linux-kernel - Re: [RFC PATCH 1/5] pci/p2p: add a function to test peer to peer capability

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Date:   Tue, 29 Jan 2019 16:00:08 -0500
From:   Jerome Glisse <jglisse@...hat.com>
To:     Logan Gunthorpe <logang@...tatee.com>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        "Rafael J . Wysocki" <rafael@...nel.org>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Christian Koenig <christian.koenig@....com>,
        Felix Kuehling <Felix.Kuehling@....com>,
        Jason Gunthorpe <jgg@...lanox.com>, linux-pci@...r.kernel.org,
        dri-devel@...ts.freedesktop.org, Christoph Hellwig <hch@....de>,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        Robin Murphy <robin.murphy@....com>,
        Joerg Roedel <jroedel@...e.de>,
        iommu@...ts.linux-foundation.org
Subject: Re: [RFC PATCH 1/5] pci/p2p: add a function to test peer to peer
 capability

On Tue, Jan 29, 2019 at 01:44:09PM -0700, Logan Gunthorpe wrote:
> 
> 
> On 2019-01-29 12:44 p.m., Greg Kroah-Hartman wrote:
> > On Tue, Jan 29, 2019 at 11:24:09AM -0700, Logan Gunthorpe wrote:
> >>
> >>
> >> On 2019-01-29 10:47 a.m., jglisse@...hat.com wrote:
> >>> +bool pci_test_p2p(struct device *devA, struct device *devB)
> >>> +{
> >>> +	struct pci_dev *pciA, *pciB;
> >>> +	bool ret;
> >>> +	int tmp;
> >>> +
> >>> +	/*
> >>> +	 * For now we only support PCIE peer to peer but other inter-connect
> >>> +	 * can be added.
> >>> +	 */
> >>> +	pciA = find_parent_pci_dev(devA);
> >>> +	pciB = find_parent_pci_dev(devB);
> >>> +	if (pciA == NULL || pciB == NULL) {
> >>> +		ret = false;
> >>> +		goto out;
> >>> +	}
> >>> +
> >>> +	tmp = upstream_bridge_distance(pciA, pciB, NULL);
> >>> +	ret = tmp < 0 ? false : true;
> >>> +
> >>> +out:
> >>> +	pci_dev_put(pciB);
> >>> +	pci_dev_put(pciA);
> >>> +	return false;
> >>> +}
> >>> +EXPORT_SYMBOL_GPL(pci_test_p2p);
> >>
> >> This function only ever returns false....
> > 
> > I guess it was nevr actually tested :(
> > 
> > I feel really worried about passing random 'struct device' pointers into
> > the PCI layer.  Are we _sure_ it can handle this properly?
> 
> Yes, there are a couple of pci_p2pdma functions that take struct devices
> directly simply because it's way more convenient for the caller. That's
> what find_parent_pci_dev() takes care of (it returns false if the device
> is not a PCI device). Whether that's appropriate here is hard to say
> seeing we haven't seen any caller code.

Caller code as a reference (i already given that link in other part of
thread but just so that people don't have to follow all branches).

https://cgit.freedesktop.org/~glisse/linux/commit/?h=hmm-p2p&id=401a567696eafb1d4faf7054ab0d7c3a16a5ef06

Cheers,
Jérôme