[PATCH v3 0/2] cxl: Allow passthrough decoders with >16K granularity

[PATCH v3 0/2] cxl: Allow passthrough decoders with >16K granularity

[Alison Schofield]

Changes in v3:
- Program the real IG encoding for an encodable non-interleaving granularity
  instead of always writing EIG=0 (RichardC, Sashiko)
- Update Patch 1 commit msg text and this cover letter text per above change
Link to v2:
https://lore.kernel.org/linux-cxl/cover.1783795720.git.alison.schofield@intel.com/

Changes in v2:
- Rebased on Linux 7.2-rc1
- Only v1 feedback was from Sashiko and that was not an issue
Link to v1:
https://lore.kernel.org/linux-cxl/cover.1781136281.git.alison.schofield@intel.com/

Sashiko reported [1] that region setup rejects valid topologies
containing a passthrough decoder beneath a wide parent interleave. In
these configurations, a passthrough decoder can compute a granularity
above the 16K maximum encodable in the interleave granularity (IG)
field even though non-interleaving decoders do not consume that field.

The first patch avoids validating the interleave granularity of
non-interleaving decoders, encodes the IG field as a don't-care
only when the computed granularity is not encodable, and updates
the sysfs ABI documentation accordingly.

The second patch extends cxl-test with a topology that allows a CXL
unit test to cover this case.

A unit test update for this case is posted for review [2].

[1] https://lore.kernel.org/all/20260530002613.682E71F00893@smtp.kernel.org/
[2] https://lore.kernel.org/all/cover.1781136221.git.alison.schofield@intel.com/


Alison Schofield (2):
  cxl/region: Allow passthrough decoders with >16K granularity
  cxl/test: Add a 2-way 16K root decoder for passthrough testing

 Documentation/ABI/testing/sysfs-bus-cxl |  5 ++-
 drivers/cxl/core/hdm.c                  |  8 ++++-
 drivers/cxl/core/region.c               | 47 ++++++++++++++----------
 tools/testing/cxl/test/cxl.c            | 48 +++++++++++++++++++++++--
 4 files changed, 85 insertions(+), 23 deletions(-)


base-commit: dc59e4fea9d83f03bad6bddf3fa2e52491777482
-- 
2.37.3

[PATCH v3 1/2] cxl/region: Allow passthrough decoders with >16K granularity

[Alison Schofield]

Region configuration rejects valid topologies that contain a
passthrough decoder beneath a wide parent interleave.

For example, a passthrough switch below an 8-way root decoder with
4K granularity computes a 32K granularity. That exceeds the maximum
encodable value of the HDM Decoder Control IG field, causing region
setup to fail even though a non-interleaving decoder does not consume
the IG field.

Only require the granularity to be encodable for interleaving decoders,
both where it is inherited from the parent and where it is computed for
the current decoder. Keep the computed value for passthrough decoders so
it can seed descendant decoder setup.

When committing a non-interleaving decoder, still program the granularity
if it is encodable; use a don't-care IG encoding only when it is not. This
preserves an encodable passthrough granularity across re-enumeration, so
descendants do not inherit a stale value.

As a consequence, the interleave_granularity attribute of a decoder
whose interleave_ways is 1 may report a value above 16K. Document that
the reported granularity for non-interleaving decoders is a don't-care
value that may exceed the maximum encodable in hardware.

Fixes: 18f35dc9314d ("cxl/region: Refactor granularity select in cxl_port_setup_targets()")
Suggested-by: Sashiko AI Review <sashiko-bot@kernel.org>
Assisted-by: Claude:Opus-4-8
Signed-off-by: Alison Schofield <alison.schofield@intel.com>
---
 Documentation/ABI/testing/sysfs-bus-cxl |  5 ++-
 drivers/cxl/core/hdm.c                  |  8 ++++-
 drivers/cxl/core/region.c               | 47 +++++++++++++++----------
 3 files changed, 39 insertions(+), 21 deletions(-)

diff --git a/Documentation/ABI/testing/sysfs-bus-cxl b/Documentation/ABI/testing/sysfs-bus-cxl
index 16a9b3d2e2c0..3f96e5bf1fd3 100644
--- a/Documentation/ABI/testing/sysfs-bus-cxl
+++ b/Documentation/ABI/testing/sysfs-bus-cxl
@@ -407,7 +407,10 @@ Description:
 		space this decoder claims at address N before the decode rotates
 		to the next target in the interleave at address N +
 		interleave_granularity (assuming N is aligned to
-		interleave_granularity).
+		interleave_granularity). When 'interleave_ways' is 1, the
+		decoder does not interleave and the reported granularity is
+		a don't-care value that may exceed the maximum encodable in
+		hardware.
 
