Re: [PATCH v8 08/10] cxl: Allow auto-committed BI hdm decoders

[Jonathan Cameron <jic23@kernel.org>]

On Wed,  9 Sep 2026 10:03:00 -0700
Davidlohr Bueso <dave@stgolabs.net> wrote:

> Allow auto-committed BI hdm decoders on sane platforms, rejecting
> only broken paths (ie: one that cannot route BISnp, or BI paired
> with a host-only target range type).
> 
> The respective region creation is done like any other committed
> decoder - with cxlds->bi set by the time an decoder attaches.
> 
> Skipping the commit does not weaken the rule stated in "cxl/pci: Add
> BI topology enable/disable". Table 8-152 and Table 8-156 key it on a
> new BI device being enabled anywhere below the port, and a level
> firmware already brought up for this device is not seeing one.
> 
> Whether it did is a property of the path, not of the dport: a level's
> committed state only proves firmware committed for some device below
> it, which need not be this one. So adoption starts from the
> endpoint's own BI Enable - only a device firmware itself enabled can
> have had its BI-ID accounted for above - and a level is then taken
> as found when nothing this driver routed sits below it, the control
> value this driver would write is the one already there, the decoder
> is committed, and the switch's route table - which carries a commit
> of its own that firmware may not have performed - is committed too.
> The first level that falls short ends it: from there up the driver
> is enabling something new, and programs and commits as for any
> other device.
> 
> A committed decoder is refused when the window's restrictions do not
> permit its coherency model, on either axis: a BI decoder under a
> window without the BI restriction, or a host-only or device-coherent
> decoder under a window exposing only the other model (undefined
> behavior per the CFMWS Window Restrictions). Assembly so far never
> consulted the type bits, so a platform that sets them wrongly loses
> auto-assembly of the affected decoders, with the refusal naming the
> window.
> 
> A committed decoder cannot inherit a region's coherency model the way
> a decoder this driver programs does, so the type-mismatch refusal
> that "cxl: Add HDM-DB region creation" removed from
> cxl_region_attach() in favor of inheritance returns there for
> committed decoders, on both axes: the committed Target Range Type,
> kept as the decoder's target_type since enumeration, must match the
> region's, and the committed BI bit, read back from the decoder since
> the driver keeps no copy of it, must match the root's. The refusal
> names both models.
> 
> Reviewed-by: Dave Jiang <dave.jiang@intel.com>
> Signed-off-by: Davidlohr Bueso <dave@stgolabs.net>
I'm less confident about the flows where the firmware did stuff in
advance in general as never used them in practice, but far as I can
see this is fine.

Reviewed-by: Jonathan Cameron <jonathan.cameron@oss.qualcomm.com>