perf tools: Don't clone maps from parent when synthesizing forks

[linux/fpc-iii.git] / drivers / pci / p2pdma.c

// SPDX-License-Identifier: GPL-2.0
/*
 * PCI Peer 2 Peer DMA support.
 *
 * Copyright (c) 2016-2018, Logan Gunthorpe
 * Copyright (c) 2016-2017, Microsemi Corporation
 * Copyright (c) 2017, Christoph Hellwig
 * Copyright (c) 2018, Eideticom Inc.
 */

#define pr_fmt(fmt) "pci-p2pdma: " fmt
#include <linux/ctype.h>
#include <linux/pci-p2pdma.h>
#include <linux/module.h>
#include <linux/slab.h>
#include <linux/genalloc.h>
#include <linux/memremap.h>
#include <linux/percpu-refcount.h>
#include <linux/random.h>
#include <linux/seq_buf.h>

struct pci_p2pdma {
        struct percpu_ref devmap_ref;
        struct completion devmap_ref_done;
        struct gen_pool *pool;
        bool p2pmem_published;
};

static ssize_t size_show(struct device *dev, struct device_attribute *attr,
                         char *buf)
{
        struct pci_dev *pdev = to_pci_dev(dev);
        size_t size = 0;

        if (pdev->p2pdma->pool)
                size = gen_pool_size(pdev->p2pdma->pool);

        return snprintf(buf, PAGE_SIZE, "%zd\n", size);
}
static DEVICE_ATTR_RO(size);

static ssize_t available_show(struct device *dev, struct device_attribute *attr,
                              char *buf)
{
        struct pci_dev *pdev = to_pci_dev(dev);
        size_t avail = 0;

        if (pdev->p2pdma->pool)
                avail = gen_pool_avail(pdev->p2pdma->pool);

        return snprintf(buf, PAGE_SIZE, "%zd\n", avail);
}
static DEVICE_ATTR_RO(available);

static ssize_t published_show(struct device *dev, struct device_attribute *attr,
                              char *buf)
{
        struct pci_dev *pdev = to_pci_dev(dev);

        return snprintf(buf, PAGE_SIZE, "%d\n",
                        pdev->p2pdma->p2pmem_published);
}
static DEVICE_ATTR_RO(published);

static struct attribute *p2pmem_attrs[] = {
        &dev_attr_size.attr,
        &dev_attr_available.attr,
        &dev_attr_published.attr,
        NULL,
};

static const struct attribute_group p2pmem_group = {
        .attrs = p2pmem_attrs,
        .name = "p2pmem",
};

static void pci_p2pdma_percpu_release(struct percpu_ref *ref)
{
        struct pci_p2pdma *p2p =
                container_of(ref, struct pci_p2pdma, devmap_ref);

        complete_all(&p2p->devmap_ref_done);
}

static void pci_p2pdma_percpu_kill(void *data)
{
        struct percpu_ref *ref = data;

        /*
         * pci_p2pdma_add_resource() may be called multiple times
         * by a driver and may register the percpu_kill devm action multiple
         * times. We only want the first action to actually kill the
         * percpu_ref.
         */
        if (percpu_ref_is_dying(ref))
                return;

        percpu_ref_kill(ref);
}

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);
        sysfs_remove_group(&pdev->dev.kobj, &p2pmem_group);
        pdev->p2pdma = NULL;
}

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;

        error = devm_add_action_or_reset(&pdev->dev, pci_p2pdma_release, pdev);
        if (error)
                goto out_pool_destroy;

        pdev->p2pdma = p2p;

        error = sysfs_create_group(&pdev->dev.kobj, &p2pmem_group);
        if (error)
                goto out_pool_destroy;

        return 0;

out_pool_destroy:
        pdev->p2pdma = NULL;
        gen_pool_destroy(p2p->pool);
out:
        devm_kfree(&pdev->dev, p2p);
        return error;
}