 
 What:		/sys/bus/cxl/devices/decoderX.Y/create_{pmem,ram}_region
diff --git a/drivers/cxl/core/hdm.c b/drivers/cxl/core/hdm.c
index 0c80b76a5f9b..d2ada82fe3c7 100644
--- a/drivers/cxl/core/hdm.c
+++ b/drivers/cxl/core/hdm.c
@@ -691,7 +691,13 @@ static void cxld_set_interleave(struct cxl_decoder *cxld, u32 *ctrl)
 	if (WARN_ONCE(ways_to_eiw(cxld->interleave_ways, &eiw),
 		      "invalid interleave_ways: %d\n", cxld->interleave_ways))
 		return;
-	if (WARN_ONCE(granularity_to_eig(cxld->interleave_granularity, &eig),
+
+	/*
+	 * A non-interleaving decoder ignores the IG field, so an
+	 * unencodable granularity is a don't-care rather than a failure.
+	 */
+	if (granularity_to_eig(cxld->interleave_granularity, &eig) &&
+	    WARN_ONCE(cxld->interleave_ways > 1,
 		      "invalid interleave_granularity: %d\n",
 		      cxld->interleave_granularity))
 		return;
diff --git a/drivers/cxl/core/region.c b/drivers/cxl/core/region.c
index 1e211542b6b6..5a443551288c 100644
--- a/drivers/cxl/core/region.c
+++ b/drivers/cxl/core/region.c
@@ -1521,12 +1521,20 @@ static int cxl_port_setup_targets(struct cxl_port *port,
 		parent_iw = parent_cxld->interleave_ways;
 	}
 
-	rc = granularity_to_eig(parent_ig, &peig);
-	if (rc) {
-		dev_dbg(&cxlr->dev, "%s:%s: invalid parent granularity: %d\n",
-			dev_name(parent_port->uport_dev),
-			dev_name(&parent_port->dev), parent_ig);
-		return rc;
+	/*
+	 * A non-interleaving parent does not encode its granularity, so its
+	 * stored value may exceed the maximum encodable and need not be
+	 * validated here.
+	 */
+	if (parent_iw > 1) {
+		rc = granularity_to_eig(parent_ig, &peig);
+		if (rc) {
+			dev_dbg(&cxlr->dev,
+				"%s:%s: invalid parent granularity: %d\n",
+				dev_name(parent_port->uport_dev),
+				dev_name(&parent_port->dev), parent_ig);
+			return rc;
+		}
 	}
 
 	rc = ways_to_eiw(parent_iw, &peiw);
@@ -1549,20 +1557,21 @@ static int cxl_port_setup_targets(struct cxl_port *port,
 	 * Interleave granularity is a multiple of @parent_port granularity.
 	 * Multiplier is the parent port interleave ways.
 	 */
-	rc = granularity_to_eig(parent_ig * parent_iw, &eig);
-	if (rc) {
-		dev_dbg(&cxlr->dev,
-			"%s: invalid granularity calculation (%d * %d)\n",
-			dev_name(&parent_port->dev), parent_ig, parent_iw);
-		return rc;
-	}
+	ig = parent_ig * parent_iw;
 
-	rc = eig_to_granularity(eig, &ig);
-	if (rc) {
-		dev_dbg(&cxlr->dev, "%s:%s: invalid interleave: %d\n",
-			dev_name(port->uport_dev), dev_name(&port->dev),
-			256 << eig);
-		return rc;
+	/*
+	 * Keep the computed granularity for descendant setup. Only
+	 * interleaving decoders require an encodable granularity.
+	 */
+	if (iw > 1) {
+		rc = granularity_to_eig(ig, &eig);
+		if (rc) {
+			dev_dbg(&cxlr->dev,
+				"%s: invalid granularity calculation (%d * %d)\n",
+				dev_name(&parent_port->dev), parent_ig,
+				parent_iw);
+			return rc;
+		}
 	}
 
 	if (iw > 8 || iw > cxlsd->nr_targets) {
-- 
2.37.3

Re: [PATCH v3 1/2] cxl/region: Allow passthrough decoders with >16K granularity

[sashiko-bot]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [High] During region auto-discovery, a passthrough decoder's interleave granularity is not updated to its correct topological value, causing valid interleaving descendants to fail auto-discovery.
--

