Re: [PATCH] cxl/hdm: Add support for 32 switch decoders

[Alison Schofield <alison.schofield@intel.com>]

On Thu, Mar 19, 2026 at 07:11:34PM +0800, Li Ming wrote:
> 
> 在 2026/3/18 23:40, Dave Jiang 写道:
> > 
> > On 3/18/26 6:00 AM, Li Ming wrote:
> > > Per CXL r4.0 section 8.2.4.20.1. CXL host bridge and switch ports can
> > > support 32 HDM decoders. Current implementation misses some decoders on
> > > CXL host bridge and switch in the case that the value of Decoder Count
> > > field in CXL HDM decoder Capability Register is greater than or equal to
> > > 9.
> > > 
> > > Update calculation implementation to ensure the decoder count calculation
> > > is correct for CXL host bridge/switch ports.
> > > 
> > > Signed-off-by: Li Ming <ming.li@zohomail.com>
> > > ---
> > >   drivers/cxl/cxl.h | 7 ++++++-
> > >   1 file changed, 6 insertions(+), 1 deletion(-)
> > > 
> > > diff --git a/drivers/cxl/cxl.h b/drivers/cxl/cxl.h
> > > index 9b947286eb9b..466b8eeefed7 100644
> > > --- a/drivers/cxl/cxl.h
> > > +++ b/drivers/cxl/cxl.h
> > > @@ -77,7 +77,12 @@ static inline int cxl_hdm_decoder_count(u32 cap_hdr)
> > >   {
> > >   	int val = FIELD_GET(CXL_HDM_DECODER_COUNT_MASK, cap_hdr);
> > > -	return val ? val * 2 : 1;
> > > +	if (!val)
> > > +		return 1;
> > > +	else if (val <= 8)
> > > +		return val * 2;
> > > +	else
> > > +		return 4 * (val - 4);
> > That looks a bit messy. How about:
> > 
> > switch (val) {
> > case 0:
> > 	return 1;
> > case 1..8:
> > 	return val * 2;
> > case 9..12:
> > 	return val 4 * (val - 4);
> > default:
> > 	return -ENXIO;
> > }
> > 
> > DJ
> 
> Sure, will do that in v2.

Hi Ming,

You may have already spied this while implementing. Before this change
this function could only return 1 or val*2, so never negative. Adding
-ENXIO changes that and could wrap to huge value in the decoder for
loops. Change decoder_count field to int to clean that up.

Or, you could do something like this:
	WARN_ONCE(1, "reserved HDM decoder count field: %d\n", val);
        return 0;

I like the ENXIO and type cleanup or maybe you have something else?

--Alison

> 
> 
> Ming
>