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

[neil.armstrong at linaro.org]

Hi Marek,

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 ?

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.

Neil

> 
> Esp. the "disable" part. That to me reads as "don't send HBP packet".
> 
> Where do you see that quote above in the DSI spec (which chapter and which version do you read) ?
> 
>>>> It should be replacing those periods with LP11 too.
>>>>
>>>> The traffic mode being used on this bridge is MIPI_DSI_MODE_VIDEO_SYNC_PULSE which is "Non-Burst Mode with Sync Pulses".
>>>>
>>>> As per this traffic mode in the DSI spec,
>>>>
>>>> "Normally, periods shown as HSA (Horizontal Sync Active), HBP (Horizontal Back Porch) and HFP (Horizontal Front Porch) are filled by Blanking Packets, with lengths (including packet overhead) calculated to match the period specified by the peripheral’s data sheet. Alternatively, if there is sufficient time to transition from HS to LP mode and back again, a timed interval in LP mode may substitute for a Blanking Packet, thus saving power. During HSA, HBP and HFP periods, the bus should stay in the LP-11 state."
>>>>
>>>> So we can either send the blanking packets or transition to LP state and those 3 flags are controlling exactly that during those periods for MSM driver.
>>>>
>>>> If you stop sending the blanking packets, you need to replace that gap with LP.
>>>
>>> I don't think that's what MIPI_DSI_MODE_VIDEO_NO_HBP means, the way I understand MIPI_DSI_MODE_VIDEO_NO_HBP is that it skips the HBP completely. So if you want HBP, then do not set MIPI_DSI_MODE_VIDEO_NO_HBP . And if you want LP11 during HBP, that is I think up to the controller (or maybe another new flag?).
>>>
>>
>> No, there is no need of another new flag. There are only two options as per the spec.
>>
>> In fact, as per my checking with more folks, requiring LP11 during those periods is something very rare.
>>
>> Because usually horizontal period is usually a very short period, most of the time we do not use the LP11 option and send the blanking packets instead.
>>
>> So its something very unusual for DSIM.
>>
>> That being said, I still think my previous question is important.
>>
>> 1) What is the difference between the resolution you are trying Vs what Amit is trying?
>>
>> 2) Are you both using just standard HDMI monitors?
> 
> What is a "standard HDMI monitor" ?
> I use DELL U2713HM .
> 
> [...]