--- Patch [1]: [PATCH v3 1/2] cxl/region: Allow passthrough decoders with >16K granularity ---
commit d213bad3e4cb8647e64aafb5eacaa82258aa533b
Author: Alison Schofield <alison.schofield@intel.com>

cxl/region: Allow passthrough decoders with >16K granularity

This commit allows passthrough decoders to have computed granularities larger
than 16K by treating the interleave granularity as a don't-care value for
non-interleaving decoders. It updates the region configuration to skip
validation and hardware encodability checks for these granularities, while
keeping the computed value to seed descendant decoder setup.

> diff --git a/drivers/cxl/core/region.c b/drivers/cxl/core/region.c
> --- a/drivers/cxl/core/region.c
> +++ b/drivers/cxl/core/region.c
[ ... ]
> @@ -1549,20 +1557,21 @@ static int cxl_port_setup_targets(struct cxl_port *port,
[ ... ]
> +	ig = parent_ig * parent_iw;
>  
> -	rc = eig_to_granularity(eig, &ig);
> -	if (rc) {
> -		dev_dbg(&cxlr->dev, "%s:%s: invalid interleave: %d\n",
> -			dev_name(port->uport_dev), dev_name(&port->dev),
> -			256 << eig);
> -		return rc;
> +	/*
> +	 * Keep the computed granularity for descendant setup. Only
> +	 * interleaving decoders require an encodable granularity.
> +	 */
> +	if (iw > 1) {
> +		rc = granularity_to_eig(ig, &eig);
> +		if (rc) {
> +			dev_dbg(&cxlr->dev,
> +				"%s: invalid granularity calculation (%d * %d)\n",
> +				dev_name(&parent_port->dev), parent_ig,
> +				parent_iw);
> +			return rc;
> +		}
>  	}
>  
>  	if (iw > 8 || iw > cxlsd->nr_targets) {

[Severity: High]
When auto-discovering a region with a passthrough decoder (iw == 1), does this
skip updating the decoder's interleave granularity to the computed 'ig'?

Looking at cxl_port_setup_targets(), the auto-discovery path handles
non-interleaving decoders by bypassing the granularity mismatch check:

    if (test_bit(CXL_REGION_F_AUTO, &cxlr->flags)) {
            if (cxld->interleave_ways != iw ||
                (iw > 1 && cxld->interleave_granularity != ig) || ...

However, because cxld->interleave_granularity is never updated to 'ig' for
auto-discovered passthrough decoders, won't it retain its stale hardware
value (which could be a don't-care value like 0)?

If so, when descendant decoders load their parent's interleave granularity
earlier in cxl_port_setup_targets():

    parent_rr = cxl_rr_load(parent_port, cxlr);
    parent_cxld = parent_rr->decoder;
    parent_ig = parent_cxld->interleave_granularity;

won't they inherit this stale hardware don't-care value? This appears to
cause their own expected 'ig' calculation to fail validation during
auto-discovery, resulting in the region being rejected.

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/cover.1784940306.git.alison.schofield@intel.com?part=1

Re: [PATCH v3 1/2] cxl/region: Allow passthrough decoders with >16K granularity

[Alison Schofield]

On Sat, Jul 25, 2026 at 01:26:37AM +0000, sashiko-bot@kernel.org wrote:
> Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
> - [High] During region auto-discovery, a passthrough decoder's interleave granularity is not updated to its correct topological value, causing valid interleaving descendants to fail auto-discovery.

Same NO as in v1[1] (and in v2).
Gratuitously updating the field is not the right thing to do.


