RE: [PATCH v4 02/27] cxl/regs: Skip sub-block region request for BAR-owning drivers

[Manish Honap]

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> From: Alex Williamson <alex@xxxxxxxxxxx>
> Sent: Wednesday, August 26, 2026 2:56 AM
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> Subject: Re: [PATCH v4 02/27] cxl/regs: Skip sub-block region request for BAR-
> owning drivers
> 
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> 
> On Thu, 13 Aug 2026 15:06:06 +0530
> <mhonap@xxxxxxxxxx> wrote:
> 
> > From: Manish Honap <mhonap@xxxxxxxxxx>
> >
> > cxl_map_component_regs() claims each mapped sub-block with
> > devm_request_mem_region(). A driver that owns the whole component
> > register BAR, such as vfio-cxl, has already claimed the full BAR, so
> > the per-sub-block claim collides and the mapping fails.
> >
> > Add a bar_owned parameter to cxl_pci_setup_regs() and record it in
> > skip_sub_bar_request on the register map. When set, cxl-core maps the
> > sub-block without requesting the region and leaves ownership with the
> > upper driver. cxl_pci passes false and keeps its existing claim,
> > preserving /dev/mem tooling access to the rest of the component space.
> 
> The bool arg itself is an undesirable shape, but then threading it through
> @bar_owned to @skip_sub_bar_request to @request is difficult to follow with
> limited utility.
> 
> What if instead cxl allowed drivers to register the resources they've already
> requested into the reg_map, ex:
> 
> int cxl_reg_map_add_owned_resource(struct cxl_register_map *reg_map,
>                                    struct resource *res)
> 
> Then before cxl does any devm_request_mem_region() calls it creates a
> temporary struct resource for the range it wants to request and compares it to
> the resources the driver already reported as owned via resource_contains()?
> 
> I'm picking reg_map vs cxlds because it seems easier to thread through to
> where we need it.  In this flow, devm_cxl_iomap_block() could be split into
> devm_cxl_request_block() and devm_cxl_ioremap_block(), where
> cxl_map_component_regs() would conditionally call the former when
> resource_contains() finds no matches for driver owned resources, and the
> latter is called unconditionally.  devm_cxl_iomap_block() could remain as the
> unconditional user of both.  Thanks,
> 
> Alex

Thanks, agreed on both counts; I will switch to the registered-resource
cxl_reg_map_add_owned_resource approach you described.

Manish