Re: [PATCH v7 2/2] tools/testing/cxl: Enable zero sized decoders under hb0

[Richard Cheng <icheng@nvidia.com>]

On Wed, Jul 08, 2026 at 11:35:55AM +0800, Alison Schofield wrote:
> On Wed, Jul 08, 2026 at 06:40:23PM +0800, Richard Cheng wrote:
> > The kernel now allows committed zero-size HDM decoders so BIOS can lock
> > empty decoders; cxl_test needs to exercise that path.
> > 
> > Add a mock_zero_size_decoders module parameter (default off). When set,
> > the special endpoints under host-bridge0 (cxl_mem.0 and cxl_mem.4)
> > commit decoders 1 and 2 as zero-size + locked above the decoder[0]
> > auto-region, mirrored on the parent switch and host bridge. The mocks
> > take a real zero-size DPA reservation, like enumeration of real
> > hardware, so commit_end lands on a zero-size decoder and the
> > reservation, poison-by-endpoint, and teardown paths all run.
> 
> At v5 review we chatted about a unit test for this.
> Not sure what has happened since wrt the poison tail mappings, but
> the check in cxl-topology.sh 'wants' to be in place to merge this.
> 
> https://lore.kernel.org/linux-cxl/ajozhDmPogKi9hb5@MWDK4CY14F/
> 
> -- Alison
>

Hi Alison,

Sorry I forgot about that,

Just have the work sent out here.

https://lore.kernel.org/linux-cxl/20260709022857.18732-1-icheng@nvidia.com/

Does this match what you expect ?

