On Thu, 5 Feb 2026 at 13:01, John Hubbard <[email protected]> wrote: > > On 2/4/26 1:56 PM, Dave Airlie wrote: > > On Thu, 5 Feb 2026 at 07:36, Timur Tabi <[email protected]> wrote: > ... > >> So are you saying that some RPC commands need to have a sequence number > >> set, and some do not? > > > > I'm copying the behaviour of opengpu here, > > src/kernel/gpu/gsp/kernel_gsp.c > > if (pSequence) > > vgpu_rpc_message_header_v->sequence = *pSequence = pRpc->sequence++; > > else > > vgpu_rpc_message_header_v->sequence = 0; > > > > src/kernel/vgpu/rpc.c:_issueRpcAsync > > doesn't pass pSequence > > _issueRpcAndWait does pass it. > > > > The SetSystemInfo and SetRegistry are the two async calls in nouveau. > > > > So I'm not saying some RPC commands need to have a sequence number and > > some don't, that would be up to someone who can access GSP source > > code, I'm saying that opengpu does this and I'm copying it :-) > > > > lol I love it. If only someone here had access to the GSP code and > the GSP team! :) > > OK, then, I just had a very enlightening call with one of our GSP > RPC experts, in order to learn what the sequence number story really > is. The notes below are sort of Nova-centric, but it should apply > equally to Nouveau. > > Let me Cc Eliot, because he was also fixing up other aspects > of GSP RPC calls on Nova, in case this overlaps. > > ========================================================== > Background > ========== > > Today there are some loose ends and inconsistencies even in the > Open RM + GSP scenario, for how sequence numbers are used. And these > are being cleaned up and fixed. In fact, I was even able to request, > and receive some nice clean behavior, which will be implemented in > GSP soon (we'll get it when we upgrade, likely sometime this year). > > Today, there are 2+ sequence number spaces, one for send-receive pairs > (command + response) RPC calls, and another for GSP-initiated ("async") > messages to the CPU. > > The "2+" is because there is an inconsistency (it will be fixed in > GSP), leading to the first two very early RPC calls being in yet another > unaccounted for number space. These: > > NovaCore 0000:01:00.0: GSP RPC: send: seq# 0, function=GSP_SET_SYSTEM_INFO, > length=0x3f0 > NovaCore 0000:01:00.0: GSP RPC: send: seq# 1, function=SET_REGISTRY, > length=0xc5 > > ...are not included in the counting, by GSP. > > The GSP finally starts counting up when it gets the first "non-async" > (command/response) message, here: > > NovaCore 0000:01:00.0: GSP RPC: send: seq# 2, function=GET_GSP_STATIC_INFO, > length=0x6c8 > NovaCore 0000:01:00.0: GSP RPC: receive: seq# 0, > function=Ok(GetGspStaticInfo), length=0x6c8 > > > But even here, it's not what I think we want, because we want the CPU to > get back the same seq num that it sent, for command/response pairs. But > that's not what actually happens (at least not directly). > > So for now, seq numbers on Nova and Nouveau are generally "do what > you want, it will work ok". But actually, we will soon be able to > use them for > > a) debugging aids, > b) detecting missing messages, and > c) recovering from "CPU sent a message, timed out waiting for > a response" cases. > > ========================================================== > Next steps for Nova (and maybe Nouveau, if not already done) > ============================================================ > > a) Change debug output to print the seq number numeric space, > which is either "async message from GSP" or "command response > from GSP". > > b) Put a comment in the code to indicate that GSP_SET_SYSTEM_INFO > and SET_REGISTRY do not yet participate in the incrementing seq > number system, but will in future GSP versions. > > I'll send patches for Nova soon, to do the above.
So just FYI nova gets this wrong as well. There are two sequence numbers: the one in GSP_MSG_QUEUE_ELEMENT and the one in rpc_message_header_v The rules for the first seem to be just increment, the rules for the 2nd seems to be along the lines above, two msgs get 0, then start from 0. Currently we never init the rpc message header sequence to anything but 0. Dave.