/**
 * pci_p2pdma_add_resource - add memory for use as p2p memory
 * @pdev: the device to add the memory to
 * @bar: PCI BAR to add
 * @size: size of the memory to add, may be zero to use the whole BAR
 * @offset: offset into the PCI BAR
 *
 * The memory will be given ZONE_DEVICE struct pages so that it may
 * be used with any DMA request.
 */
int pci_p2pdma_add_resource(struct pci_dev *pdev, int bar, size_t size,
                            u64 offset)
{
        struct dev_pagemap *pgmap;
        void *addr;
        int error;

        if (!(pci_resource_flags(pdev, bar) & IORESOURCE_MEM))
                return -EINVAL;

        if (offset >= pci_resource_len(pdev, bar))
                return -EINVAL;

        if (!size)
                size = pci_resource_len(pdev, bar) - offset;

        if (size + offset > pci_resource_len(pdev, bar))
                return -EINVAL;

        if (!pdev->p2pdma) {
                error = pci_p2pdma_setup(pdev);
                if (error)
                        return error;
        }

        pgmap = devm_kzalloc(&pdev->dev, sizeof(*pgmap), GFP_KERNEL);
        if (!pgmap)
                return -ENOMEM;

        pgmap->res.start = pci_resource_start(pdev, bar) + offset;
        pgmap->res.end = pgmap->res.start + size - 1;
        pgmap->res.flags = pci_resource_flags(pdev, bar);
        pgmap->ref = &pdev->p2pdma->devmap_ref;
        pgmap->type = MEMORY_DEVICE_PCI_P2PDMA;
        pgmap->pci_p2pdma_bus_offset = pci_bus_address(pdev, bar) -
                pci_resource_start(pdev, bar);

        addr = devm_memremap_pages(&pdev->dev, pgmap);
        if (IS_ERR(addr)) {
                error = PTR_ERR(addr);
                goto pgmap_free;
        }

        error = gen_pool_add_virt(pdev->p2pdma->pool, (unsigned long)addr,
                        pci_bus_address(pdev, bar) + offset,
                        resource_size(&pgmap->res), dev_to_node(&pdev->dev));
        if (error)
                goto pgmap_free;

        error = devm_add_action_or_reset(&pdev->dev, pci_p2pdma_percpu_kill,
                                          &pdev->p2pdma->devmap_ref);
        if (error)
                goto pgmap_free;

        pci_info(pdev, "added peer-to-peer DMA memory %pR\n",
                 &pgmap->res);

        return 0;

pgmap_free:
        devm_kfree(&pdev->dev, pgmap);
        return error;
}
EXPORT_SYMBOL_GPL(pci_p2pdma_add_resource);

/*
 * Note this function returns the parent PCI device with a
 * reference taken. It is the caller's responsibily to drop
 * the reference.
 */
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;
}

/*
 * Check if a PCI bridge has its ACS redirection bits set to redirect P2P
 * TLPs upstream via ACS. Returns 1 if the packets will be redirected
 * upstream, 0 otherwise.
 */
static int pci_bridge_has_acs_redir(struct pci_dev *pdev)
{
        int pos;
        u16 ctrl;

        pos = pci_find_ext_capability(pdev, PCI_EXT_CAP_ID_ACS);
        if (!pos)
                return 0;

        pci_read_config_word(pdev, pos + PCI_ACS_CTRL, &ctrl);

        if (ctrl & (PCI_ACS_RR | PCI_ACS_CR | PCI_ACS_EC))
                return 1;

