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

["Lisovskiy, Stanislav" <stanislav.lisovskiy@intel.com>]

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.

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