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

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

Hi Laurent & Daniel :-)

On 01/29/2018 11:40 AM, Laurent Pinchart wrote:
> 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.
> 

Laurent, do you think we can use "adjusted mode" here in this small 
patch as the actual 2 "users" of this bridge (rockchip & stm) use both 
"adjusted mode" in their crtc?

>> Many thanks,
>> Philippe :-)
>> [1] https://patchwork.freedesktop.org/patch/200240/
>> [2] https://patchwork.freedesktop.org/patch/200720/
>>
>>> Daniel, what's your opinion on this ?
>>>

Daniel, any opinion on the adjusted_mode usage?

Many thanks to both of you,
Philippe :-)

>>>> ---
>>>> 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);
>