[1] https://lore.kernel.org/linux-cxl/alKP8OP4DNT2nJnu@aschofie-mobl2.lan/#r
No. A downstream decoder can't inherit an unecodable value into a failing
check. A >16K granularity only appears below a wide interleave, and
everything below that inherits an equal-or-larger value that an interleaving
decoder couldn't encode. So every descendant is necessarily passthrough and
never validates granularity. There is no path where updating the F_AUTO
value would prevent a failure because the failure cannot happen.










> --
> 
> --- Patch [1]: [PATCH v3 1/2] cxl/region: Allow passthrough decoders with >16K granularity ---
> commit d213bad3e4cb8647e64aafb5eacaa82258aa533b
> Author: Alison Schofield <alison.schofield@intel.com>
> 
> cxl/region: Allow passthrough decoders with >16K granularity
> 
> This commit allows passthrough decoders to have computed granularities larger
> than 16K by treating the interleave granularity as a don't-care value for
> non-interleaving decoders. It updates the region configuration to skip
> validation and hardware encodability checks for these granularities, while
> keeping the computed value to seed descendant decoder setup.
> 
> > diff --git a/drivers/cxl/core/region.c b/drivers/cxl/core/region.c
> > --- a/drivers/cxl/core/region.c
> > +++ b/drivers/cxl/core/region.c
> [ ... ]
> > @@ -1549,20 +1557,21 @@ static int cxl_port_setup_targets(struct cxl_port *port,
> [ ... ]
> > +	ig = parent_ig * parent_iw;
> >  
> > -	rc = eig_to_granularity(eig, &ig);
> > -	if (rc) {
> > -		dev_dbg(&cxlr->dev, "%s:%s: invalid interleave: %d\n",
> > -			dev_name(port->uport_dev), dev_name(&port->dev),
> > -			256 << eig);
> > -		return rc;
> > +	/*
> > +	 * Keep the computed granularity for descendant setup. Only
> > +	 * interleaving decoders require an encodable granularity.
> > +	 */
> > +	if (iw > 1) {
> > +		rc = granularity_to_eig(ig, &eig);
> > +		if (rc) {
> > +			dev_dbg(&cxlr->dev,
> > +				"%s: invalid granularity calculation (%d * %d)\n",
> > +				dev_name(&parent_port->dev), parent_ig,
> > +				parent_iw);
> > +			return rc;
> > +		}
> >  	}
> >  
> >  	if (iw > 8 || iw > cxlsd->nr_targets) {
> 
> [Severity: High]
> When auto-discovering a region with a passthrough decoder (iw == 1), does this
> skip updating the decoder's interleave granularity to the computed 'ig'?
> 
> Looking at cxl_port_setup_targets(), the auto-discovery path handles
> non-interleaving decoders by bypassing the granularity mismatch check:
> 
>     if (test_bit(CXL_REGION_F_AUTO, &cxlr->flags)) {
>             if (cxld->interleave_ways != iw ||
>                 (iw > 1 && cxld->interleave_granularity != ig) || ...
> 
> However, because cxld->interleave_granularity is never updated to 'ig' for
> auto-discovered passthrough decoders, won't it retain its stale hardware
> value (which could be a don't-care value like 0)?
> 
> If so, when descendant decoders load their parent's interleave granularity
> earlier in cxl_port_setup_targets():
> 
>     parent_rr = cxl_rr_load(parent_port, cxlr);
>     parent_cxld = parent_rr->decoder;
>     parent_ig = parent_cxld->interleave_granularity;
> 
> won't they inherit this stale hardware don't-care value? This appears to
> cause their own expected 'ig' calculation to fail validation during
> auto-discovery, resulting in the region being rejected.
> 
> -- 
> Sashiko AI review · https://sashiko.dev/#/patchset/cover.1784940306.git.alison.schofield@intel.com?part=1

[PATCH v3 2/2] cxl/test: Add a 2-way 16K root decoder for passthrough testing

[Alison Schofield]

The cxl-test topologies do not provide a configuration where pass-
through decoders compute a granularity above the 16K max encodable
in hardware. The widest power-of-2 root decoder currently available
is 2-way at 4K granularity, so a passthrough decoder below it
computes at most 16K.

Add a CFMWS entry for a 2-way root decoder at 16K granularity. This
topology supports a region configuration spanning the two multi-port
host bridges and placing one endpoint under each bridge. The
intermediate switch decoders are then passthrough decoders with a
computed granularity of 32K.

