Re: [Intel-gfx] [PATCH] drm/i915/display: Reset message bus after each read/write operation

[Rodrigo Vivi <rodrigo.vivi@intel.com>]

On Thu, Oct 05, 2023 at 12:40:35PM -0300, Gustavo Sousa wrote:
> Quoting Rodrigo Vivi (2023-10-05 12:13:34-03:00)
> >On Thu, Oct 05, 2023 at 03:05:31AM -0400, Kahola, Mika wrote:
> >> > -----Original Message-----
> >> > From: Vivi, Rodrigo <rodrigo.vivi@intel.com>
> >> > Sent: Wednesday, October 4, 2023 3:56 PM
> >> > To: Kahola, Mika <mika.kahola@intel.com>
> >> > Cc: intel-gfx@lists.freedesktop.org
> >> > Subject: Re: [Intel-gfx] [PATCH] drm/i915/display: Reset message bus after each read/write operation
> >> > 
> >> > On Wed, Oct 04, 2023 at 01:25:04PM +0300, Mika Kahola wrote:
> >> > > Every know and then we receive the following error when running for
> >> > > example IGT test kms_flip.
> >> > >
> >> > > [drm] *ERROR* PHY G Read 0d80 failed after 3 retries.
> >> > > [drm] *ERROR* PHY G Write 0d81 failed after 3 retries.
> >> > >
> >> > > Since the error is sporadic in nature, the patch proposes to reset the
> >> > > message bus after every successful or unsuccessful read or write
> >> > > operation. However, testing revealed that this alone is not sufficient
> >> > > method an additiona delay is also introduces anything from 200us to
> >> > > 300us. This delay is experimental value and has no specification to
> >> > > back it up.
> >> > 
> >> > have you tried the delays without the bus_reset?
> >> Yes, we have bumped up the delay, first from 0x100 to 0x200 and then as per 
> >> BSpec change 0xa000 and I have tried 0xf000. Increasing the timeout reduces
> >> the frequency of this error but doesn't solve this issue.
> >
> >what is exactly this BSPec's 0xa000? where can I see it? So maybe you can
> >update the message above removing the 'no specification to back it up'.
> 
> (Resending this because I got a delivery failure notification)
> 
> I think we are confusing "delay" with the "timeout parameter" of the msgbus.
> 
> The PHY has a register to control the timeout parameter of msgbus transactions
> (BSpec 65156). It's default value is 0x100. With commit e028d7a4235d
> ("drm/i915/cx0: Check and increase msgbus timeout threshold"), we had integrated
> a workaround that bumped the timeout value to 0x200 in case timeouts were
> observed. Later on, there was a BSpec update with the formal timeout value to be
> programmed to 0xa000, which was incorporated with commit e35628968032
> ("drm/i915/cx0: Add step for programming msgbus timer").
> 
> I *believe* what Rodrigo has asked was about the usleep_range() calls added with
> this patch, if we tried to only keep the usleed_range() without the bus reset.

yes, that was my original question.

> 
> --
> Gustavo Sousa
> 
> >
> >Oh, and my english is bad, but it looks to me that 'empirical' might
> >sound better than 'experimental' for this case, since you really did
> >a lot of experiments before coming to this final conclusion.
> >
> >> 
> >> > have you talked to hw architects about this?
> >> Yes, HW guys requested traces which I provided but based on these the sequence we use in i915
> >> is correct.
> >> 
> >> > 
> >> > I wonder if we should add the delay inside the bus_reset itself?
> >> > although the bit 15 clear check should be enough by itself and it doesn't look like it is a hw/fw reset involved to justify the extra
> >> > delay.
> >> That should be enough. To me, it looks like when reading/writing to the bus maybe too fast, the hw cannot handle that and we need
> >> to reset and let things settle down before trying again.
> >> 
> >> > 
> >> > well, at least some /* FIXME: */ or /* XXX: */ comments is desired along with the messages if we are going with this hack without
> >> > understanding why...
> >> True, I will add these the the patch.
> >> 
> >> Thanks for review!
> >> 
> >> -Mika-
> >> > 
> >> > >
> >> > > Signed-off-by: Mika Kahola <mika.kahola@intel.com>
> >> > > ---
> >> > >  drivers/gpu/drm/i915/display/intel_cx0_phy.c | 6 ++++++
> >> > >  1 file changed, 6 insertions(+)
> >> > >
> >> > > diff --git a/drivers/gpu/drm/i915/display/intel_cx0_phy.c
> >> > > b/drivers/gpu/drm/i915/display/intel_cx0_phy.c
> >> > > index abd607b564f1..a71b8a29d6b0 100644
> >> > > --- a/drivers/gpu/drm/i915/display/intel_cx0_phy.c
> >> > > +++ b/drivers/gpu/drm/i915/display/intel_cx0_phy.c
> >> > > @@ -220,9 +220,12 @@ static u8 __intel_cx0_read(struct drm_i915_private *i915, enum port port,
> >> > >          /* 3 tries is assumed to be enough to read successfully */
> >> > >          for (i = 0; i < 3; i++) {
> >> > >                  status = __intel_cx0_read_once(i915, port, lane, addr);
> >> > > +                intel_cx0_bus_reset(i915, port, lane);
> >> > >
> >> > >                  if (status >= 0)
> >> > >                          return status;
> >> > > +
> >> > > +                usleep_range(200, 300);
> >> > >          }
> >> > >
> >> > >          drm_err_once(&i915->drm, "PHY %c Read %04x failed after %d
> >> > > retries.\n", @@ -299,9 +302,12 @@ static void __intel_cx0_write(struct drm_i915_private *i915, enum port port,
> >> > >          /* 3 tries is assumed to be enough to write successfully */
> >> > >          for (i = 0; i < 3; i++) {
> >> > >                  status = __intel_cx0_write_once(i915, port, lane, addr, data,
> >> > > committed);
> >> > > +                intel_cx0_bus_reset(i915, port, lane);
> >> > >
> >> > >                  if (status == 0)
> >> > >                          return;
> >> > > +
> >> > > +                usleep_range(200, 300);
> >> > >          }
> >> > >
> >> > >          drm_err_once(&i915->drm,
> >> > > --
> >> > > 2.34.1
> >> > >