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

[neil.armstrong at linaro.org]

Hi Marek,

On 02/08/2023 14:25, Marek Vasut wrote:
> On 8/2/23 10:39, neil.armstrong at linaro.org wrote:
>> Hi Marek,
> 
> Hi,
> 
>> On 13/07/2023 20:28, Marek Vasut wrote:
>>
>> <snip>
>>
>>>>>
>>>>> MIPI_DSI_MODE_VIDEO_NO_HFP means the HBP period is just skipped by DSIM.
>>>>>
>>>>> Maybe there is a need for new set of flags which differentiate between HBP skipped (i.e. NO HBP) and HBP LP11 ?
>>>>>
>>>>
>>>> No, the section of the MIPI DSI spec I posted below clearly states there are two options:
>>>>
>>>> 1) send blanking packets during those periods
>>>> 2) transition to LP11 during those periods
>>>>
>>>> There is no 3rd option in the spec of not doing both like what you are suggesting. So DSIM should also be only transitioning to LP11 during those periods if its not sending the blanking packets with those flags set.
>>>>
>>>> So, there is no need for any new set of flags to differentiate.
>>>>
>>>> The flags and their interpretation is correct in MSM driver. I cannot comment on what exactly DSIM does with those flags.
>>>
>>> How do you explain the comment in include/drm/drm_mipi_dsi.h:
>>>
>>> 128 /* disable hback-porch area */
>>> 129 #define MIPI_DSI_MODE_VIDEO_NO_HBP      BIT(6)
>>
>> Can you specify how you determined those flags were needed on DSIM ? a vendor tree ? a datasheet ?
> 
> The following upstream commit:
> 
> 996e1defca344 ("drm: exynos: dsi: Fix MIPI_DSI*_NO_* mode flags")
> 
>> In the meantime, we should revert this patch because it regresses some Qcom
>> based platforms until we figure out what's missing to make DSIM based boards
>> happy.
>>
>> I'll send a revert change afterwards.
> 
> That change would break existing use case on i.MX8M then, I disagree with that revert.

As I understand the timeline is :

- 996e1defca344 was merged in v6.2-rc2 and caused regression on NXP platforms

- 8ddce13ae696 was merged in v6.5-rc1 to fix that but caused regression on QCOM platforms

Did I miss something ?

I don't know how to handle this apart reverting 8ddce13ae696 and trying to find a proper fix that doesn't regress QCOM.

So, The main issue is around the real meaning of the IPI_DSI_MODE_VIDEO_NO_* flags,
Exynos DRM removed the HSA, HBP and HFP packets, Qcom DSI moves the DSI lanes
state to LP-11 during the period.

The behavior is significantly different and the naming doesn't suggest any
correct behavior.

The only solution is to find out why :
- On Qcom platforms, having the HSA, HBP and HFP periods is OK, but not on DSIM
- On DSIM, removing the HSA, HBP and HFP periods is fine
- What's the exact requirement of the lt9611 bridge concerning those periods

Neil