        return 0;
}

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));
}

/*
 * Find the distance through the nearest common upstream bridge between
 * two PCI devices.
 *
 * If the two devices are the same device then 0 will be returned.
 *
 * If there are two virtual functions of the same device behind the same
 * bridge port then 2 will be returned (one step down to the PCIe switch,
 * then one step back to the same device).
 *
 * In the case where two devices are connected to the same PCIe switch, the
 * value 4 will be returned. This corresponds to the following PCI tree:
 *
 *     -+  Root Port
 *      \+ Switch Upstream Port
 *       +-+ Switch Downstream Port
 *       + \- Device A
 *       \-+ Switch Downstream Port
 *         \- Device B
 *
 * The distance is 4 because we traverse from Device A through the downstream
 * port of the switch, to the common upstream port, back up to the second
 * downstream port and then to Device B.
 *
 * Any two devices that don't have a common upstream bridge will return -1.
 * In this way devices on separate PCIe root ports will be rejected, which
 * is what we want for peer-to-peer seeing each PCIe root port defines a
 * separate hierarchy domain and there's no way to determine whether the root
 * complex supports forwarding between them.
 *
 * In the case where two devices are connected to different PCIe switches,
 * this function will still return a positive distance as long as both
 * switches eventually have a common upstream bridge. Note this covers
 * the case of using multiple PCIe switches to achieve a desired level of
 * fan-out from a root port. The exact distance will be a function of the
 * number of switches between Device A and Device B.
 *
 * If a bridge which has any ACS redirection bits set is in the path
 * then this functions will return -2. This is so we reject any
 * cases where the TLPs are forwarded up into the root complex.
 * In this case, a list of all infringing bridge addresses will be
 * populated in acs_list (assuming it's non-null) for printk purposes.
 */
static int upstream_bridge_distance(struct pci_dev *a,
                                    struct pci_dev *b,
                                    struct seq_buf *acs_list)
{
        int dist_a = 0;
        int dist_b = 0;
        struct pci_dev *bb = NULL;
        int acs_cnt = 0;

        /*
         * Note, we don't need to take references to devices returned by
         * pci_upstream_bridge() seeing we hold a reference to a child
         * device which will already hold a reference to the upstream bridge.
         */

        while (a) {
                dist_b = 0;

                if (pci_bridge_has_acs_redir(a)) {
                        seq_buf_print_bus_devfn(acs_list, a);
                        acs_cnt++;
                }

                bb = b;

                while (bb) {
                        if (a == bb)
                                goto check_b_path_acs;

                        bb = pci_upstream_bridge(bb);
                        dist_b++;
                }

                a = pci_upstream_bridge(a);
                dist_a++;
        }

        return -1;

check_b_path_acs:
        bb = b;

        while (bb) {
                if (a == bb)
                        break;

                if (pci_bridge_has_acs_redir(bb)) {
                        seq_buf_print_bus_devfn(acs_list, bb);
                        acs_cnt++;
                }

                bb = pci_upstream_bridge(bb);
        }

        if (acs_cnt)
                return -2;

        return dist_a + dist_b;
}

static int upstream_bridge_distance_warn(struct pci_dev *provider,
                                         struct pci_dev *client)
{
        struct seq_buf acs_list;
        int ret;

        seq_buf_init(&acs_list, kmalloc(PAGE_SIZE, GFP_KERNEL), PAGE_SIZE);
        if (!acs_list.buffer)
                return -ENOMEM;

        ret = upstream_bridge_distance(provider, client, &acs_list);
        if (ret == -2) {
                pci_warn(client, "cannot be used for peer-to-peer DMA as ACS redirect is set between the client and provider (%s)\n",
                         pci_name(provider));
                /* Drop final semicolon */
                acs_list.buffer[acs_list.len-1] = 0;
                pci_warn(client, "to disable ACS redirect for this path, add the kernel parameter: pci=disable_acs_redir=%s\n",
                         acs_list.buffer);

        } else if (ret < 0) {
                pci_warn(client, "cannot be used for peer-to-peer DMA as the client and provider (%s) do not share an upstream bridge\n",
                         pci_name(provider));
        }

        kfree(acs_list.buffer);

        return ret;
}

/**
 * pci_p2pdma_distance_many - Determive the cumulative distance between
 *      a p2pdma provider and the clients in use.
 * @provider: p2pdma provider to check against the client list
 * @clients: array of devices to check (NULL-terminated)
 * @num_clients: number of clients in the array
 * @verbose: if true, print warnings for devices when we return -1
 *
 * Returns -1 if any of the clients are not compatible (behind the same
 * root port as the provider), otherwise returns a positive number where
 * a lower number is the preferrable choice. (If there's one client
 * that's the same as the provider it will return 0, which is best choice).
 *
 * For now, "compatible" means the provider and the clients are all behind
 * the same PCI root port. This cuts out cases that may work but is safest
 * for the user. Future work can expand this to white-list root complexes that
 * can safely forward between each ports.
 */
int pci_p2pdma_distance_many(struct pci_dev *provider, struct device **clients,
                             int num_clients, bool verbose)
{
        bool not_supported = false;
        struct pci_dev *pci_client;
        int distance = 0;
        int i, ret;

