Re: [PATCH v14 1/2] cxl/core/regs: Add rcd_pcie_cap initialization

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

Dan Williams wrote:
> Kobayashi,Daisuke wrote:
> > Add rcd_pcie_cap and its initialization to cache the offset of cxl1.1 
> > device link status information. By caching it, avoid the walking 
> > memory map area to find the offset when output the register value.
> > 
> > Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
> > Signed-off-by: "Kobayashi,Daisuke" <kobayashi.da-06@fujitsu.com>
> > ---
> >  drivers/cxl/core/core.h |  6 ++++
> >  drivers/cxl/core/regs.c | 61 +++++++++++++++++++++++++++++++++++++++++
> >  drivers/cxl/cxl.h       |  9 ++++++
> >  drivers/cxl/pci.c       |  8 ++++--
> >  4 files changed, 82 insertions(+), 2 deletions(-)
> > 
[..]
> > diff --git a/drivers/cxl/pci.c b/drivers/cxl/pci.c
> > index 2ff361e756d6..bbc55732d6c1 100644
> > --- a/drivers/cxl/pci.c
> > +++ b/drivers/cxl/pci.c
> > @@ -512,11 +512,15 @@ static int cxl_pci_setup_regs(struct pci_dev *pdev, enum cxl_regloc_type type,
> >  	 * is an RCH and try to extract the Component Registers from
> >  	 * an RCRB.
> >  	 */
> > -	if (rc && type == CXL_REGLOC_RBI_COMPONENT && is_cxl_restricted(pdev))
> > +	if (rc && type == CXL_REGLOC_RBI_COMPONENT && is_cxl_restricted(pdev)) {
> >  		rc = cxl_rcrb_get_comp_regs(pdev, map);
> > +		if (rc)
> > +			return rc;
> >  
> > -	if (rc)
> > +		cxl_dport_map_rcd_linkcap(pdev);
> 
[..]
> Ugh, I was going to say copy what cxl_mem_probe() does around locking
> endpoint_parent before attaching further ports, but that also appears to
> not handle the same race. I.e. I think cxl_mem_probe() needs a fix to do
> this as well. I will copy you on a proposed patch for that.

I attempted to add the proper locking to keep cxl_dport live, but that
runs into lockdep issues.

So I think a better fix is rework dport lifetime to stay alive until the
final put_device() of the port. In other words dport instances get added
dynamically to the cxl_port, but only get destroyed after all port
references are dropped. Then the @dport result from find_cxl_port() is
not ephemeral.

Given this is a latent bug that affects all current
cxl_{mem,pci}_find_port() users, the planned fix is to just make dport
lifetime longer, and that I will not have time to do that rework before
v6.11 merge window, then I am ok for this lnkcap code to introduce
another instance of the same bug.

So, just make cxl_rcrb_get_comp_regs() and cxl_dport_map_rcd_linkcap()
share the same port reference from one cxl_pci_find_port() call.