[PATCH v2] cxl/pci: Fix NULL pointer dereference in reset detection

__cxl_endpoint_decoder_reset_detected() samples the Committed bit of an
enabled endpoint decoder by reading the decoder control register at
cxlhdm->regs.hdm_decoder, without testing that pointer.
should_emulate_decoders(), the other consumer of the same field, does test
it, because the field is optional.

A memory device that exposes no component registers and describes its HDM
ranges through the CXL DVSEC range registers takes the early return in
devm_cxl_setup_hdm(), which leaves regs.hdm_decoder NULL and derives the
decoder count from the DVSEC ranges instead.
cxl_setup_hdm_decoder_from_dvsec() then publishes those emulated decoders
with CXL_DECODER_F_ENABLE set, and CXL_DECODER_F_ENABLE is the only filter
cxl_reset_done() applies when it walks the endpoint's decoders after an FLR
or an SBR. So on such a device every reset reaches the readl() with a NULL
base and oopses in the PCI reset completion path.

Report "no reset detected" when the endpoint has no HDM decoder registers,
as should_emulate_decoders() already does for the same condition. A range
described by the DVSEC registers has no Committed bit that a reset could
clear, so there is nothing for the post-reset warning to observe.

Fixes: 934edcd436dc ("cxl: Add post-reset warning if reset results in loss of previously committed HDM decoders")
Signed-off-by: Guixin Liu <[email protected]>
---
This was patch 4/8 of the "cxl: Assorted fixes" series [1]. Per review
feedback that series is not being reworked as a whole; the fixes are resent
individually instead. Patches 1, 2 and 7 of the series are dropped, as those
issues are already fixed in cxl/next.

v1->v2:
- rebase onto cxl/next
- rewrite the commit message to describe the behaviour rather than narrate
  the code change (Alison Schofield)

[1] https://lore.kernel.org/linux-cxl/[email protected]/

 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 9d807c1a002c..611175f29d24 100644
--- a/drivers/cxl/core/pci.c
+++ b/drivers/cxl/core/pci.c
@@ -685,6 +685,14 @@ 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);

base-commit: 7098e9cd98a05c0c5de2fae0c2465f9d966fdd07
-- 
2.43.7