[Intel-gfx] [PATCH] drm/i915/pcode: Wait 10 seconds for pcode to settle
Andi Shyti
andi.shyti at linux.intel.com
Mon Jan 30 20:41:34 UTC 2023
Hi Rodrigo,
first of all, thanks for looking into this!
> > > > > > In the call flow invoked by intel_pcode_init(), I've added brief comments
> > > > > > where further clarification is needed in this scenario, and a description of
> > > > > > the suspicious scenario at the bottom.
> > > > > >
> > > > > > -------------------------------------------------------------------------
> > > > > > intel_pcode_init()
> > > > > > |
> > > > > > +-> skl_pcode_request(uncore, DG1_PCODE_STATUS,
> > > > > > DG1_UNCORE_GET_INIT_STATUS,
> > > > > > DG1_UNCORE_INIT_STATUS_COMPLETE,
> > > > > > DG1_UNCORE_INIT_STATUS_COMPLETE, 180000);
> > > > > > |
> > > > > > +-> skl_pcode_try_request()
> > > > > > |
> > > > > > +-> *status = __snb_pcode_rw(uncore, mbox, &request, NULL,
> > > > > > 500, 0, true);
> > > > > >
> > > > > > -------------------------------------------------------------------------
> > > > > > static int __snb_pcode_rw(struct intel_uncore *uncore, u32 mbox,
> > > > > > u32 *val, u32 *val1,
> > > > > > int fast_timeout_us, int slow_timeout_ms,
> > > > > > bool is_read)
> > > > > > {
> > > > > > ...
> > > > > > /* Before writing a value to the GEN6_PCODE_DATA register,
> > > > > > check if the bit in the GEN6_PCODE_MAILBOX register indicates
> > > > > > BUSY. */
> > > > > > if (intel_uncore_read_fw(uncore, GEN6_PCODE_MAILBOX) & GEN6_PCODE_READY)
> > > > > > return -EAGAIN;
> > > > >
> > > > > what if we fail here because the punit is still initializing and
> > > > > will be ready, say, in 10 seconds?
> > > > >
> > > > > GG, without going any further, we fail here! The -EAGAIN we
> > > > > receive from the test comes from this point. We don't fail with
> > > > > -ETIMEDOUT, but with -EAGAIN and the reason is because the punit
> > > > > is not ready to perform the very fist communication and we fail
> > > > > the probing.
> > > > >
> > > > > It doesn't mean, though, that there is anything wrong, we just
> > > > > need to wait a bit before "taking drastic decisions"!
> > > > >
> > > > > This patch is suggesting to wait up to 10s for the punit to be
> > > > > ready and eventually try to probe again... and, indeed, it works!
> > > >
> > > > As GG, what I still don't understand is how this extra 10 seconds
> > > > wait helps... have you tried to simple add the 10 to the 180 and
> > > > make the code 190 sec instead?
> > >
> > > maybe I haven't been able to explain the issue properly.
> > >
> > > I can even set that timer to 2hrs and a half and nothing changes
> > > because we fail before.
> > >
> > > Here it's not a matter of how much do I wait but when do I check
> > > the pcode readiness (i.e. signalled by the GEN6_PCODE_READY bit
> > > in the GEN6_PCODE_MAILBOX register).
> > >
> > > During a normal run we are always sure that communicating with
> > > the punit works, because we made it sure during the previous
> > > transaction.
> > >
> > > During probe there is no previous transaction and we start
> > > communicating with the punit without making sure that it is
> > > ready. And indeed some times it is not, so that we suppress the
> > > probing on purpose instead of giving it another chance.
> > >
> > > I admit that the commit message is not written properly and
> > > rather misleading, but here it's not at all a matter of how much
> > > do I wait.
> >
> > The commit message was initially confused because it looks like
> > we are just adding a wait, without doing anything....
> >
> > But looking to the code we can see that it will wait until
> > pcode is ready with a timeout of 10 seconds.
> >
> > But if pcode is ready in 10 seconds, why pcode is not ready
> > in 190 seconds. We are doing absolutely nothing more that could
> > make pcode ready in 10 seconds that won't be in 190.
> >
> > This is what we are missing here... The code as is doesn't make
> > a lot of sense to us and it looks like it is solving the issue
> > by the 10 extra seconds and not by some special status checking.
>
> Okay, after an offline talk I am convinced now that we need some
> check like this in some place.
>
> But the commit message needs be be fully re-written.
>
> It needs to be clear that underneath, the pcode communication
> functions will do a check for ready without any wait, what will
> make desired timeout to never really wait for the pcode done
> and prematurely return.
