Re: [PATCH 1/4] cxl: Remove the CXL_DECODER_MIXED mistake

[Dan Williams <dan.j.williams@intel.com>]

Alejandro Lucero Palau wrote:
> 
> On 1/17/25 06:10, Dan Williams wrote:
> > CXL_DECODER_MIXED is a safety mechanism introduced for the case where
> > platform firmware has programmed an endpoint decoder that straddles a
> > DPA partition boundary. While the kernel is careful to only allocate DPA
> > capacity within a single partition there is no guarantee that platform
> > firmware, or anything that touched the device before the current kernel,
> > gets that right.
> >
> > However, __cxl_dpa_reserve() will never get to the CXL_DECODER_MIXED
> > designation because of the way it tracks partition boundaries. A
> > request_resource() that spans ->ram_res and ->pmem_res fails with the
> > following signature:
> >
> >      __cxl_dpa_reserve: cxl_port endpoint15: decoder15.0: failed to reserve allocation
> >
> > CXL_DECODER_MIXED is dead defensive programming after the driver has
> > already given up on the device. It has never offered any protection in
> > practice, just delete it.
> 
> 
> I wonder if the reason for adding this CXL_DECODER_MIXED  does still 
> worth it for fixing __cxl_dpa_reserve instead of just not supporting 
> this case.

See where that "failed to reserve allocation" message is printed. That
leads to the driver giving up on the device before the bad decoder
setting can confuse other code paths.