        if (num_clients == 0)
                return -1;

        for (i = 0; i < num_clients; i++) {
                pci_client = find_parent_pci_dev(clients[i]);
                if (!pci_client) {
                        if (verbose)
                                dev_warn(clients[i],
                                         "cannot be used for peer-to-peer DMA as it is not a PCI device\n");
                        return -1;
                }

                if (verbose)
                        ret = upstream_bridge_distance_warn(provider,
                                                            pci_client);
                else
                        ret = upstream_bridge_distance(provider, pci_client,
                                                       NULL);

                pci_dev_put(pci_client);

                if (ret < 0)
                        not_supported = true;

                if (not_supported && !verbose)
                        break;

                distance += ret;
        }

        if (not_supported)
                return -1;

        return distance;
}
EXPORT_SYMBOL_GPL(pci_p2pdma_distance_many);

/**
 * pci_has_p2pmem - check if a given PCI device has published any p2pmem
 * @pdev: PCI device to check
 */
bool pci_has_p2pmem(struct pci_dev *pdev)
{
        return pdev->p2pdma && pdev->p2pdma->p2pmem_published;
}
EXPORT_SYMBOL_GPL(pci_has_p2pmem);

/**
 * pci_p2pmem_find - find a peer-to-peer DMA memory device compatible with
 *      the specified list of clients and shortest distance (as determined
 *      by pci_p2pmem_dma())
 * @clients: array of devices to check (NULL-terminated)
 * @num_clients: number of client devices in the list
 *
 * If multiple devices are behind the same switch, the one "closest" to the
 * client devices in use will be chosen first. (So if one of the providers are
 * the same as one of the clients, that provider will be used ahead of any
 * other providers that are unrelated). If multiple providers are an equal
 * distance away, one will be chosen at random.
 *
 * Returns a pointer to the PCI device with a reference taken (use pci_dev_put
 * to return the reference) or NULL if no compatible device is found. The
 * found provider will also be assigned to the client list.
 */
struct pci_dev *pci_p2pmem_find_many(struct device **clients, int num_clients)
{
        struct pci_dev *pdev = NULL;
        int distance;
        int closest_distance = INT_MAX;
        struct pci_dev **closest_pdevs;
        int dev_cnt = 0;
        const int max_devs = PAGE_SIZE / sizeof(*closest_pdevs);
        int i;

        closest_pdevs = kmalloc(PAGE_SIZE, GFP_KERNEL);
        if (!closest_pdevs)
                return NULL;

        while ((pdev = pci_get_device(PCI_ANY_ID, PCI_ANY_ID, pdev))) {
                if (!pci_has_p2pmem(pdev))
                        continue;

                distance = pci_p2pdma_distance_many(pdev, clients,
                                                    num_clients, false);
                if (distance < 0 || distance > closest_distance)
                        continue;

                if (distance == closest_distance && dev_cnt >= max_devs)
                        continue;

                if (distance < closest_distance) {
                        for (i = 0; i < dev_cnt; i++)
                                pci_dev_put(closest_pdevs[i]);

                        dev_cnt = 0;
                        closest_distance = distance;
                }

                closest_pdevs[dev_cnt++] = pci_dev_get(pdev);
        }

        if (dev_cnt)
                pdev = pci_dev_get(closest_pdevs[prandom_u32_max(dev_cnt)]);

        for (i = 0; i < dev_cnt; i++)
                pci_dev_put(closest_pdevs[i]);

        kfree(closest_pdevs);
        return pdev;
}
EXPORT_SYMBOL_GPL(pci_p2pmem_find_many);