--Richard
 
> 
> > 
> > Signed-off-by: Vishal Aslot <vaslot@nvidia.com>
> > Signed-off-by: Richard Cheng <icheng@nvidia.com>
> > ---
> > Changelog:
> > 
> > v6->v7:
> >     - Rebase onto cxl/for-7.3/cxl-type2-test.
> > 	- Integrate mock_zero_size_decoders with special-topology parameter
> > 	  validation and reject its incompatible use with fail_autoassemble.
> > 
> > v5->v6:
> >      - Reword a mock comment, no functional change.
> > v4->v5:
> >       - Mirror the v5 core semantics: mock zero-size decoders take a real
> >         zero-size DPA reservation via devm_cxl_dpa_reserve() so
> >         dpa_res/hdm_end/cxled->part match real hardware enumeration and
> >         the poison-by-endpoint walk reaches commit_end.
> >       - Set cxled->state = CXL_DECODER_STATE_AUTO to match
> >         init_hdm_decoder()'s fall-through (undoes the v2 switch to
> >         MANUAL).
> >       - cxld_registry_restore(): reserve unconditionally for enabled
> >         endpoint decoders; enabled now implies reserved, sized or not.
> > 
> > v3->v4:
> >      - No change.
> > 
> > v2->v3:
> >       - Gate the zero-size + locked decoder injection behind a new
> >         mock_zero_size_decoders module parameter (default off). v2
> >         applied it unconditionally on the host-bridge0 auto-region
> >         endpoints, which the region test suite reuses, regressing 7 of
> >         17 cxl unit tests; defaulting off leaves the shared topology
> >         untouched.
> > 
> > v1->v2:
> >       - Replace second_decoder(), third_decoder() with a single
> >         match_decoder_by_index() helper, so all lookups share one
> >         matcher.
> >       - Use DEFINE_RANGE() for the empty range instead of an open-coded
> >         struct.
> >       - Set cxled->state = CXL_DECODER_STATE_MANUAL rather than
> >         STATE_AUTO.
> >       - Set CXL_DECODER_F_LOCK on the mock zero-size decoders to model
> >         the BIOS-burns-slots case.
> > ---
> >  tools/testing/cxl/test/cxl.c | 109 ++++++++++++++++++++++++++++++-----
> >  1 file changed, 94 insertions(+), 15 deletions(-)
> > 
> > diff --git a/tools/testing/cxl/test/cxl.c b/tools/testing/cxl/test/cxl.c
> > index 8ab2ce1262f3..fff2004a08c8 100644
> > --- a/tools/testing/cxl/test/cxl.c
> > +++ b/tools/testing/cxl/test/cxl.c
> > @@ -18,6 +18,7 @@ static int interleave_arithmetic;
> >  static bool extended_linear_cache;
> >  static bool fail_autoassemble;
> >  static bool type2_test;
> > +static bool mock_zero_size_decoders;
> >  
> >  #define FAKE_QTG_ID	42
> >  
> > @@ -872,14 +873,13 @@ static int cxld_registry_restore(struct cxl_decoder *cxld,
> >  		cxld_copy(cxld, &td->cxled.cxld);
> >  		cxled->state = td->cxled.state;
> >  		cxled->skip = td->cxled.skip;
> > -		if (range_len(&td->dpa_range)) {
> > -			rc = devm_cxl_dpa_reserve(cxled, td->dpa_range.start,
> > -						  range_len(&td->dpa_range),
> > -						  td->cxled.skip);
> > -			if (rc) {
> > -				init_disabled_mock_decoder(cxld);
> > -				return rc;
> > -			}
> > +		/* enabled endpoint decoders hold a reservation, sized or not */
> > +		rc = devm_cxl_dpa_reserve(cxled, td->dpa_range.start,
> > +					  range_len(&td->dpa_range),
> > +					  td->cxled.skip);
> > +		if (rc) {
> > +			init_disabled_mock_decoder(cxld);
> > +			return rc;
> >  		}
> >  		port->commit_end = cxld->id;
> >  	}
> > @@ -1072,16 +1072,49 @@ static void default_mock_decoder(struct cxl_decoder *cxld)
> >  	WARN_ON_ONCE(!cxld_registry_new(cxld));
> >  }
> >  
> > -static int first_decoder(struct device *dev, const void *data)
> > +static int match_decoder_by_index(struct device *dev, const void *data)
> >  {
> > +	int target_id = *(const int *)data;
> >  	struct cxl_decoder *cxld;
> >  
> >  	if (!is_switch_decoder(dev))
> >  		return 0;
> >  	cxld = to_cxl_decoder(dev);
> > -	if (cxld->id == 0)
> > -		return 1;
> > -	return 0;
> > +	return cxld->id == target_id;
> > +}
> > +
> > +/*
> > + * Mock a committed, locked, empty decoder
> > + * (CXL r4.0 8.2.4.20.12). Gated by the mock_zero_size_decoders module
> > + * param so the default cxl_test topology, shared by the region test
> > + * suite, is left undisturbed.
> > + */
> > +static void size_zero_mock_decoder_ep(struct cxl_decoder *cxld, u64 base)
> > +{
> > +	struct cxl_endpoint_decoder *cxled = to_cxl_endpoint_decoder(&cxld->dev);
> > +
> > +	cxld->hpa_range = DEFINE_RANGE(base, base - 1);
> > +	cxld->interleave_ways = 2;
> > +	cxld->interleave_granularity = 4096;
> > +	cxld->target_type = CXL_DECODER_HOSTONLYMEM;
> > +	cxld->flags = CXL_DECODER_F_ENABLE | CXL_DECODER_F_LOCK;
> > +	cxled->state = CXL_DECODER_STATE_AUTO;
> > +	/* decoder[0] reserved [0, size/2), empty decoders sit at that watermark */
> > +	devm_cxl_dpa_reserve(cxled, mock_auto_region_size / 2, 0, 0);
> > +	cxld->commit = mock_decoder_commit;
> > +	cxld->reset = mock_decoder_reset;
