Re: [Intel-gfx] [PATCH 1/2] drm/i915: Fix skl_pcode_try_request function

["Ville Syrjälä" <ville.syrjala@linux.intel.com>]

On Fri, Apr 08, 2022 at 06:46:00PM +0300, Ville Syrjälä wrote:
> On Fri, Apr 08, 2022 at 03:51:59PM +0300, Stanislav Lisovskiy wrote:
> > Currently skl_pcode_try_request function doesn't
> > properly handle return value it gets from
> > snb_pcode_rw, but treats status != 0 as success,
> > returning true, which basically doesn't allow
> > to use retry/timeout mechanisms if PCode happens
> > to be busy and returns EGAIN or some other status
> > code not equal to 0.
> > 
> > We saw this on real hw and also tried simulating this
> > by always returning -EAGAIN from snb_pcode_rw for 6 times, which
> > currently will just result in false success, while it should
> > have tried until timeout is reached:
> > 
> > [   22.357729] i915 0000:00:02.0: [drm:intel_cdclk_dump_config [i915]] Changing CDCLK to
> > 307200 kHz, VCO 614400 kHz, ref 38400 kHz, bypass 19200 kHz, voltage level 0
> > [   22.357831] i915 0000:00:02.0: [drm:__snb_pcode_rw [i915]] Returning EAGAIN retry 1
> > [   22.357892] i915 0000:00:02.0: [drm:skl_pcode_request [i915]] Success, exiting
> > [   22.357936] i915 0000:00:02.0: [drm] ERROR Failed to inform PCU about cdclk change (err -11, freq 307200)
> > 
> > We see en error because higher level api, still notices that status was wrong,
> > however we still did try only once.
> > 
> > We fix it by requiring _both_ the status to be 0 and
> > request/reply match for success(true) and function
> > should return failure(false) if either status turns
> > out to be EAGAIN, EBUSY or whatever or reply/request
> > masks do not match.
> > 
> > So now we see this in the logs:
> > 
> > [   22.318667] i915 0000:00:02.0: [drm:intel_cdclk_dump_config [i915]] Changing CDCLK to
> > 307200 kHz, VCO 614400 kHz, ref 38400 kHz, bypass 19200 kHz, voltage level 0
> > [   22.318782] i915 0000:00:02.0: [drm:__snb_pcode_rw [i915]] Returning EAGAIN retry 1

Hmm. That is weird. The timestamp difference is only ~100 usec even
though we are supposed to use that 500 usec timeout. So did some
previous pcode access already timeout and leave the mailbox busy 
before we even do this request, or what is going on?

> > [   22.318849] i915 0000:00:02.0: [drm:__snb_pcode_rw [i915]] Returning EAGAIN retry 2
> > [   22.319006] i915 0000:00:02.0: [drm:__snb_pcode_rw [i915]] Returning EAGAIN retry 3
> > [   22.319091] i915 0000:00:02.0: [drm:__snb_pcode_rw [i915]] Returning EAGAIN retry 4
> > [   22.319158] i915 0000:00:02.0: [drm:__snb_pcode_rw [i915]] Returning EAGAIN retry 5
> > [   22.319224] i915 0000:00:02.0: [drm:__snb_pcode_rw [i915]] Returning EAGAIN retry 6
> > 
> > Reviewed-by: Vinod Govindapillai <vinod.govindapillai@intel.com>
> > Signed-off-by: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>
> > ---
> >  drivers/gpu/drm/i915/intel_pcode.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/drivers/gpu/drm/i915/intel_pcode.c b/drivers/gpu/drm/i915/intel_pcode.c
> > index 391a37492ce5..fb6c43e8a02f 100644
> > --- a/drivers/gpu/drm/i915/intel_pcode.c
> > +++ b/drivers/gpu/drm/i915/intel_pcode.c
> > @@ -136,7 +136,7 @@ static bool skl_pcode_try_request(struct drm_i915_private *i915, u32 mbox,
> >  {
> >  	*status = __snb_pcode_rw(i915, mbox, &request, NULL, 500, 0, true);
> >  
> > -	return *status || ((request & reply_mask) == reply);
> > +	return (*status == 0) && ((request & reply_mask) == reply);
> 
> The problem with this is that now we'll keep pointlessly banging it
> even if it returns a real error.
> 
> We should never really see that -EAGAIN since it indicates that our
> timeout is too short. So the real fix should be to increase that
> timeout. But I guess we could do a belt-and-suspenders approach
> where we also keep repeating on -EGAIN. But I'm thinking -EAGAIN
> should WARN as well to make sure we notice that our timeout is wrong.
> 
> >  }
> >  
> >  /**
> > -- 
> > 2.24.1.485.gad05a3d8e5
> 
> -- 
> Ville Syrjälä
> Intel

-- 
Ville Syrjälä
Intel