Re: [dpdk-dev] [PATCH v2 3/4] eal: enable WC during resources mapping

[Ferruh Yigit <ferruh.yigit@intel.com>]

On 6/28/2018 2:15 PM, Rafal Kozik wrote:
> Write combining (WC) increases NIC performance by making better
> utilization of PCI bus, but cannot be used by all PMDs.
> 
> It will be enabled only if RTE_PCI_DRV_WC_ACTIVATE will be set in
> drivers flags. For proper work also igb_uio driver must be loaded with
> wc_activate set to 1.
> 
> When mapping PCI resources, firstly try to us WC.
> If it is not supported it will fallback to normal mode.
> 
> Signed-off-by: Rafal Kozik <rk@semihalf.com>
> Acked-by: Bruce Richardson <bruce.richardson@intel.com>
> ---
>  drivers/bus/pci/linux/pci_uio.c | 41 +++++++++++++++++++++++++++++------------
>  drivers/bus/pci/rte_bus_pci.h   |  2 ++
>  2 files changed, 31 insertions(+), 12 deletions(-)
> 
> diff --git a/drivers/bus/pci/linux/pci_uio.c b/drivers/bus/pci/linux/pci_uio.c
> index d423e4b..e3947c2 100644
> --- a/drivers/bus/pci/linux/pci_uio.c
> +++ b/drivers/bus/pci/linux/pci_uio.c
> @@ -282,22 +282,19 @@ int
>  pci_uio_map_resource_by_index(struct rte_pci_device *dev, int res_idx,
>  		struct mapped_pci_resource *uio_res, int map_idx)
>  {
> -	int fd;
> +	int fd = -1;
>  	char devname[PATH_MAX];
>  	void *mapaddr;
>  	struct rte_pci_addr *loc;
>  	struct pci_map *maps;
> +	int wc_activate = 0;
> +
> +	if (dev->driver != NULL)
> +		wc_activate = dev->driver->drv_flags & RTE_PCI_DRV_WC_ACTIVATE;
>  
>  	loc = &dev->addr;
>  	maps = uio_res->maps;
>  
> -	/* update devname for mmap  */
> -	snprintf(devname, sizeof(devname),
> -			"%s/" PCI_PRI_FMT "/resource%d",
> -			rte_pci_get_sysfs_path(),
> -			loc->domain, loc->bus, loc->devid,
> -			loc->function, res_idx);
> -
>  	/* allocate memory to keep path */
>  	maps[map_idx].path = rte_malloc(NULL, strlen(devname) + 1, 0);
>  	if (maps[map_idx].path == NULL) {
> @@ -309,11 +306,31 @@ pci_uio_map_resource_by_index(struct rte_pci_device *dev, int res_idx,
>  	/*
>  	 * open resource file, to mmap it
>  	 */
> -	fd = open(devname, O_RDWR);
> -	if (fd < 0) {
> -		RTE_LOG(ERR, EAL, "Cannot open %s: %s\n",
> +	if (wc_activate) {
> +		/* update devname for mmap  */
> +		snprintf(devname, sizeof(devname),
> +			"%s/" PCI_PRI_FMT "/resource%d_wc",
> +			rte_pci_get_sysfs_path(),
> +			loc->domain, loc->bus, loc->devid,
> +			loc->function, res_idx);
> +
> +		fd = open(devname, O_RDWR);

What do you think adding an error log here? If opening resource$_wc fails and
fallback to resource# file, there won't be any way for user to know about it.

> +	}
> +
> +	if (!wc_activate || fd < 0) {
> +		snprintf(devname, sizeof(devname),
> +			"%s/" PCI_PRI_FMT "/resource%d",
> +			rte_pci_get_sysfs_path(),
> +			loc->domain, loc->bus, loc->devid,
> +			loc->function, res_idx);
> +
> +		/* then try to map resource file */
> +		fd = open(devname, O_RDWR);
> +		if (fd < 0) {
> +			RTE_LOG(ERR, EAL, "Cannot open %s: %s\n",
>  				devname, strerror(errno));
> -		goto error;
> +			goto error;
> +		}
>  	}
>  
>  	/* try mapping somewhere close to the end of hugepages */
> diff --git a/drivers/bus/pci/rte_bus_pci.h b/drivers/bus/pci/rte_bus_pci.h
> index 458e6d0..828acc5 100644
> --- a/drivers/bus/pci/rte_bus_pci.h
> +++ b/drivers/bus/pci/rte_bus_pci.h
> @@ -135,6 +135,8 @@ struct rte_pci_bus {
>  
>  /** Device needs PCI BAR mapping (done with either IGB_UIO or VFIO) */
>  #define RTE_PCI_DRV_NEED_MAPPING 0x0001
> +/** Device needs PCI BAR mapping with enabled write combining (wc) */
> +#define RTE_PCI_DRV_WC_ACTIVATE 0x0002
>  /** Device driver supports link state interrupt */
>  #define RTE_PCI_DRV_INTR_LSC	0x0008
>  /** Device driver supports device removal interrupt */
>