[PATCH 2/2] drm/bridge: lt9611: Do not generate HFP/HBP/HSA and EOT packet

[Neil Armstrong]

On 07/07/2023 09:18, Neil Armstrong wrote:
> Hi,
> 
> On 06/07/2023 11:20, Amit Pundir wrote:
>> On Wed, 5 Jul 2023 at 11:09, Dmitry Baryshkov
>> <dmitry.baryshkov at linaro.org> wrote:
>>>
>>> [Adding freedreno@ to cc list]
>>>
>>> On Wed, 5 Jul 2023 at 08:31, Jagan Teki <jagan at amarulasolutions.com> wrote:
>>>>
>>>> Hi Amit,
>>>>
>>>> On Wed, Jul 5, 2023 at 10:15 AM Amit Pundir <amit.pundir at linaro.org> wrote:
>>>>>
>>>>> Hi Marek,
>>>>>
>>>>> On Wed, 5 Jul 2023 at 01:48, Marek Vasut <marex at denx.de> wrote:
>>>>>>
>>>>>> Do not generate the HS front and back porch gaps, the HSA gap and
>>>>>> EOT packet, as these packets are not required. This makes the bridge
>>>>>> work with Samsung DSIM on i.MX8MM and i.MX8MP.
>>>>>
>>>>> This patch broke display on Dragonboard 845c (SDM845) devboard running
>>>>> AOSP. This is what I see
>>>>> https://people.linaro.org/~amit.pundir/db845c-userdebug/v6.5-broken-display/PXL_20230704_150156326.jpg.
>>>>> Reverting this patch fixes this regression for me.
>>>>
>>>> Might be msm dsi host require proper handling on these updated
>>>> mode_flags? did they?
>>>
>>> The msm DSI host supports those flags. Also, I'd like to point out
>>> that the patch didn't change the rest of the driver code. So even if
>>> drm/msm ignored some of the flags, it should not have caused the
>>> issue. Most likely the issue is on the lt9611 side. I's suspect that
>>> additional programming is required to make it work with these flags.
>>
>> I spent some time today on smoke testing these flags (individually and
>> in limited combination) on DB845c, to narrow down this breakage to one
>> or more flag(s) triggering it. Here are my observations in limited
>> testing done so far.
>>
>> There is no regression with MIPI_DSI_MODE_NO_EOT_PACKET when enabled
>> alone and system boots to UI as usual.
>>
>> MIPI_DSI_MODE_VIDEO_NO_HFP always trigger the broken display as in the
>> screenshot[1] shared earlier as well.
>>
>> Adding either of MIPI_DSI_MODE_VIDEO_NO_HSA and
>> MIPI_DSI_MODE_VIDEO_NO_HBP always result in no display, unless paired
>> with MIPI_DSI_MODE_VIDEO_NO_HFP and in that case we get the broken
>> display as reported.
>>
>> In short other than MIPI_DSI_MODE_NO_EOT_PACKET flag, all other flags
>> added in this commit break the display on DB845c one way or another.
> 
> I think the investigation would be to understand why samsung-dsim requires
> such flags and/or what are the difference in behavior between MSM DSI and samsung DSIM
> for those flags ?
> 
> If someone has access to the lt9611 datasheet, so it requires HSA/HFP/HBP to be
> skipped ? and does MSM DSI and samsung DSIM skip them in the same way ?

I think there's a mismatch, where on one side this flags sets the link in LP-11 while
in HSA/HFP/HPB while on the other it completely removes those blanking packets.

The name MIPI_DSI_MODE_VIDEO_NO_HBP suggests removal of HPB, not LP-11 while HPB.
the registers used in both controllers are different:
- samsung-dsim: DSIM_HBP_DISABLE_MODE
- msm dsi: DSI_VID_CFG0_HBP_POWER_STOP

The first one suggest removing the packet, while the second one suggests powering
off the line while in the blanking packet period.

@Abhinav, can you comment on that ?

@Jagan, Andrezej So you have any documentation on what DSIM_xxx_DISABLE_MODE does ?

@Dmitry, so you have access to the lt9611 datasheet to know what's needed here ?

Neil

> 
> Neil
> 
>>
>> Regards,
>> Amit Pundir
>> [1] https://people.linaro.org/~amit.pundir/db845c-userdebug/v6.5-broken-display/PXL_20230704_150156326.jpg
>>
>>>
>>> -- 
>>> With best wishes
>>> Dmitry
>