netdev - Re: [RFC] PCI: Allow sysfs control over totalvfs

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Date:   Tue, 20 Sep 2016 21:36:45 +0200
From:   Jiri Pirko <jiri@...nulli.us>
To:     Yuval Mintz <Yuval.Mintz@...gic.com>
Cc:     linux-pci@...r.kernel.org, netdev@...r.kernel.org,
        derek.chickles@...iumnetworks.com,
        Yuval Mintz <Yuval.Mintz@...iumnetworks.com>
Subject: Re: [RFC] PCI: Allow sysfs control over totalvfs

Tue, Sep 20, 2016 at 05:49:48PM CEST, Yuval.Mintz@...gic.com wrote:
>[Sorry in advance if this was already discussed in the past]
>
>Some of the HW capable of SRIOV has resource limitations, where the
>PF and VFs resources are drawn from a common pool.
>In some cases, these limitations have to be considered early during
>chip initialization and can only be changed by tearing down the
>configuration and re-initializing.
>As a result, drivers for such HWs sometimes have to make unfavorable
>compromises where they reserve sufficient resources to accomadate
>the maximal number of VFs that can be created - at the expanse of
>resources that could have been used by the PF.
>
>If users were able to provide 'hints' regarding the required number
>of VFs *prior* to driver attachment, then such compromises could be
>avoided. As we already have sysfs nodes that can be queried for the
>number of totalvfs, it makes sense to let the user reduce the number
>of said totalvfs using same infrastrucure.
>Then, we can have drivers supporting SRIOV take that value into account
>when deciding how much resources to reserve, allowing the PF to benefit
>from the difference between the configuration space value and the actual
>number needed by user.

One of the motivations for introducing devlink interface was to allow
user to pass some kind of well defined option parameters or as you call
it hints to driver module. That would allow to replace module options
and introduce similar possibility to pre-configure hardware on probe time.
We plan to use devlink to allow user to change resource allocation for
mlxsw devices.

The plan is to allow to pre-create devlink instance before driver module
is loaded. Then the user will use this placeholder to do the options
setting. Once the driver module is loaded, it will fetch the options
from devlink core and process it accordingly.

I believe this is exactly what you need.


>
>Signed-off-by: Yuval Mintz <Yuval.Mintz@...iumnetworks.com>
>---
> drivers/pci/pci-sysfs.c | 28 +++++++++++++++++++++++++++-
> 1 file changed, 27 insertions(+), 1 deletion(-)
>
>diff --git a/drivers/pci/pci-sysfs.c b/drivers/pci/pci-sysfs.c
>index bcd10c7..c1546f8 100644
>--- a/drivers/pci/pci-sysfs.c
>+++ b/drivers/pci/pci-sysfs.c
>@@ -449,6 +449,30 @@ static ssize_t sriov_totalvfs_show(struct device *dev,
> 	return sprintf(buf, "%u\n", pci_sriov_get_totalvfs(pdev));
> }
> 
>+static ssize_t sriov_totalvfs_store(struct device *dev,
>+				    struct device_attribute *attr,
>+				    const char *buf, size_t count)
>+{
>+	struct pci_dev *pdev = to_pci_dev(dev);
>+	u16 max_vfs;
>+	int ret;
>+
>+	ret = kstrtou16(buf, 0, &max_vfs);
>+	if (ret < 0)
>+		return ret;
>+
>+	if (pdev->driver) {
>+		dev_info(&pdev->dev,
>+			 "Can't change totalvfs while driver is attached\n");
>+		return -EUSERS;
>+	}
>+
>+	ret = pci_sriov_set_totalvfs(pdev, max_vfs);
>+	if (ret)
>+		return ret;
>+
>+	return count;
>+}
> 
> static ssize_t sriov_numvfs_show(struct device *dev,
> 				 struct device_attribute *attr,
>@@ -516,7 +540,9 @@ static ssize_t sriov_numvfs_store(struct device *dev,
> 	return count;
> }
> 
>-static struct device_attribute sriov_totalvfs_attr = __ATTR_RO(sriov_totalvfs);
>+static struct device_attribute sriov_totalvfs_attr =
>+		__ATTR(sriov_totalvfs, (S_IRUGO|S_IWUSR|S_IWGRP),
>+		       sriov_totalvfs_show, sriov_totalvfs_store);
> static struct device_attribute sriov_numvfs_attr =
> 		__ATTR(sriov_numvfs, (S_IRUGO|S_IWUSR|S_IWGRP),
> 		       sriov_numvfs_show, sriov_numvfs_store);
>-- 
>1.9.3
>