[PATCH v2] cxl/region: Add cxl_decoder_is_passthrough() helper

[PATCH v2] cxl/region: Add cxl_decoder_is_passthrough() helper

[Richard Cheng]

commit_decoder() open-codes the passthrough test.
A NULL ->commit() plus a switch decoder with at most one target. The
call site is hard to read, and it suggests a NULL ->commit() alone
identifies a passthrough decoder. It does not.
DVSEC-emulated endpoint decoders also leave ->commit() NULL, and root
decoders never set it. A future caller testing ->commit() alone would
silently include them.

Move the test into cxl_decoder_is_passthrough() and document what each
condition rules out. The helper runs the same tests in the same order,
no functional change.

Signed-off-by: Richard Cheng <icheng@nvidia.com>
---
Changelog:

v1 -> v2:
    - Dropped the passthrough F_ENABLE restore patch. The bug was
      already fixed.
    - What remains is only the clarify helper function patch, so the
      patch is retitled accordingly

v1:
https://lore.kernel.org/linux-cxl/20260727103743.63343-1-icheng@nvidia.com/

Best regards,
Richard Cheng.
---
 drivers/cxl/core/region.c | 28 ++++++++++++++++++++++------
 1 file changed, 22 insertions(+), 6 deletions(-)

diff --git a/drivers/cxl/core/region.c b/drivers/cxl/core/region.c
index 27e63e6dab7c..cd73684fbe97 100644
--- a/drivers/cxl/core/region.c
+++ b/drivers/cxl/core/region.c
@@ -293,17 +293,33 @@ static void cxl_region_decode_reset(struct cxl_region *cxlr, int count)
 	clear_bit(CXL_REGION_F_NEEDS_RESET, &cxlr->flags);
 }
 
-static int commit_decoder(struct cxl_decoder *cxld)
+/*
+ * A single-dport host-bridge need not publish an HDM decoder capability
+ * when passthrough decode can be assumed. The resulting decoder is a
+ * software-only construct with no registers to program, so it carries no
+ * ->commit() operation, see devm_cxl_add_passthrough_decoder().
+ *
+ * A NULL ->commit() alone does not identify one, it is also NULL for
+ * DVSEC-emulated endpoint decoders. Test the decoder type and target
+ * count as well.
+ */
+static bool cxl_decoder_is_passthrough(struct cxl_decoder *cxld)
 {
-	struct cxl_switch_decoder *cxlsd = NULL;
+	if (cxld->commit)
+		return false;
+
+	if (!is_switch_decoder(&cxld->dev))
+		return false;
+
+	return to_cxl_switch_decoder(&cxld->dev)->nr_targets <= 1;
+}
 
+static int commit_decoder(struct cxl_decoder *cxld)
+{
 	if (cxld->commit)
 		return cxld->commit(cxld);
 
-	if (is_switch_decoder(&cxld->dev))
-		cxlsd = to_cxl_switch_decoder(&cxld->dev);
-
-	if (dev_WARN_ONCE(&cxld->dev, !cxlsd || cxlsd->nr_targets > 1,
+	if (dev_WARN_ONCE(&cxld->dev, !cxl_decoder_is_passthrough(cxld),
 			  "->commit() is required\n"))
 		return -ENXIO;
 	return 0;

base-commit: 212e015fc34712c849653cdb3179cd643c915015
-- 
2.43.0

Re: [PATCH v2] cxl/region: Add cxl_decoder_is_passthrough() helper

[sashiko-bot]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [Medium] `cxl_decoder_is_passthrough()` incorrectly identifies single-target root decoders as passthrough decoders, contradicting the commit message.
--

--- Patch [1]: [PATCH v2] cxl/region: Add cxl_decoder_is_passthrough() helper ---
commit 9b921fd6e2f183466d76ed78181b72b15a45105b
Author: Richard Cheng <icheng@nvidia.com>
cxl/region: Add cxl_decoder_is_passthrough() helper

This commit abstracts the passthrough decoder check into a standalone
helper function. The commit message explicitly notes this is necessary
because checking only for a NULL ->commit() would incorrectly identify
root decoders and DVSEC-emulated endpoint decoders as passthrough.

Link: https://lore.kernel.org/linux-cxl/20260727103743.63343-1-icheng@nvidia.com/

> 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
[ ... ]
> +/*
> + * A single-dport host-bridge need not publish an HDM decoder capability
> + * when passthrough decode can be assumed. The resulting decoder is a
> + * software-only construct with no registers to program, so it carries no
> + * ->commit() operation, see devm_cxl_add_passthrough_decoder().
> + *
> + * A NULL ->commit() alone does not identify one, it is also NULL for
> + * DVSEC-emulated endpoint decoders. Test the decoder type and target
> + * count as well.
> + */
> +static bool cxl_decoder_is_passthrough(struct cxl_decoder *cxld)
> +{
> +	if (cxld->commit)
> +		return false;
> +
> +	if (!is_switch_decoder(&cxld->dev))
> +		return false;
> +
> +	return to_cxl_switch_decoder(&cxld->dev)->nr_targets <= 1;

[Severity: Medium]
Does this helper correctly exclude root decoders as intended?

