On Tue, 2018-09-25 at 10:22 -0600, Logan Gunthorpe wrote:

[ ... ]

Hi Logan,

It's great to see this patch series making progress. Unfortunately I didn't
have the time earlier to have a closer look at this patch series. I hope that
you don't mind that I ask a few questions about the implementation?

+static void pci_p2pdma_percpu_kill(void *data)
+{
+	struct percpu_ref *ref = data;
+
+	if (percpu_ref_is_dying(ref))
+		return;
+
+	percpu_ref_kill(ref);
+}

The percpu_ref_is_dying() test should either be removed or a comment should be
added above it that explains why it is necessary. Is the purpose of that call
perhaps to protect against multiple calls of pci_p2pdma_percpu_kill()? If so,
which mechanism serializes these multiple calls?

+static void pci_p2pdma_release(void *data)
+{
+	struct pci_dev *pdev = data;
+
+	if (!pdev->p2pdma)
+		return;
+
+	wait_for_completion(&pdev->p2pdma->devmap_ref_done);
+	percpu_ref_exit(&pdev->p2pdma->devmap_ref);
+
+	gen_pool_destroy(pdev->p2pdma->pool);
+	pdev->p2pdma = NULL;
+}

Which code frees the memory pdev->p2pdma points at? Other functions similar to
pci_p2pdma_release() call devm_remove_action(), e.g. hmm_devmem_ref_exit().

+static int pci_p2pdma_setup(struct pci_dev *pdev)
+{
+	int error = -ENOMEM;
+	struct pci_p2pdma *p2p;
+
+	p2p = devm_kzalloc(&pdev->dev, sizeof(*p2p), GFP_KERNEL);
+	if (!p2p)
+		return -ENOMEM;
+
+	p2p->pool = gen_pool_create(PAGE_SHIFT, dev_to_node(&pdev->dev));
+	if (!p2p->pool)
+		goto out;
+
+	init_completion(&p2p->devmap_ref_done);
+	error = percpu_ref_init(&p2p->devmap_ref,
+			pci_p2pdma_percpu_release, 0, GFP_KERNEL);
+	if (error)
+		goto out_pool_destroy;
+
+	percpu_ref_switch_to_atomic_sync(&p2p->devmap_ref);

Why are percpu_ref_init() and percpu_ref_switch_to_atomic_sync() called
separately instead of passing PERCPU_REF_INIT_ATOMIC to percpu_ref_init()?
Would using PERCPU_REF_INIT_ATOMIC eliminate a call_rcu_sched() call and
hence make this function faster?

+static struct pci_dev *find_parent_pci_dev(struct device *dev)
+{
+	struct device *parent;
+
+	dev = get_device(dev);
+
+	while (dev) {
+		if (dev_is_pci(dev))
+			return to_pci_dev(dev);
+
+		parent = get_device(dev->parent);
+		put_device(dev);
+		dev = parent;
+	}
+
+	return NULL;
+}

The above function increases the reference count of the device it returns a
pointer to. It is a good habit to explain such behavior above the function
definition.

+static void seq_buf_print_bus_devfn(struct seq_buf *buf, struct pci_dev *pdev)
+{
+	if (!buf)
+		return;
+
+	seq_buf_printf(buf, "%s;", pci_name(pdev));
+}

NULL checks in functions that print to a seq buffer are unusual. Is it
possible that a NULL pointer gets passed as the first argument to
seq_buf_print_bus_devfn()?

+struct pci_p2pdma_client {
+	struct list_head list;
+	struct pci_dev *client;
+	struct pci_dev *provider;
+};

Is there a reason that the peer-to-peer client and server code exist in the
same source file? If not, have you considered to split the p2pdma.c file into
two files - one with the code for devices that provide p2p functionality and
another file with the code that supports p2p users? I think that would make it
easier to follow the code.

+/**
+ * pci_free_p2pmem - allocate peer-to-peer DMA memory
+ * @pdev: the device the memory was allocated from
+ * @addr: address of the memory that was allocated
+ * @size: number of bytes that was allocated
+ */
+void pci_free_p2pmem(struct pci_dev *pdev, void *addr, size_t size)
+{
+	gen_pool_free(pdev->p2pdma->pool, (uintptr_t)addr, size);
+	percpu_ref_put(&pdev->p2pdma->devmap_ref);
+}
+EXPORT_SYMBOL_GPL(pci_free_p2pmem);

Please fix the header of this function - there is a copy-paste error in the
function header.

Thanks,

Bart.