/**
 * pci_alloc_p2p_mem - allocate peer-to-peer DMA memory
 * @pdev: the device to allocate memory from
 * @size: number of bytes to allocate
 *
 * Returns the allocated memory or NULL on error.
 */
void *pci_alloc_p2pmem(struct pci_dev *pdev, size_t size)
{
        void *ret;

        if (unlikely(!pdev->p2pdma))
                return NULL;

        if (unlikely(!percpu_ref_tryget_live(&pdev->p2pdma->devmap_ref)))
                return NULL;

        ret = (void *)gen_pool_alloc(pdev->p2pdma->pool, size);

        if (unlikely(!ret))
                percpu_ref_put(&pdev->p2pdma->devmap_ref);

        return ret;
}
EXPORT_SYMBOL_GPL(pci_alloc_p2pmem);

/**
 * pci_free_p2pmem - free 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);

/**
 * pci_virt_to_bus - return the PCI bus address for a given virtual
 *      address obtained with pci_alloc_p2pmem()
 * @pdev: the device the memory was allocated from
 * @addr: address of the memory that was allocated
 */
pci_bus_addr_t pci_p2pmem_virt_to_bus(struct pci_dev *pdev, void *addr)
{
        if (!addr)
                return 0;
        if (!pdev->p2pdma)
                return 0;

        /*
         * Note: when we added the memory to the pool we used the PCI
         * bus address as the physical address. So gen_pool_virt_to_phys()
         * actually returns the bus address despite the misleading name.
         */
        return gen_pool_virt_to_phys(pdev->p2pdma->pool, (unsigned long)addr);
}
EXPORT_SYMBOL_GPL(pci_p2pmem_virt_to_bus);

/**
 * pci_p2pmem_alloc_sgl - allocate peer-to-peer DMA memory in a scatterlist
 * @pdev: the device to allocate memory from
 * @nents: the number of SG entries in the list
 * @length: number of bytes to allocate
 *
 * Returns 0 on success
 */
struct scatterlist *pci_p2pmem_alloc_sgl(struct pci_dev *pdev,
                                         unsigned int *nents, u32 length)
{
        struct scatterlist *sg;
        void *addr;

        sg = kzalloc(sizeof(*sg), GFP_KERNEL);
        if (!sg)
                return NULL;

        sg_init_table(sg, 1);

        addr = pci_alloc_p2pmem(pdev, length);
        if (!addr)
                goto out_free_sg;

        sg_set_buf(sg, addr, length);
        *nents = 1;
        return sg;

out_free_sg:
        kfree(sg);
        return NULL;
}
EXPORT_SYMBOL_GPL(pci_p2pmem_alloc_sgl);

/**
 * pci_p2pmem_free_sgl - free a scatterlist allocated by pci_p2pmem_alloc_sgl()
 * @pdev: the device to allocate memory from
 * @sgl: the allocated scatterlist
 */
void pci_p2pmem_free_sgl(struct pci_dev *pdev, struct scatterlist *sgl)
{
        struct scatterlist *sg;
        int count;

        for_each_sg(sgl, sg, INT_MAX, count) {
                if (!sg)
                        break;

                pci_free_p2pmem(pdev, sg_virt(sg), sg->length);
        }
        kfree(sgl);
}
EXPORT_SYMBOL_GPL(pci_p2pmem_free_sgl);

/**
 * pci_p2pmem_publish - publish the peer-to-peer DMA memory for use by
 *      other devices with pci_p2pmem_find()
 * @pdev: the device with peer-to-peer DMA memory to publish
 * @publish: set to true to publish the memory, false to unpublish it
 *
 * Published memory can be used by other PCI device drivers for
 * peer-2-peer DMA operations. Non-published memory is reserved for
 * exlusive use of the device driver that registers the peer-to-peer
 * memory.
 */
void pci_p2pmem_publish(struct pci_dev *pdev, bool publish)
{
        if (pdev->p2pdma)
                pdev->p2pdma->p2pmem_published = publish;
}
EXPORT_SYMBOL_GPL(pci_p2pmem_publish);

