Re: [dpdk-dev] [PATCH 1/4] igb_uio: add wc option

["Rafał Kozik" <rk@semihalf.com>]

2018-06-27 18:34 GMT+02:00 Ferruh Yigit <ferruh.yigit@intel.com>:
> On 4/11/2018 3:07 PM, Rafal Kozik wrote:
>> Write combining (wc) increase NIC performance by making better
>> utilization of PCI bus, but cannot be use by all PMD.
>>
>> Parameter wc_activate add possibility to enable it for
>> those PMD that could support it.
>>
>> Signed-off-by: Rafal Kozik <rk@semihalf.com>
>
> Hi Rafal,
>
> First, sorry for delayed comment.
>
>> ---
>>  kernel/linux/igb_uio/igb_uio.c | 17 ++++++++++++++---
>>  1 file changed, 14 insertions(+), 3 deletions(-)
>>
>> diff --git a/kernel/linux/igb_uio/igb_uio.c b/kernel/linux/igb_uio/igb_uio.c
>> index 4cae4dd..42e3b3f 100644
>> --- a/kernel/linux/igb_uio/igb_uio.c
>> +++ b/kernel/linux/igb_uio/igb_uio.c
>> @@ -30,6 +30,7 @@ struct rte_uio_pci_dev {
>>       int refcnt;
>>  };
>>
>> +static int wc_activate;
>>  static char *intr_mode;
>>  static enum rte_intr_mode igbuio_intr_mode_preferred = RTE_INTR_MODE_MSIX;
>>  /* sriov sysfs */
>> @@ -375,9 +376,13 @@ igbuio_pci_setup_iomem(struct pci_dev *dev, struct uio_info *info,
>>       len = pci_resource_len(dev, pci_bar);
>>       if (addr == 0 || len == 0)
>>               return -1;
>> -     internal_addr = ioremap(addr, len);
>> -     if (internal_addr == NULL)
>> -             return -1;
>> +     if (wc_activate == 0) {
>> +             internal_addr = ioremap(addr, len);
>> +             if (internal_addr == NULL)
>> +                     return -1;
>> +     } else {
>> +             internal_addr = NULL;
>> +     }
>
> Why we need this update at all?
> What is the relation between internal_address and write combined?
>
> As far as I can see we are not using internal_addr at all, what do you observe
> in write combined usage without this modification?
>

To get internal_addr memory need to be mapped. But as memory could not be
mapped twice: with and without WC it should be skipped for WC. [1]
WC does not work without this modification.

[1] https://www.kernel.org/doc/ols/2008/ols2008v2-pages-135-144.pdf
        section 5.3 and 5.4

>>       info->mem[n].name = name;
>>       info->mem[n].addr = addr;
>>       info->mem[n].internal_addr = internal_addr;
>> @@ -638,6 +643,12 @@ MODULE_PARM_DESC(intr_mode,
>>  "    " RTE_INTR_MODE_LEGACY_NAME "     Use Legacy interrupt\n"
>>  "\n");
>>
>> +module_param(wc_activate, int, 0);
>> +MODULE_PARM_DESC(wc_activate,
>> +"Activate support for write combining (WC) (default=0)\n"
>> +"    0 - disable\n"
>> +"    other - enable\n");
>> +
>>  MODULE_DESCRIPTION("UIO driver for Intel IGB PCI cards");
>>  MODULE_LICENSE("GPL");
>>  MODULE_AUTHOR("Intel Corporation");
>>
>