaches the readl() with a NULL base and oopses.

Bail out when there are no HDM decoder registers, mirroring the existing
!hdm test in should_emulate_decoders(). Without those registers there is no
Committed bit to sample, so report "no reset detected" rather than
dereferencing NULL.

Fixes: 934edcd436dc ("cxl: Add post-reset warning if reset results in loss of previously committed HDM decoders")
Signed-off-by: Guixin Liu <kanie@linux.alibaba.com>
---
 drivers/cxl/core/pci.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/drivers/cxl/core/pci.c b/drivers/cxl/core/pci.c
index e4338fd7e01b..2515ef4282f2 100644
--- a/drivers/cxl/core/pci.c
+++ b/drivers/cxl/core/pci.c
@@ -684,6 +684,14 @@ static int __cxl_endpoint_decoder_reset_detected(struct device *dev, void *data)
 
 	cxlhdm = dev_get_drvdata(&port->dev);
 	hdm = cxlhdm->regs.hdm_decoder;
+
+	/*
+	 * Devices that describe their HDM ranges with the DVSEC range
+	 * registers have no HDM decoder registers to consult.
+	 */
+	if (!hdm)
+		return 0;
+
 	ctrl = readl(hdm + CXL_HDM_DECODER0_CTRL_OFFSET(cxld->id));
 
 	return !FIELD_GET(CXL_HDM_DECODER0_CTRL_COMMITTED, ctrl);
-- 
2.43.7