This allows a CXL unit test to be added for this case.

Place the new window in the XOR topology set so the default modulo
topology used by most cxl-test coverage is unchanged.

Assisted-by: Claude:Opus-4-8
Signed-off-by: Alison Schofield <alison.schofield@intel.com>
---
 tools/testing/cxl/test/cxl.c | 48 ++++++++++++++++++++++++++++++++++--
 1 file changed, 46 insertions(+), 2 deletions(-)

diff --git a/tools/testing/cxl/test/cxl.c b/tools/testing/cxl/test/cxl.c
index ef92dd35e030..5b87a3f5673f 100644
--- a/tools/testing/cxl/test/cxl.c
+++ b/tools/testing/cxl/test/cxl.c
@@ -188,10 +188,18 @@ static struct {
 		struct acpi_cedt_cfmws cfmws;
 		u32 target[3];
 	} cfmws8;
+	struct {
+		struct acpi_cedt_cfmws cfmws;
+		u32 target[2];
+	} cfmws9;
 	struct {
 		struct acpi_cedt_cxims cxims;
 		u64 xormap_list[2];
 	} cxims0;
+	struct {
+		struct acpi_cedt_cxims cxims;
+		u64 xormap_list[1];
+	} cxims1;
 } __packed mock_cedt = {
 	.cedt = {
 		.header = {
@@ -371,6 +379,28 @@ static struct {
 		},
 		.target = { 0, 1, 2, },
 	},
+	/*
+	 * A 2-way root at 16K granularity. A passthrough decoder below this
+	 * root computes a 32K granularity (16K * 2), which exceeds the maximum
+	 * encodable in hardware. It exercises the passthrough granularity path
+	 * in cxl_port_setup_targets().
+	 */
+	.cfmws9 = {
+		.cfmws = {
+			.header = {
+				.type = ACPI_CEDT_TYPE_CFMWS,
+				.length = sizeof(mock_cedt.cfmws9),
+			},
+			.interleave_arithmetic = ACPI_CEDT_CFMWS_ARITHMETIC_XOR,
+			.interleave_ways = 1,
+			.granularity = 6,
+			.restrictions = ACPI_CEDT_CFMWS_RESTRICT_HOSTONLYMEM |
+					ACPI_CEDT_CFMWS_RESTRICT_PMEM,
+			.qtg_id = FAKE_QTG_ID,
+			.window_size = SZ_256M * 8UL,
+		},
+		.target = { 0, 1, },
+	},
 	.cxims0 = {
 		.cxims = {
 			.header = {
@@ -382,6 +412,18 @@ static struct {
 		},
 		.xormap_list = { 0x404100, 0x808200, },
 	},
+	/* CXIMS for the 16K (hbig = 6) 2-way root, cfmws9 */
+	.cxims1 = {
+		.cxims = {
+			.header = {
+				.type = ACPI_CEDT_TYPE_CXIMS,
+				.length = sizeof(mock_cedt.cxims1),
+			},
+			.hbig = 6,
+			.nr_xormaps = 1,
+		},
+		.xormap_list = { 0x0, },
+	},
 };
 
 struct acpi_cedt_cfmws *mock_cfmws[] = {
@@ -395,6 +437,7 @@ struct acpi_cedt_cfmws *mock_cfmws[] = {
 	[6] = &mock_cedt.cfmws6.cfmws,
 	[7] = &mock_cedt.cfmws7.cfmws,
 	[8] = &mock_cedt.cfmws8.cfmws,
+	[9] = &mock_cedt.cfmws9.cfmws,
 };
 
 static int cfmws_start;
@@ -402,10 +445,11 @@ static int cfmws_end;
 #define CFMWS_MOD_ARRAY_START 0
 #define CFMWS_MOD_ARRAY_END   5
 #define CFMWS_XOR_ARRAY_START 6
-#define CFMWS_XOR_ARRAY_END   8
+#define CFMWS_XOR_ARRAY_END   9
 
-struct acpi_cedt_cxims *mock_cxims[1] = {
+struct acpi_cedt_cxims *mock_cxims[2] = {
 	[0] = &mock_cedt.cxims0.cxims,
+	[1] = &mock_cedt.cxims1.cxims,
 };
 
 struct cxl_mock_res {
-- 
2.37.3