/**
 * pci_p2pdma_map_sg - map a PCI peer-to-peer scatterlist for DMA
 * @dev: device doing the DMA request
 * @sg: scatter list to map
 * @nents: elements in the scatterlist
 * @dir: DMA direction
 *
 * Scatterlists mapped with this function should not be unmapped in any way.
 *
 * Returns the number of SG entries mapped or 0 on error.
 */
int pci_p2pdma_map_sg(struct device *dev, struct scatterlist *sg, int nents,
                      enum dma_data_direction dir)
{
        struct dev_pagemap *pgmap;
        struct scatterlist *s;
        phys_addr_t paddr;
        int i;

        /*
         * p2pdma mappings are not compatible with devices that use
         * dma_virt_ops. If the upper layers do the right thing
         * this should never happen because it will be prevented
         * by the check in pci_p2pdma_add_client()
         */
        if (WARN_ON_ONCE(IS_ENABLED(CONFIG_DMA_VIRT_OPS) &&
                         dev->dma_ops == &dma_virt_ops))
                return 0;

        for_each_sg(sg, s, nents, i) {
                pgmap = sg_page(s)->pgmap;
                paddr = sg_phys(s);

                s->dma_address = paddr - pgmap->pci_p2pdma_bus_offset;
                sg_dma_len(s) = s->length;
        }

        return nents;
}
EXPORT_SYMBOL_GPL(pci_p2pdma_map_sg);

/**
 * pci_p2pdma_enable_store - parse a configfs/sysfs attribute store
 *              to enable p2pdma
 * @page: contents of the value to be stored
 * @p2p_dev: returns the PCI device that was selected to be used
 *              (if one was specified in the stored value)
 * @use_p2pdma: returns whether to enable p2pdma or not
 *
 * Parses an attribute value to decide whether to enable p2pdma.
 * The value can select a PCI device (using it's full BDF device
 * name) or a boolean (in any format strtobool() accepts). A false
 * value disables p2pdma, a true value expects the caller
 * to automatically find a compatible device and specifying a PCI device
 * expects the caller to use the specific provider.
 *
 * pci_p2pdma_enable_show() should be used as the show operation for
 * the attribute.
 *
 * Returns 0 on success
 */
int pci_p2pdma_enable_store(const char *page, struct pci_dev **p2p_dev,
                            bool *use_p2pdma)
{
        struct device *dev;

        dev = bus_find_device_by_name(&pci_bus_type, NULL, page);
        if (dev) {
                *use_p2pdma = true;
                *p2p_dev = to_pci_dev(dev);

                if (!pci_has_p2pmem(*p2p_dev)) {
                        pci_err(*p2p_dev,
                                "PCI device has no peer-to-peer memory: %s\n",
                                page);
                        pci_dev_put(*p2p_dev);
                        return -ENODEV;
                }

                return 0;
        } else if ((page[0] == '0' || page[0] == '1') && !iscntrl(page[1])) {
                /*
                 * If the user enters a PCI device that  doesn't exist
                 * like "0000:01:00.1", we don't want strtobool to think
                 * it's a '0' when it's clearly not what the user wanted.
                 * So we require 0's and 1's to be exactly one character.
                 */
        } else if (!strtobool(page, use_p2pdma)) {
                return 0;
        }

        pr_err("No such PCI device: %.*s\n", (int)strcspn(page, "\n"), page);
        return -ENODEV;
}
EXPORT_SYMBOL_GPL(pci_p2pdma_enable_store);

/**
 * pci_p2pdma_enable_show - show a configfs/sysfs attribute indicating
 *              whether p2pdma is enabled
 * @page: contents of the stored value
 * @p2p_dev: the selected p2p device (NULL if no device is selected)
 * @use_p2pdma: whether p2pdme has been enabled
 *
 * Attributes that use pci_p2pdma_enable_store() should use this function
 * to show the value of the attribute.
 *
 * Returns 0 on success
 */
ssize_t pci_p2pdma_enable_show(char *page, struct pci_dev *p2p_dev,
                               bool use_p2pdma)
{
        if (!use_p2pdma)
                return sprintf(page, "0\n");

        if (!p2p_dev)
                return sprintf(page, "1\n");

        return sprintf(page, "%s\n", pci_name(p2p_dev));
}
EXPORT_SYMBOL_GPL(pci_p2pdma_enable_show);