Re: [dpdk-dev] [PATCH] pci/uio: enable prefetchable resources mapping

[Andrew Rybchenko <arybchenko@solarflare.com>]

On 02/01/2018 12:59 PM, Bruce Richardson wrote:
> On Thu, Feb 01, 2018 at 09:18:22AM +0800, Changpeng Liu wrote:
>> For PCI prefetchable resources, Linux will create a
>> write combined file as well, the library will try
>> to map resourceX_wc file first, if the file does
>> not exist, then it will map resourceX as usual.
>>
>> Signed-off-by: Changpeng Liu <changpeng.liu@intel.com>
>> ---
>>   drivers/bus/pci/linux/pci_uio.c | 19 ++++++++++++++-----
>>   1 file changed, 14 insertions(+), 5 deletions(-)
>>
> Hi,
>
> Given the lack of ordering guarantees with write-combined memory, I
> would have thought that this is very risky to do without a complete set
> of changes inside the PMDs to add in the necessary memory barriers to
> ensure ordering of operations to the BARs.  Therefore, instead of
> mapping one file or another, I think the change should be made to map
> *both* in DPDK if available. Then each driver can chose whether to write
> a given device register using uncacheable memory type or write-combining
> memory type + any appropriate barriers.
>
> For example, with many NICs the initialization of the device involves
> many register writes in a pretty defined order, so wc operations are
> probably to suitable as performance is not a concern. However, for data
> path operations, a driver may chose to use wc memory for the occasional
> device writes there, for performance reasons.

+1

I think so too that it would be useful to have both mappings available and
allow driver to choose which one to use.