Re: [PATCH v3 4/4] drm/msm/dpu: fix video mode DSC INTF timing width calculation

[Dmitry Baryshkov <dmitry.baryshkov@oss.qualcomm.com>]

On Fri, Mar 20, 2026 at 12:25:00AM -0400, Jonathan Marek wrote:
> On 3/19/26 9:45 PM, Dmitry Baryshkov wrote:
> > On Thu, Mar 19, 2026 at 01:23:03PM -0400, Jonathan Marek wrote:
> ...
> > > 
> > > That's not how it works. INTF (which feeds DSI) is after DSC compression.
> > > 
> > > INTF timings are always in RGB888 (24-bit) units. Ignoring widebus details,
> > > the INTF timing should match what is programmed on the DSI side (hdisplay,
> > > which is calculated as bytes per line / 3).
> > > 
> > > (fwiw, the current "timing->width = ..." calculation here blames to me, but
> > > what I wrote originally was just "timing->width = timing->width / 3" with a
> > > comment about being incomplete.)
> > > 
> > Okay. After reading the docs (sorry, it took a while).
> > 
> > - When widebus is not enabled, the transfer is always 24 bit of
> >    compressed data. Thus if it is not in play, pclk and timing->width
> >    should be scaled by source_pixel_depth / compression_ratio / 24. In
> >    case of the code it is 'drm_dsc_get_bpp_int(dsc) / 24'.
> > 
> >    For RGB101010 / 8bpp DSC this should result in the PCLK being lowered
> >    by the factor of 3 (= 24 / (30 / 3.75))
> > 
> > - When widebus is in play (MDSS 6.x+, DSI 2.4+), the transfer takes
> >    more than 24 bits. In this case the PCLK and timing->width should be
> >    scaled exactly by the DSC compression ratio, which is
> >    'drm_dsc_get_bpp_int(dsc) / (3 * dsc->bits_per_component).
> > 
> > So, this piece of code needs to be adjusted to check for the widebus
> > being enabled or not.
> > 
> 
> The widebus adjustment on the MDP/INTF side is already in
> dpu_hw_intf_setup_timing_engine: the "data width" is divided by 2 for 48-bit
> widebus instead of 24-bit. there shouldn't be any other adjustment
> (downstream doesn't have any other adjustment).
> 
> a relevant downstream comment: "In DATABUS-WIDEN mode, MDP always sends out
> 48-bit compressed data per pclk and on average, DSI consumes an amount of
> compressed data equivalent to the uncompressed pixel depth per pclk."
> 
> Based on that comment, this patch is correct, and the
> ''drm_dsc_get_bpp_int(dsc) / (3 * dsc->bits_per_component)' adjustment only
> applies to DSI. (note: newer downstream looks like it would divide by 3.75
> here, which doesn't make sense. older downstream would divide by 3 here. I
> guess downstream is broken now and video mode + 10-bit dsc doesn't get
> tested?)

I guess, the downstream might be broken wrt. the widebus being enabled
or not.

> 
> on DSI side, "uncompressed pixel depth" shouldn't matter either: DSI only
> sees the compressed data. But based on that comment, when widebus is
> enabled, by setting DSI_VID_CFG0_DST_FORMAT(?) to 30bpp, then the DSI pclk
> is in 30-bit units instead of 24-bits. And with this series DSI side ends up
> with the right result if 30bpp format and widebus is enabled.

-- 
With best wishes
Dmitry