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SubjectRe: [PATCH] drm/bridge/synopsys: dsi: use adjusted_mode in mode_set
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Hi Philippe,

On Monday, 29 January 2018 12:17:37 EET Philippe CORNU wrote:
> On 01/29/2018 10:46 AM, Laurent Pinchart wrote:
> > On Thursday, 25 January 2018 17:55:04 EET Philippe Cornu wrote:
> >
> >> The "adjusted_mode" clock value (ie the real pixel clock) is more
> >> accurate than "mode" clock value (ie the panel/bridge requested
> >> clock value). It offers a better preciseness for timing
> >> computations and allows to reduce the extra dsi bandwidth in
> >> burst mode (from ~20% to ~10-12%, hw platform dependant).
> >>
> >> Signed-off-by: Philippe Cornu <philippe.cornu@st.com>
> >
> > The adjusted mode is documented as
> >
> > /**
> > * @adjusted_mode:
> > *
> > * Internal display timings which can be used by the driver to handle
> > * differences between the mode requested by userspace in @mode and what
> > * is actually programmed into the hardware. It is purely driver
> > * implementation defined what exactly this adjusted mode means. Usually
> > * it is used to store the hardware display timings used between the
> > * CRTC and encoder blocks.
> > */
> >
> > This is easy to handle when the CRTC and encoder are controlled by the
> > same driver, as the field is "implementation defined" by a single driver
> > . However, when using bridges, there are two drivers involved, and they
> > must both agree to meaningfully use the adjusted mode. I can't see how to
> > do so without standardizing the meaning of the adjusted mode field.
>
> This is exactly the reason why my first implementation used the dsi
> bridge "optional pixel clock" instead of the adjusted_mode (see [1])
>
> But after digging more into the drm source code, I think using
> adjusted_mode instead of the pixel clock here brings more advantages
> because:
> * adjusted_mode is an argument of bridge mode_set() probably for being
> used in any manner, maybe like this :)
> * if the bridge "user" (crtc or a master bridge drivers) does not need
> to modify its adjusted_mode then mode & adjusted_mode mode_set()
> arguments will have the same values so "no consequence" for the bridge.
> * if the bridge "user" (crtc or master bridge drivers) needs to adjust
> any value of the mode then this adjustment is available for the bridge.

Remember that there can be multiple chained bridges, and a single adjusted
mode field.

> * rockchip crtc updates a part of the mode (the clock), stm is doing the
> same (see [2]) but any future "user" of the dw_mipi_dsi bridge can
> adjust something else (blankings...) and the dw_mipi_dsi bridge will be
> then aware of...
>
> But maybe it is a wrong usage of the "adjusted_mode offer"...

I don't disagree that there's a need for using adjusted values, but I believe
we need to create a clear API to do so. Using the adjust_mode field as-is when
it's clearly documented as being implementation-defined is asking for trouble.

> Many thanks,
> Philippe :-)
> [1] https://patchwork.freedesktop.org/patch/200240/
> [2] https://patchwork.freedesktop.org/patch/200720/
>
> > Daniel, what's your opinion on this ?
> >
> >> ---
> >> Note: This patch replaces "drm/bridge/synopsys: dsi: add optional pixel
> >> clock"
> >>
> >> drivers/gpu/drm/bridge/synopsys/dw-mipi-dsi.c | 12 ++++++------
> >> 1 file changed, 6 insertions(+), 6 deletions(-)
> >>
> >> diff --git a/drivers/gpu/drm/bridge/synopsys/dw-mipi-dsi.c
> >> b/drivers/gpu/drm/bridge/synopsys/dw-mipi-dsi.c index
> >> ed8af32f8e52..b926b62e9e33 100644
> >> --- a/drivers/gpu/drm/bridge/synopsys/dw-mipi-dsi.c
> >> +++ b/drivers/gpu/drm/bridge/synopsys/dw-mipi-dsi.c
> >> @@ -707,20 +707,20 @@ static void dw_mipi_dsi_bridge_mode_set(struct
> >> drm_bridge *bridge,
> >>
> >> clk_prepare_enable(dsi->pclk);
> >>
> >> - ret = phy_ops->get_lane_mbps(priv_data, mode, dsi->mode_flags,
> >> + ret = phy_ops->get_lane_mbps(priv_data, adjusted_mode,
> >> dsi->mode_flags,
> >> dsi->lanes, dsi->format, &dsi->lane_mbps);
> >> if (ret)
> >> DRM_DEBUG_DRIVER("Phy get_lane_mbps() failed\n");
> >>
> >> pm_runtime_get_sync(dsi->dev);
> >> dw_mipi_dsi_init(dsi);
> >>
> >> - dw_mipi_dsi_dpi_config(dsi, mode);
> >> + dw_mipi_dsi_dpi_config(dsi, adjusted_mode);
> >> dw_mipi_dsi_packet_handler_config(dsi);
> >> dw_mipi_dsi_video_mode_config(dsi);
> >> - dw_mipi_dsi_video_packet_config(dsi, mode);
> >> + dw_mipi_dsi_video_packet_config(dsi, adjusted_mode);
> >> dw_mipi_dsi_command_mode_config(dsi);
> >> - dw_mipi_dsi_line_timer_config(dsi, mode);
> >> - dw_mipi_dsi_vertical_timing_config(dsi, mode);
> >> + dw_mipi_dsi_line_timer_config(dsi, adjusted_mode);
> >> + dw_mipi_dsi_vertical_timing_config(dsi, adjusted_mode);
> >> dw_mipi_dsi_dphy_init(dsi);
> >> dw_mipi_dsi_dphy_timing_config(dsi);
> >>
> >> @@ -734,7 +734,7 @@ static void dw_mipi_dsi_bridge_mode_set(struct
> >> drm_bridge *bridge,
> >>
> >> dw_mipi_dsi_dphy_enable(dsi);
> >>
> >> - dw_mipi_dsi_wait_for_two_frames(mode);
> >> + dw_mipi_dsi_wait_for_two_frames(adjusted_mode);
> >>
> >> /* Switch to cmd mode for panel-bridge pre_enable & panel prepare */
> >> dw_mipi_dsi_set_mode(dsi, 0);

--
Regards,

Laurent Pinchart

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