[PATCH] drm/i915/dg2: Catch and log more unexpected values in DG1_MSTR_TILE_INTR

[Tvrtko Ursulin]

On 27/05/2022 19:42, Matt Roper wrote:
> On Thu, May 26, 2022 at 11:18:17AM +0100, Tvrtko Ursulin wrote:
>> On 25/05/2022 19:05, Matt Roper wrote:
>>> On Wed, May 25, 2022 at 05:03:13PM +0100, Tvrtko Ursulin wrote:
>>>>
>>>> On 24/05/2022 18:51, Matt Roper wrote:
>>>>> On Tue, May 24, 2022 at 10:43:39AM +0100, Tvrtko Ursulin wrote:
>>>>>> From: Tvrtko Ursulin <tvrtko.ursulin at intel.com>
>>>>>>
>>>>>> Catch and log any garbage in the register, including no tiles marked, or
>>>>>> multiple tiles marked.
>>>>>>
>>>>>> Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin at intel.com>
>>>>>> Cc: Matt Roper <matthew.d.roper at intel.com>
>>>>>> ---
>>>>>> We caught garbage in DG1_MSTR_TILE_INTR with DG2 (actual value 0xF9D2C008)
>>>>>> during glmark and more badness. So I thought lets log all possible failure
>>>>>> modes from here and also use per device logging.
>>>>>> ---
>>>>>>     drivers/gpu/drm/i915/i915_irq.c | 33 ++++++++++++++++++++++-----------
>>>>>>     drivers/gpu/drm/i915/i915_reg.h |  1 +
>>>>>>     2 files changed, 23 insertions(+), 11 deletions(-)
>>>>>>
>>>>>> diff --git a/drivers/gpu/drm/i915/i915_irq.c b/drivers/gpu/drm/i915/i915_irq.c
>>>>>> index 73cebc6aa650..79853d3fc1ed 100644
>>>>>> --- a/drivers/gpu/drm/i915/i915_irq.c
>>>>>> +++ b/drivers/gpu/drm/i915/i915_irq.c
>>>>>> @@ -2778,24 +2778,30 @@ static irqreturn_t dg1_irq_handler(int irq, void *arg)
>>>>>>     	u32 gu_misc_iir;
>>>>>>     	if (!intel_irqs_enabled(i915))
>>>>>> -		return IRQ_NONE;
>>>>>> +		goto none;
>>>>>>     	master_tile_ctl = dg1_master_intr_disable(regs);
>>>>>> -	if (!master_tile_ctl) {
>>>>>> -		dg1_master_intr_enable(regs);
>>>>>> -		return IRQ_NONE;
>>>>>> +	if (!master_tile_ctl)
>>>>>> +		goto enable_none;
>>>>>> +
>>>>>> +	if (master_tile_ctl & ~(DG1_MSTR_IRQ | DG1_MSTR_TILE_MASK)) {
>>>>>> +		drm_warn(&i915->drm, "Garbage in master_tile_ctl: 0x%08x!\n",
>>>>>> +			 master_tile_ctl);
>>>>>
>>>>> I know we have a bunch of them already, but shouldn't we be avoiding
>>>>> printk-based stuff like this inside interrupt handlers?  Should we be
>>>>> migrating all these error messages over to trace_printk or something
>>>>> similar that's safer to use?
>>>>
>>>> Not sure - I kind of think some really unexpected and worrying situations
>>>> should be loud and on by default. Risk is then spam if not ratelimited.
>>>> Maybe we should instead ratelimit most errors/warnings coming for irq
>>>> handlers?
>>>
>>> It's not the risk of spam that's the problem, but rather that
>>> printk-based stuff eventually calls into the console code to flush its
>>> buffers.  That's way more overhead than you want in an interrupt handler
>>> so it's bad on its own, but if you're using something slow like a serial
>>> console, it becomes even more of a problem.
>>
>> Is it a problem for messages which we never expect to see?
> 
> Kind of.  While not as catastrophic, it's the same argument for why we
> don't use BUG() anymore...when the impossible does manage to happen
> there's unnecessary collateral damage on things outside of graphics.  If
> we're adding huge delays inside an interrupt handler (while other
> interrupts are disabled) that impacts the system-wide usability, not
> just our own driver.
> 
> I'd also argue that these messages actually are semi-expected.  Random
> bits being set shouldn't happen, but in the world of dgpu's, we do
> occasionally see cases where the PCI link itself goes down for reasons
> outside our control and then all registers read back as 0xFFFFFFFF,
> which will probably trigger error messages here (as well as a bunch of
> other places).

Could you expand a bit on what is semi-expected and when? I mean the 
circumstances of PCI link going down. We certainly don't have any code 
to survive that.

>>> While the unexpected bits in the master tile register are strange and
>>> may point to a bigger problem somewhere else, they're also harmless on
>>> their own since we should just ignore those bits and only process the
>>> valid tiles.
>>
>> Yes, I was expecting that a patch belonging to multi-tile enablement would
>> be incoming soon, which would be changing:
>>
>> +	if (REG_FIELD_GET(DG1_MSTR_TILE_MASK, master_tile_ctl) !=
>> +	    DG1_MSTR_TILE(0)) {
>> +		drm_warn(&i915->drm, "Unexpected irq from tile %u!\n",
>> +			 ilog2(REG_FIELD_GET(DG1_MSTR_TILE_MASK,
>> +					     master_tile_ctl)));
>> +		goto enable_none;
>>   	}
>>
>>  From this patch, into something completely different like walking bit by
>> bit, handling the present tiles, and warning on unexpected ones. What should
>> remain though is warning on no tiles signaled (which what we saw, together
>> with garbage in reserved bits).
> 
> Yeah.  Although I still feel the interrupt handler should really just be
> flagging the errors so that the actual prints themselves can happen
> outside the interrupt.
> 
>>
>>>> In this particular case at least DRM_ERROR with no device info is the odd
>>>> one out in the entire file so I'd suggest changing at least that, if the
>>>> rest of my changes is of questionable benefit.
>>>
>>> Changing DRM_ERROR -> drm_err would probably be fine in the short term
>>> since it doesn't really make us any worse off.  Changing to drm_warn
>>> might not be great since we're generating a lot more lines of output and
>>
>> Sorry I don't follow - why does replacing drm_err with drm_warn generate (a
>> lot) more lines of output?
> 
> Sorry, my mistake; I had it in my mind that we were talking about a
> drm_WARN_ON rather than just drm_warn (i.e., including a big stacktrace
> and such).  DRM_ERROR -> drm_warn alone shouldn't have any extra
> negative impact.
> 
>>
>> But it can be drm_err for all I care, I don't think we really have
>> consistent story between errors and warnings in this area.
>>
>>> probably multiplying the already bad overhead that shouldn't be
>>> happening in an interrupt handler.  But if we could update the interrupt
>>> handler to just save away the details and do the actual drm_warn later,
>>> outside the interrupt handler code, that would be okay.  We should
>>> probably work toward something like that for all of our interrupt
>>> handler warning/error messages.
>>
>> Not sure I agree - for messages which we don't expect to see it doesn't
>> really matter that there will be overhead when they are hit. Presumably bad
>> things are already happening there so spending effort to optimise those path
>> is questionable.
> 
> Something bad is happening to graphics is we hit one of these cases.
> But if we start doing prints while interrupts are disabled, we start
> having more of a negative impact on the rest of the system too.

Truly for the case of this particular patch I don't think we should 
care. Rate limiting should be all that is needed in the short term to 
strike a balance between effort and benefit. But lets first clarify the 
PCI link going down problem.

Regards,

Tvrtko