>
> at __snb_pcode_rw():
>
> if (intel_uncore_read_fw(uncore, GEN6_PCODE_MAILBOX) & GEN6_PCODE_READY)
> return -EAGAIN;
>
> So, for this reason we need to ensure that pcode is really ready
> before we wait.
>
> Other options are to handle the EAGAIN return and then wait.
> Or even change the __snb_pcode_rw to ensure that it is ready...
>
> Something like:
>
> - if (intel_uncore_read_fw(uncore, GEN6_PCODE_MAILBOX) & GEN6_PCODE_READY)
> - return -EAGAIN;
> + if (__intel_wait_for_register_fw(uncore, GEN6_PCODE_MAILBOX,
> + GEN6_PCODE_READY, GEN6_PCODE_READY,
> + fast_timeout_us,
> + slow_timeout_ms,
> + NULL))
> + return -ETIMEDOUT;
This works, but the difference is that you are putting it on any
call to the __snb_pcode_rw(), even when not necessary.
Putting it in the schema we used in our offline chat, the
original implementation was:
+-----------------------+
| r/w -> check if ready |-> r/w -> cir -> r/w -> cir -> r/w -> ...
+-----------------------+
probe time
(where cir stands for "check if ready"). Because in the first
r/w we didn't check the status of the punit we added:
+-------------------+
| cir -> r/w -> cir |-> r/w -> cir -> r/w -> cir -> r/w -> ...
+-------------------+
probe time
So that we are sure that the first r/w works. What you are
suggesting is:
+-------------------+
| cir -> r/w -> cir |-> cir -> r/w -> cir -> cir -> r/w -> cir ->
--------------------+
probe time
As you can se we have two "check if ready" in a raw, which might
be a bit of an overkill, it doesn't hurt much but it would look
like:
if (__intel_wait_for_register_fw())
return -EAGAIN;
intel_uncore_write_fw......
if (__intel_wait_for_register_fw())
return -TIMEDOUT;
and this for every single snb_pcode_r/w.
Besides some functions might need the first wait, some might not.
Check, for example icl_tc_cold_exit() where the -EAGAIN is
handled. With your version the retries would be quite ponderous.
I'm sorry, but I can't take your suggestion as it can have major
consequences along i915, it requires more changes and and it
needs to be carefully tested.
On top of that I like Ashutosh refactoring that is quite an
elegant way to gain more flexibility at boot time without any
further impact.
> In the best regular case it will not change anything since the wait
> for will return 0 immediatelly... in the worst case we would double
> the timeout value, but this would be only in this insane case of
> 180 seconds anyway and if it goes really really nasty...
>
> But anyway, now the motivation and the source of the issue is pretty
> clear, I can add my rv-b there, but I really want a better commit msg
> at least...
I definitely need to make a better commit message :)
Thanks a lot,
Andi
> >
> > >
> > > Thanks, Rodrigo!
> > > Andi
> > >
> > > > >
> > > > > Andi
> > > > >
> > > > > >
> > > > > > /* write value to GEN6_PCODE_DATA register */
> > > > > > intel_uncore_write_fw(uncore, GEN6_PCODE_DATA, *val);
> > > > > >
> > > > > > intel_uncore_write_fw(uncore, GEN6_PCODE_DATA1, val1 ? *val1 : 0);
> > > > > >
> > > > > > /* In this scenario, the value
> > > > > > "DG1_PCODE_STATUS | GEN6_PCODE_READY"
> > > > > > is written to the GEN6_PCODE_MAILBOX register,
> > > > > > so that the Busy status of the GEN6_PCODE_MAILBOX register
> > > > > > can be checked later.
> > > > > > (When the value of the GEN6_PCODE_READY bit of the
> > > > > > GEN6_PCODE_MAILBOX register changes to 0, the operation can
> > > > > > be considered completed.) */
> > > > > > intel_uncore_write_fw(uncore,
> > > > > > GEN6_PCODE_MAILBOX, GEN6_PCODE_READY | mbox);
> > > > > >
> > > > > > /* In this scenario, verify that the BUSY status bit in the
> > > > > > GEN6_PCODE_MAILBOX register turns off for up to 500us. */
> > > > > > if (__intel_wait_for_register_fw(uncore,
> > > > > > GEN6_PCODE_MAILBOX,
> > > > > > GEN6_PCODE_READY, 0,
> > > > > > fast_timeout_us,
> > > > > > slow_timeout_ms,
> > > > > > &mbox))
> > > > > > return -ETIMEDOUT;
> > > > > > /* If there is a failure here, it may be considered that the
> > > > > > "DG1_PCODE_STATUS | GEN6_PCODE_READY" operation was not
> > > > > > completed within 500us */
> > > > > > ...
