Re: [PATCH] drm: lcdif: Set and enable FIFO Panic threshold

[Marco Felsch <m.felsch@pengutronix.de>]

On 22-11-01, Marek Vasut wrote:
> On 11/1/22 15:04, Marco Felsch wrote:
> > Hi Marek, Liu,
> 
> Hi,
> 
> [...]
> 
> > > > > Also IMHO the threshold should be taken wisely to not enter panic
> > > > > mode
> > > > > to early to not block others from the bus e.g. the GPU.
> > > > 
> > > > As far as I understand the PANIC0_THRES, both thresholds are really
> > > > watermarks in the FIFO, 0=EMPTY, 1/3=LOW, 2/3=HIGH, 3/3=FULL. Under
> > > > normal conditions, the FIFO is filled above 1/3. When the FIFO fill
> > > > drops below LOW=1/3, the "PANIC" signal is asserted so the FIFO can
> > > > be
> > > > refilled faster. When the FIFO fill raises above HIGH=2/3, the
> > > > "PANIC"
> > > > signal is deasserted so the FIFO refills at normal rate again.
> > 
> > This matches exactly my picture of the hardware.
> > 
> > > > It seems to me the LOW=1/3 and HIGH=2/3 thresholds are the kind of
> > > > good
> > > > balance. The LOW=2/3 and HIGH=FULL=3/3 seems like it would keep the
> > > > "PANIC" signal asserted much longer, which could indeed block others
> > > > from the bus.
> > 
> > Also I understood the thresholds in such a way, that the FIFO watermark
> > must be higher but there is no place left when it is set to 3/3. In such
> > case this means that the PANIC will never left once it was entered.
> 
> I think this part is wrong.
> 
> Consider that the FIFO fill drops below 2/3 so PANIC signal asserts. 

? I thought the PANIC is triggered if the FIFO drops below the 1/3
threshold and is active till the 2/3 threshold.

> After a bit of time, the FIFO fill reaches full 3/3 (maybe during
> blanking period, where the data can be read in quickly without being
> scanned out again), and the PANIC signal de-asserts.
> 
> So the LCDIF won't be in constant PANIC asserted, but it will be there for
> quite a bit longer.
> 
> > > > It also seems to me that tuning these thresholds might be related to
> > > > some special use-case of the SoC, and it is most likely not just the
> > > > LCDIF thresholds which have been adjusted in such case, I would
> > > > expect
> > > > the NOC and GPV NIC priorities to be adjusted at that point too.
> > 
> > As far as I understood, the PANIC signal triggers the NOC to use the
> > PANIC signal priorities instead of the normal ones. I found a patch
> > laying in our downstream [1] repo which configures the threshold. This
> > raises the question which PANIC prio do you use? Do you have a patch for
> > this? If I remember correctly some TF-A's like the NXP downstream one
> > configure this but the upstream TF-A don't. Which TF-A do you use?
> 
> Upstream 2.6 or 2.7 , so this tuning does not apply.

So your panic priority is what?

> > > > So unless there are further details for that use-case which justify
> > > > making this somehow configurable, then maybe we should just stick to
> > > > 1/3 and 2/3 for now. And once there is a valid use-case which does
> > > > justify making this configurable, then we can add the DT properties
> > > > and all.
> > > > 
> > > > What do you think ?
> > > 
> > > No strong opinion from me on using LOW=1/3 and HIGH=2/3 thresholds for
> > > now if they satisfy all current users of the upstream kernel.  Tuning
> > > them in a certain way will be indeed needed once an usecase comes along
> > > and still suffers from the FIFO underflows with those thresholds.
> > 
> > I think that 1/3 and 2/3 should be fine for now too.
> 
> All right, lemme re-test this, send V2, and then we can go from there.

Okay :)

> btw. can you resend that [PATCH] drm: lcdif: change burst size to 256B with
> Fixes tag , so it can trickle into stable releases ?

Shure I will resend it with the tag added.

Regards,
  Marco