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 17:06, Marco Felsch wrote:
> 
> Hi,
> 
> > > > 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.
> 
> Yes, although I think the ASSERT/DEASSERT are one-way switches.
> 
> > > 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?
> 
> If you tell me which register (physical address) to read, I will do that on
> this board right now.

According our patch it is:

#define HDMI_TX_CONTROL0       0x200
#define HDMI_LCDIF_NOC_HURRY_PRIO_MASK GENMASK(14, 12)
#define HDMI_LCDIF_NOC_HURRY_PRIO(p)   (((p) << 12) & HDMI_LCDIF_NOC_HURRY_PRIO_MASK)

and we set it within imx8mp_hdmi_blk_ctrl_power_on() by:

regmap_set_bits(bc->regmap, HDMI_TX_CONTROL0,
                HDMI_LCDIF_NOC_HURRY_PRIO(7));

imx8mp_hdmi_blk_ctrl_power_off():

regmap_clear_bits(bc->regmap, HDMI_TX_CONTROL0,
                HDMI_LCDIF_NOC_HURRY_PRIO(7));

> 
> > > > > > 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.
> 
> Thanks, I'll pick it via drm-misc then.

Send it as v2.

Regards,
  Marco