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To:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc:     jglisse@redhat.com, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org,
        "Rafael J . Wysocki" <rafael@kernel.org>,
        Bjorn Helgaas <bhelgaas@google.com>,
        Christian Koenig <christian.koenig@amd.com>,
        Felix Kuehling <Felix.Kuehling@amd.com>,
        Jason Gunthorpe <jgg@mellanox.com>, linux-pci@vger.kernel.org,
        dri-devel@lists.freedesktop.org, Christoph Hellwig <hch@lst.de>,
        Marek Szyprowski <m.szyprowski@samsung.com>,
        Robin Murphy <robin.murphy@arm.com>,
        Joerg Roedel <jroedel@suse.de>,
        iommu@lists.linux-foundation.org
References: <20190129174728.6430-1-jglisse@redhat.com>
 <20190129174728.6430-2-jglisse@redhat.com>
 <f66ba584-9c4a-f6cd-c647-9b32a93be807@deltatee.com>
 <20190129194426.GB32069@kroah.com>
From:   Logan Gunthorpe <logang@deltatee.com>
Message-ID: <8b4e0157-4eaf-c79a-28d0-7a266abe2207@deltatee.com>
Date:   Tue, 29 Jan 2019 13:44:09 -0700
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Subject: Re: [RFC PATCH 1/5] pci/p2p: add a function to test peer to peer
 capability
Sender: linux-kernel-owner@vger.kernel.org
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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@redhat.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.

Logan