Re: [PATCH] drm/bridge/synopsys: dsi: use adjusted_mode in mode_set

[Philippe CORNU <philippe.cornu@st.com>]

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);
>