Re: [Intel-gfx] [PATCH] drm/i915: program wm blocks to at least blocks required per line

["Govindapillai, Vinod" <vinod.govindapillai@intel.com>]

On Wed, 2022-04-06 at 17:01 +0300, Ville Syrjälä wrote:
> On Wed, Apr 06, 2022 at 04:45:26PM +0300, Lisovskiy, Stanislav wrote:
> > On Wed, Apr 06, 2022 at 03:48:02PM +0300, Ville Syrjälä wrote:
> > > On Mon, Apr 04, 2022 at 04:49:18PM +0300, Vinod Govindapillai wrote:
> > > > In configurations with single DRAM channel, for usecases like
> > > > 4K 60 Hz, FIFO underruns are observed quite frequently. Looks
> > > > like the wm0 watermark values need to bumped up because the wm0
> > > > memory latency calculations are probably not taking the DRAM
> > > > channel's impact into account.
> > > > 
> > > > As per the Bspec 49325, if the ddb allocation can hold at least
> > > > one plane_blocks_per_line we should have selected method2.
> > > > Assuming that modern HW versions have enough dbuf to hold
> > > > at least one line, set the wm blocks to equivalent to blocks
> > > > per line.
> > > > 
> > > > cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
> > > > cc: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>
> > > > 
> > > > Signed-off-by: Vinod Govindapillai <vinod.govindapillai@intel.com>
> > > > ---
> > > >  drivers/gpu/drm/i915/intel_pm.c | 19 ++++++++++++++++++-
> > > >  1 file changed, 18 insertions(+), 1 deletion(-)
> > > > 
> > > > diff --git a/drivers/gpu/drm/i915/intel_pm.c b/drivers/gpu/drm/i915/intel_pm.c
> > > > index 8824f269e5f5..ae28a8c63ca4 100644
> > > > --- a/drivers/gpu/drm/i915/intel_pm.c
> > > > +++ b/drivers/gpu/drm/i915/intel_pm.c
> > > > @@ -5474,7 +5474,24 @@ static void skl_compute_plane_wm(const struct intel_crtc_state
> > > > *crtc_state,
> > > >  		}
> > > >  	}
> > > >  
> > > > -	blocks = fixed16_to_u32_round_up(selected_result) + 1;
> > > > +	/*
> > > > +	 * Lets have blocks at minimum equivalent to plane_blocks_per_line
> > > > +	 * as there will be at minimum one line for lines configuration.
> > > > +	 *
> > > > +	 * As per the Bspec 49325, if the ddb allocation can hold at least
> > > > +	 * one plane_blocks_per_line, we should have selected method2 in
> > > > +	 * the above logic. Assuming that modern versions have enough dbuf
> > > > +	 * and method2 guarantees blocks equivalent to at least 1 line,
> > > > +	 * select the blocks as plane_blocks_per_line.
> > > > +	 *
> > > > +	 * TODO: Revisit the logic when we have better understanding on DRAM
> > > > +	 * channels' impact on the level 0 memory latency and the relevant
> > > > +	 * wm calculations.
> > > > +	 */
> > > > +	blocks = skl_wm_has_lines(dev_priv, level) ?
> > > > +			max_t(u32, fixed16_to_u32_round_up(selected_result) + 1,
> > > > +				  fixed16_to_u32_round_up(wp->plane_blocks_per_line)) :
> > > > +			fixed16_to_u32_round_up(selected_result) + 1;
> > > 
> > > That's looks rather convoluted.
> > > 
> > >   blocks = fixed16_to_u32_round_up(selected_result) + 1;
> > > + /* blah */
> > > + if (has_lines)
> > > +	blocks = max(blocks, fixed16_to_u32_round_up(wp->plane_blocks_per_line));
> > 
> > We probably need to do similar refactoring in the whole function ;-)
> > 
> > > Also since Art said nothing like this should actually be needed
> > > I think the comment should make it a bit more clear that this
> > > is just a hack to work around the underruns with some single
> > > memory channel configurations.
> > 
> > It is actually not quite a hack, because we are missing that condition
> > implementation from BSpec 49325, which instructs us to select method2
> > when ddb blocks allocation is known and that ratio is >= 1.

In the slides sent by Art, It is mentioned that driver should be using the wm results to arrive at
optimum ddb allocations. So I guess the best solution would be to identify the extra latency because
of the single DRAM channel and calculate the wm.

> 
> The ddb allocation is not yet known, so we're implementing the
> algorithm 100% correctly.
> 
> And this patch does not implement that misisng part anyway.

Thanks. Updated the patch as per your comments and V2 sent.

BR
Vinod

> > Mean this one:
> > 
> > "If ('plane buffer allocation' is known and (plane buffer allocation / plane blocks per line)
> > >=1)
> > Selected Result Blocks = Method 2"
> > 
> > Stan
> > 
> > > 
> > > >  	lines = div_round_up_fixed16(selected_result,
> > > >  				     wp->plane_blocks_per_line);
> > > >  
> > > > -- 
> > > > 2.25.1
> > > 
> > > -- 
> > > Ville Syrjälä
> > > Intel