On 7/14/2024 7:38 PM, Baochen Qiang wrote: > In transmit path, it is likely that the iova is not aligned to PCIe TLP > max payload size, which is 128 for WCN7850. Normally in such cases hardware > is expected to split the packet into several parts in a manner such that > they, other than the first one, have aligned iova. However due to hardware > limitations, WCN7850 does not behave like that properly with some specific > unaligned iova in transmit path. This easily results in target hang in a > KPI transmit test: packet send/receive failure, WMI command send timeout > etc. Also fatal error seen in PCIe level: > > ... > Capabilities: ... > ... > DevSta: ... FatalErr+ ... > ... > ... > > Work around this by manually moving/reallocating payload buffer such that > we can map it to a 128 bytes aligned iova. The moving requires sufficient > head room or tail room in skb: for the former we can do ourselves a favor > by asking some extra bytes when registering with mac80211, while for the > latter we can do nothing. > > Moving/reallocating buffer consumes additional CPU cycles, but the good news > is that an aligned iova increases PCIe efficiency. In my tests on some X86 > platforms the KPI results are almost consistent. > > Since this is seen only with WCN7850, add a new hardware parameter to > differentiate from others. I asked for expert opinion on this patch and received the following response. Baochen, can you take a look at this suggestion? > Aligning headers is sometimes done, but it appears the driver > doesn't support scatter gather? I think the author may want to advertise > scatter and linearize manually in the driver, to a correct offset. > Because now core is linearizing the skb in validate_xmit_skb() > and then the driver moves it a second time..
