On 13/06/2022 17:41, Ceraolo Spurio, Daniele wrote:
On 6/13/2022 9:31 AM, Tvrtko Ursulin wrote:
On 13/06/2022 16:39, Ceraolo Spurio, Daniele wrote:
On 6/13/2022 1:16 AM, Tvrtko Ursulin wrote:
On 10/06/2022 00:19, Daniele Ceraolo Spurio wrote:
On DG2, HuC loading is performed by the GSC, via a PXP command. The
load
operation itself is relatively simple (just send a message to the GSC
with the physical address of the HuC in LMEM), but there are timing
changes that requires special attention. In particular, to send a PXP
command we need to first export the GSC driver and then wait for the
mei-gsc and mei-pxp modules to start, which means that HuC load will
complete after i915 load is complete. This means that there is a small
window of time after i915 is registered and before HuC is loaded
during which userspace could submit and/or checking the HuC load
status,
although this is quite unlikely to happen (HuC is usually loaded
before
kernel init/resume completes).
We've consulted with the media team in regards to how to handle
this and
they've asked us to do the following:
1) Report HuC as loaded in the getparam IOCTL even if load is still in
progress. The media driver uses the IOCTL as a way to check if HuC is
enabled and then includes a secondary check in the batches to get the
actual status, so doing it this way allows userspace to keep working
without changes.
2) Stall all userspace VCS submission until HuC is loaded. Stalls are
expected to be very rare (if any), due to the fact that HuC is usually
loaded before kernel init/resume is completed.
Motivation to add these complications into i915 are not clear to me
here. I mean there is no HuC on DG2 _yet_ is the premise of the
series, right? So no backwards compatibility concerns. In this case
why jump through the hoops and not let userspace handle all of this
by just leaving the getparam return the true status?
The main areas impacted by the fact that we can't guarantee that HuC
load is complete when i915 starts accepting submissions are boot and
suspend/resume, with the latter being the main problem; GT reset is
not a concern because HuC now survives it. A suspend/resume can be
transparent to userspace and therefore the HuC status can temporarily
flip from loaded to not without userspace knowledge, especially if we
start going into deeper suspend states and start causing HuC resets
when we go into runtime suspend. Note that this is different from
what happens during GT reset for older platforms, because in that
scenario we guarantee that HuC reload is complete before we restart
the submission back-end, so userspace doesn't notice that the HuC
status change. We had an internal discussion about this problem with
both media and i915 archs and the conclusion was that the best option
is for i915 to stall media submission while HuC (re-)load is in
progress.
Resume is potentialy a good reason - I did not pick up on that from
the cover letter. I read the statement about the unlikely and small
window where HuC is not loaded during kernel init/resume and I guess
did not pick up on the resume part.
Waiting for GSC to load HuC from i915 resume is not an option?
GSC is an aux device exported by i915, so AFAIU GSC resume can't start
until i915 resume completes.
I'll dig into this in the next few days since I want to understand how
exactly it works. Or someone can help explain.
If in the end conclusion will be that i915 resume indeed cannot wait for
GSC, then I think auto-blocking of queued up contexts on media engines
indeed sounds unavoidable. Otherwise, as you explained, user experience
post resume wouldn't be good.
However, do we really need to lie in the getparam? How about extend or
add a new one to separate the loading vs loaded states? Since userspace
does not support DG2 HuC yet this should be doable.
Will there be runtime suspend happening on the GSC device behind
i915's back, or i915 and GSC will always be able to transition the
states in tandem?
They're always in sync. The GSC is part of the same HW PCI device as the
rest of the GPU, so they change HW state together.
Okay thanks, I wasn't sure if it is the same or separate device.
Regards,
Tvrtko
