[openib-general] Drop in performance on Mellanox MT25204 single port DDR HCA

[Michael Krause]

At 02:43 PM 10/2/2006, Roland Dreier wrote:
>     Robert> Yes. 1250Mbytes/sec is what we expect.  You say the 128
>     Robert> value comes from the BIOS ? If so, we need to discuss this
>     Robert> with our BIOS team to find out why they limit it to 128,
>     Robert> perhaps it is a BIOS bug.
>
>Yes, I believe that the BIOS is the only place that would set that
>value.  We know that resetting the device makes it go back to a
>different default value, and nothing in the kernel that I know of is
>going to set it down to 128.

128B is the default minimum from PCIe so likely some BIOS engineer took a 
conservative view and chose the defaults (go figure).  Setting Max Read 
Request Size = 4096 is preferred on any implementation as it is basically 
free from a chipset perspective.  The chipset will likely return in cache 
line quantities but there is some obvious optimizations to be achieved by 
issuing a single DMA Read Request.

Mike