The commit message states that root decoders leave ->commit() NULL and
that a future caller testing ->commit() alone would silently include them.
However, looking at is_switch_decoder() in drivers/cxl/core/port.c, it
returns true for root decoders:

bool is_switch_decoder(struct device *dev)
{
        return is_root_decoder(dev) || dev->type == &cxl_decoder_switch_type;
}

Since is_switch_decoder() evaluates to true for root decoders, won't this
function incorrectly return true for single-target root decoders? This
appears to create a misleading abstraction that contradicts the commit
message's claim about ruling out root decoders.

> +}

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260805065935.27837-1-icheng@nvidia.com?part=1

Re: [PATCH v2] cxl/region: Add cxl_decoder_is_passthrough() helper

[Robert Richter]

On 05.08.26 14:59:34, Richard Cheng wrote:
> commit_decoder() open-codes the passthrough test.
> A NULL ->commit() plus a switch decoder with at most one target. The
> call site is hard to read, and it suggests a NULL ->commit() alone
> identifies a passthrough decoder. It does not.
> DVSEC-emulated endpoint decoders also leave ->commit() NULL, and root
> decoders never set it. A future caller testing ->commit() alone would
> silently include them.
> 
> Move the test into cxl_decoder_is_passthrough() and document what each
> condition rules out. The helper runs the same tests in the same order,
> no functional change.
> 
> Signed-off-by: Richard Cheng <icheng@nvidia.com>
> ---
> Changelog:
> 
> v1 -> v2:
>     - Dropped the passthrough F_ENABLE restore patch. The bug was
>       already fixed.
>     - What remains is only the clarify helper function patch, so the
>       patch is retitled accordingly
> 
> v1:
> https://lore.kernel.org/linux-cxl/20260727103743.63343-1-icheng@nvidia.com/
> 
> Best regards,
> Richard Cheng.
> ---
>  drivers/cxl/core/region.c | 28 ++++++++++++++++++++++------
>  1 file changed, 22 insertions(+), 6 deletions(-)
> 
> diff --git a/drivers/cxl/core/region.c b/drivers/cxl/core/region.c
> index 27e63e6dab7c..cd73684fbe97 100644
> --- a/drivers/cxl/core/region.c
> +++ b/drivers/cxl/core/region.c
> @@ -293,17 +293,33 @@ static void cxl_region_decode_reset(struct cxl_region *cxlr, int count)
>  	clear_bit(CXL_REGION_F_NEEDS_RESET, &cxlr->flags);
>  }
>  
> -static int commit_decoder(struct cxl_decoder *cxld)
> +/*
> + * A single-dport host-bridge need not publish an HDM decoder capability
> + * when passthrough decode can be assumed. The resulting decoder is a
> + * software-only construct with no registers to program, so it carries no
> + * ->commit() operation, see devm_cxl_add_passthrough_decoder().
> + *
> + * A NULL ->commit() alone does not identify one, it is also NULL for
> + * DVSEC-emulated endpoint decoders. Test the decoder type and target
> + * count as well.
> + */
> +static bool cxl_decoder_is_passthrough(struct cxl_decoder *cxld)

This is much noise for just checking if ->commit() is required in some
cases and generate an error if not.

>  {
> -	struct cxl_switch_decoder *cxlsd = NULL;
> +	if (cxld->commit)
> +		return false;

That duplicates the test and the code is never executed.

> +
> +	if (!is_switch_decoder(&cxld->dev))
> +		return false;
> +
> +	return to_cxl_switch_decoder(&cxld->dev)->nr_targets <= 1;

cxlsd->nr_targets is easier to read and explains that nr_targets is in
struct cxl_switch_decoder, which wouln't be obious else. So the change
does not improve readability and instead does multiple things in a
single line.

> +}
>  
> +static int commit_decoder(struct cxl_decoder *cxld)
> +{
>  	if (cxld->commit)
>  		return cxld->commit(cxld);
>  
> -	if (is_switch_decoder(&cxld->dev))
> -		cxlsd = to_cxl_switch_decoder(&cxld->dev);
> -
> -	if (dev_WARN_ONCE(&cxld->dev, !cxlsd || cxlsd->nr_targets > 1,
> +	if (dev_WARN_ONCE(&cxld->dev, !cxl_decoder_is_passthrough(cxld),
>  			  "->commit() is required\n"))

The original was pretty straight forward: if it is a switch decoder
with multiple targets, raise an error. It looks like an assert here as
!cxld->commit is only created in devm_cxl_add_passthrough_decoder()
which sets the number of targets to one (not looking into test code
here, but if that causes an issue, fix it there).

So I don't see much sense in this patch.

Thanks,

-Robert

>  		return -ENXIO;
>  	return 0;
> 
> base-commit: 212e015fc34712c849653cdb3179cd643c915015
> -- 
> 2.43.0
>