On Thu, Apr 16, 2015 at 01:58:16AM +0200, Luis R. Rodriguez wrote: > > An alternative... is to just ioremap_wc() the entire region, including > MMIO registers for these old devices. I see one ethernet driver that does > this, myri10ge, and am curious how and why they ended up deciding this > and if they have run into any issues. I wonder if this is a reasonable > comrpomise for these 2 remaining corner cases. > For myri10ge, it a performance thing. Descriptor rings are in NIC memory BAR0, not in host memory. Say, to send a packet, the driver writes the send descriptor to the ioremap'd NIC memory. It is a multi-word descriptor. So, to send it as one PCIE MWr transaction, the driver maps the whole BAR0 as WC and does "copy descriptor; wmb". Without WC, descriptors would end up as multiple 4B or 8B MWr packets to the NIC, which has a pretty big performance impact on this particular NIC. Most registers that do not want WC are actually in BAR2, which is not mapped as WC. For registers that are in BAR0, we do "write to the register; wmb". If we want to wait till the NIC has seen the write, we do "write; wmb; read". This approach has worked for this device for many years. I cannot say whether it works for other devices, though. -- To unsubscribe from this list: send the line "unsubscribe linux-media" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
