linux-kernel - Re: [PATCH V6 06/10] cxl/pci: Find the DOE mailbox which supports CDAT

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Date:   Tue, 1 Feb 2022 14:18:41 -0800
From:   Ira Weiny <ira.weiny@...el.com>
To:     Ben Widawsky <ben.widawsky@...el.com>
Cc:     Dan Williams <dan.j.williams@...el.com>,
        Jonathan Cameron <Jonathan.Cameron@...wei.com>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Alison Schofield <alison.schofield@...el.com>,
        Vishal Verma <vishal.l.verma@...el.com>,
        linux-kernel@...r.kernel.org, linux-cxl@...r.kernel.org,
        linux-pci@...r.kernel.org
Subject: Re: [PATCH V6 06/10] cxl/pci: Find the DOE mailbox which supports
 CDAT

On Tue, Feb 01, 2022 at 10:49:47AM -0800, Widawsky, Ben wrote:
> On 22-01-31 23:19:48, ira.weiny@...el.com wrote:
> > From: Ira Weiny <ira.weiny@...el.com>
> > 
> > Memory devices need the CDAT data from the device.  This data is read
> > from a DOE mailbox which supports the CDAT protocol.
> > 
> > Search the DOE auxiliary devices for the one which supports the CDAT
> > protocol.  Cache that device to be used for future queries.
> > 
> > Signed-off-by: Ira Weiny <ira.weiny@...el.com>

[snip]

> >  
> > diff --git a/drivers/cxl/pci.c b/drivers/cxl/pci.c
> > index d4ae79b62a14..dcc55c4efd85 100644
> > --- a/drivers/cxl/pci.c
> > +++ b/drivers/cxl/pci.c
> > @@ -536,12 +536,53 @@ static int cxl_dvsec_ranges(struct cxl_dev_state *cxlds)
> >  	return rc;
> >  }
> >  
> > +static int cxl_match_cdat_doe_device(struct device *dev, const void *data)
> > +{
> > +	const struct cxl_dev_state *cxlds = data;
> > +	struct auxiliary_device *adev;
> > +	struct pci_doe_dev *doe_dev;
> > +
> > +	/* First determine if this auxiliary device belongs to the cxlds */
> > +	if (cxlds->dev != dev->parent)
> > +		return 0;
> 
> I don't understand auxiliary bus but I'm wondering why it's checking the parent
> of the device?

auxiliary_find_device() iterates all the auxiliary devices in the system.  This
check was a way for the match function to know if the auxiliary device belongs
to the cxlds we are interested in...

But now that I think about it we could have other auxiliary devices attached
which are not DOE...  :-/  So this check is not complete.

FWIW I'm not thrilled with the way auxiliary_find_device() is defined.  And now
that I look at it I think the only user of it currently is wrong.  They too
have a check like this but it is after another check...  :-/

I was hoping to avoid having a list of DOE devices in the cxlds and simply let
the auxiliary bus infrastructure do that somehow.  IIRC Jonathan was thinking
along the same lines.  I think he actually suggested auxiliary_find_device()...

It would be nice if I could have an aux_find_child() or something which
iterated the auxiliary devices attached to a particular parent device.  I've
just not figured out exactly how to implement that better than what I did here.

> 
> > +
> > +	adev = to_auxiliary_dev(dev);
> > +	doe_dev = container_of(adev, struct pci_doe_dev, adev);
> > +
> > +	/* If it is one of ours check for the CDAT protocol */
> > +	if (pci_doe_supports_prot(doe_dev, PCI_DVSEC_VENDOR_ID_CXL,
> > +				  CXL_DOE_PROTOCOL_TABLE_ACCESS))
> > +		return 1;
> > +
> > +	return 0;
> > +}
> > +
> >  static int cxl_setup_doe_devices(struct cxl_dev_state *cxlds)
> >  {
> >  	struct device *dev = cxlds->dev;
> >  	struct pci_dev *pdev = to_pci_dev(dev);
> > +	struct auxiliary_device *adev;
> > +	int rc;
> >  
> > -	return pci_doe_create_doe_devices(pdev);
> > +	rc = pci_doe_create_doe_devices(pdev);
> > +	if (rc)
> > +		return rc;
> > +
> > +	adev = auxiliary_find_device(NULL, cxlds, &cxl_match_cdat_doe_device);
> > +
> > +	if (adev) {
> > +		struct pci_doe_dev *doe_dev = container_of(adev,
> > +							   struct pci_doe_dev,
> > +							   adev);
> > +
> > +		/*
> > +		 * No reference need be taken.  The DOE device lifetime is
> > +		 * longer that the CXL device state lifetime
> > +		 */
> 
> You're holding a reference to the adev here. Did you mean to drop it?

Does find device get a reference? ...  Ah shoot I did not see that.

Yea the reference should be dropped somewhere.

Thanks,
Ira

> 
> > +		cxlds->cdat_doe = doe_dev;
> > +	}
> > +
> > +	return 0;
> >  }
> >  
> >  static int cxl_pci_probe(struct pci_dev *pdev, const struct pci_device_id *id)
> > -- 
> > 2.31.1
> >