> > +}
> > +
> > +static void size_zero_mock_decoder_sw(struct cxl_decoder *cxld, u64 base,
> > +				      int level)
> > +{
> > +	cxld->flags = CXL_DECODER_F_ENABLE | CXL_DECODER_F_LOCK;
> > +	cxld->target_type = CXL_DECODER_HOSTONLYMEM;
> > +	cxld->interleave_ways = level == 0 ? 2 : 1;
> > +	cxld->interleave_granularity = 4096;
> > +	cxld->hpa_range = DEFINE_RANGE(base, base - 1);
> > +	cxld->commit = mock_decoder_commit;
> > +	cxld->reset = mock_decoder_reset;
> >  }
> >  
> >  enum cxld_init_type {
> > @@ -1114,7 +1147,7 @@ static enum cxld_init_type get_decoder_init_type(struct cxl_decoder *cxld,
> >  	 * See 'cxl list -BMPu -m cxl_mem.0,cxl_mem.4'
> >  	 */
> >  	if (!is_endpoint_decoder(&cxld->dev) || !hb0 || pdev->id % 4 ||
> > -	    pdev->id > 4 || cxld->id > 0)
> > +	    pdev->id > 4 || cxld->id > (mock_zero_size_decoders ? 2 : 0))
> >  		return MOCK_DECODER_INIT_DEFAULT;
> >  
> >  	return type2_test ? MOCK_DECODER_INIT_TYPE2_AUTO :
> > @@ -1147,6 +1180,7 @@ static void mock_init_hdm_type2_cxled(struct cxl_endpoint_decoder *cxled,
> >  	struct cxl_port *root_port;
> >  	struct device *dev;
> >  	u64 base;
> > +	int id = 0;
> >  
> >  	base = window->base_hpa;
> >  	cxld->hpa_range = (struct range) {
> > @@ -1172,7 +1206,7 @@ static void mock_init_hdm_type2_cxled(struct cxl_endpoint_decoder *cxled,
> >  	 */
> >  	dport = port->parent_dport;
> >  	root_port = dport->port;
> > -	dev = device_find_child(&root_port->dev, NULL, first_decoder);
> > +	dev = device_find_child(&root_port->dev, &id, match_decoder_by_index);
> >  	/*
> >  	 * Ancestor ports are guaranteed to be enumerated before
> >  	 * @port, and all ports have at least one decoder.
> > @@ -1232,6 +1266,20 @@ static void mock_init_hdm_type3_cxled(struct cxl_endpoint_decoder *cxled,
> >  	base = window->base_hpa;
> >  	if (extended_linear_cache)
> >  		base += mock_auto_region_size;
> > +
> > +	/*
> > +	 * With mock_zero_size_decoders, decoders 1 and 2 of the special
> > +	 * endpoints mock committed, locked, empty decoders above the
> > +	 * decoder[0] auto-region (CXL r4.0 8.2.4.20.12). commit_end then
> > +	 * points at a zero-size decoder, exercising the zero-size
> > +	 * reservation and poison-by-endpoint code paths.
> > +	 */
> > +	if (cxld->id == 1 || cxld->id == 2) {
> > +		size_zero_mock_decoder_ep(cxld, base);
> > +		port->commit_end = cxld->id;
> > +		WARN_ON_ONCE(!cxld_registry_new(cxld));
> > +		return;
> > +	}
> >  	cxld->hpa_range = (struct range) {
> >  		.start = base,
> >  		.end = base + mock_auto_region_size - 1,
> > @@ -1255,9 +1303,11 @@ static void mock_init_hdm_type3_cxled(struct cxl_endpoint_decoder *cxled,
> >  	 */
> >  	iter = port;
> >  	for (i = 0; i < 2; i++) {
> > +		int id = 0;
> > +
> >  		dport = iter->parent_dport;
> >  		iter = dport->port;
> > -		dev = device_find_child(&iter->dev, NULL, first_decoder);
> > +		dev = device_find_child(&iter->dev, &id, match_decoder_by_index);
> >  		/*
> >  		 * Ancestor ports are guaranteed to be enumerated before
> >  		 * @port, and all ports have at least one decoder.
> > @@ -1307,6 +1357,26 @@ static void mock_init_hdm_type3_cxled(struct cxl_endpoint_decoder *cxled,
> >  
> >  		cxld_registry_update(cxld);
> >  		put_device(dev);
> > +
> > +		if (!mock_zero_size_decoders)
> > +			continue;
> > +
> > +		/*
> > +		 * Mirror the endpoint: commit the next two switch decoders
> > +		 * as zero-size + locked so the empty-decoder layout extends
> > +		 * end-to-end through the switch and host bridge.
> > +		 */
> > +		for (id = 1; id <= 2; id++) {
> > +			dev = device_find_child(&iter->dev, &id,
> > +						match_decoder_by_index);
> > +			if (WARN_ON(!dev))
> > +				continue;
> > +			cxld = to_cxl_decoder(dev);
> > +			size_zero_mock_decoder_sw(cxld, base, i);
> > +			iter->commit_end = id;
> > +			cxld_registry_update(cxld);
> > +			put_device(dev);
> > +		}
> >  	}
> >  }
> >  
> > @@ -2085,6 +2155,12 @@ static bool __init have_multiple_modparms(void)
> >  		count++;
> >  	if (type2_test)
> >  		count++;
> > +	if (mock_zero_size_decoders)
> > +		count++;
> > +
> > +	/* Zero-size decoders require decoder[0] to preserve commit order. */
> > +	if (fail_autoassemble && mock_zero_size_decoders)
> > +		return true;
> >  
> >  	return count > 1;
> >  }
> > @@ -2493,6 +2569,9 @@ module_param(fail_autoassemble, bool, 0444);
> >  MODULE_PARM_DESC(fail_autoassemble, "Simulate missing member of an auto-region");
> >  module_param(type2_test, bool, 0444);
> >  MODULE_PARM_DESC(type2_test, "Enable type 2 support testing");
> > +module_param(mock_zero_size_decoders, bool, 0444);
> > +MODULE_PARM_DESC(mock_zero_size_decoders,
> > +		 "Mock committed, locked, empty decoders under host-bridge0");
> >  module_init(cxl_test_init);
> >  module_exit(cxl_test_exit);
> >  MODULE_LICENSE("GPL v2");
> > -- 
> > 2.43.0
> >