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

And many thanks for your comments :)


On 01/29/2018 10:46 AM, Laurent Pinchart wrote:
> Hi Philippe,
>
> (CC'ing Daniel Vetter)
>
> Thank you for the patch.
>
> 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.
* 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"...

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);
>
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