On Tue, Jan 23, 2018 at 01:25:56PM +0000, Ron Yuan wrote: I'm reproducing Sinan's picture here so we can see what you're talking about: > >>>> root (MPS=256) > >>>> | > >>>> ------------------ > >>>> / \ > >>>> bridge0 (MPS=256) bridge1 (MPS=128) > >>>> / \ > >>>> EP0 (MPS=256) EP1 (MPS=128) > >>>> > > PERFORMANCE mode reduces MRRS not because of a starvation issue, > > but because reducing EP1's MRRS allows EP0 to use a larger MPS. > Looks like this case is talking about EP1 requests data directly > from EP0, using MRRS to control the return data payload, while still > keeping the traffic from EP0 to RC in 256B. No, this is not talking about EP1 requesting data from EP0. That would be peer-to-peer DMA, and PERFORMANCE mode explicitly assumes there is no peer-to-peer DMA. It reduces MRRS to allow EP0 to use a larger MPS. We must guarantee that no device receives a TLP larger than its MPS setting. The simple and obvious configuration is to set MPS=128 for everything in Sinan's picture. That works correctly but limits EP0's performance. What PERFORMANCE mode does is set MPS as shown in the picture and set EP1's MRRS=128. We're assuming no peer-to-peer DMA, but of course EP1 may still need to do DMA reads from system memory, and setting its MRRS=128 means those reads will be of 128 bytes or less. If we set EP1's MRRS=256, it could do a 256-byte DMA read from system memory, the root port could send a 256-byte completion, and bridge1 would treat that as a malformed TLP because its MPS=128.