> > > > > > }
> > > > > >
> > > > > > int skl_pcode_request(struct intel_uncore *uncore, u32 mbox, u32 request,
> > > > > > u32 reply_mask, u32 reply, int timeout_base_ms)
> > > > > > {
> > > > > > u32 status;
> > > > > > int ret;
> > > > > >
> > > > > > mutex_lock(&uncore->i915->sb_lock);
> > > > > >
> > > > > > #define COND \
> > > > > > skl_pcode_try_request(uncore, mbox, request, reply_mask, reply, &status)
> > > > > >
> > > > > > /* the first trial for skl_pcode_try_request() can return
> > > > > > -EAGAIN or -ETIMEDOUT. And the code did not check the error
> > > > > > code here, so we don't know how far the __snb_pcode_rw()
> > > > > > function went. It is not known whether the pcode_mailbox
> > > > > > status was busy before writing the value to the
> > > > > > GEN6_PCODE_DATA register or after.*/
> > > > > > if (COND) {
> > > > > > ret = 0;
> > > > > > goto out;
> > > > > > }
> > > > > >
> > > > > > /* In this scenario, skl_pcode_try_request() is invoked every
> > > > > > 10us for 180 seconds. When skl_pcode_try_request() returns
> > > > > > -EAGAIN and -ETIMEDOUT by _wait_for(),
> > > > > > -ETIMEDOUT is returned to a variable ret. */
> > > > > >
> > > > > > ret = _wait_for(COND, timeout_base_ms * 1000, 10, 10);
> > > > > >
> > > > > > if (!ret)
> > > > > > goto out;
> > > > > >
> > > > > > /*
> > > > > > * The above can time out if the number of requests was low (2 in the
> > > > > > * worst case) _and_ PCODE was busy for some reason even after a
> > > > > > * (queued) request and @timeout_base_ms delay. As a workaround retry
> > > > > > * the poll with preemption disabled to maximize the number of
> > > > > > * requests. Increase the timeout from @timeout_base_ms to 50ms to
> > > > > > * account for interrupts that could reduce the number of these
> > > > > > * requests, and for any quirks of the PCODE firmware that delays
> > > > > > * the request completion.
> > > > > > */
> > > > > > drm_dbg_kms(&uncore->i915->drm,
> > > > > > "PCODE timeout, retrying with preemption disabled\n");
> > > > > > drm_WARN_ON_ONCE(&uncore->i915->drm, timeout_base_ms > 3);
> > > > > > preempt_disable();
> > > > > > ret = wait_for_atomic(COND, 50);
> > > > > >
> > > > > > preempt_enable();
> > > > > >
> > > > > > out:
> > > > > > mutex_unlock(&uncore->i915->sb_lock);
> > > > > > return status ? status : ret;
> > > > > > #undef COND
> > > > > > }
> > > > > >
> > > > > > ---------------------------------------------------------
> > > > > >
> > > > > > If you try skl_pcode_try_request() for 180 seconds in skl_pcode_request(),
> > > > > > and the first "intel_uncore_read_fw(uncore, GEN6_PCODE_MAILBOX) &
> > > > > > GEN6_PCODE_READY)" call in __snb_pcode_rw() that skl_pcode_try_request()
> > > > > > invokes always fails. if then it does not make sense to me why this patch
> > > > > > fixes it by just waiting 10 seconds.This is because if it was called with
> > > > > > the flow described above, 180 seconds is longer than 10 seconds, so the
> > > > > > scenario you mentioned is also covered in the existing code.
> > > > > >
> > > > > > To describe in more detail the second scenario I previously commented on:
> > > > > > skl_pcode_request() tries skl_pcode_try_request() for 180 seconds
> > > > > > 1) In skl_pcode_try_request(), the GEN6_PCODE_MAILBOX register bit is not
> > > > > > BUSY, so the value is written to the GEN6_PCODE_DATA1 register.
> > > > > > 2) skl_pcode_try_request() returns -ETIMEDOUT value because the operation
> > > > > > of 1) does not complete within 500us.
> > > > > > 3) Scenario in which the GEN6_PCODE_MAILBOX register bit is checked as BUSY
> > > > > > and returns -EAGAIN in the last call of skl_pcode_try_request() invoked by
> > > > > > skl_pcode_request()
> > > > > >
> > > > > > If the reason why this problem occurred is because of this scenario,
> > > > > > shouldn't there be an attempt to increase fast_timeout_us used as an
> > > > > > argument of __snb_pcode_rw() to 500us or more when skl_pcode_try_request()
> > > > > > returns -ETIMEDOUT?
> > > > > >
> > > > > > Br,
> > > > > > G.G.
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