[PATCH V2 0/3] DSI host and peripheral initialisation ordering

[Frieder Schrempf]

Am 10.06.22 um 09:52 schrieb Lucas Stach:
> Hi,
> 
> Am Mittwoch, dem 11.05.2022 um 16:58 +0200 schrieb Marek Szyprowski:
>> Hi Dave,
>>
>> On 05.04.2022 13:43, Dave Stevenson wrote:
>>> On Fri, 18 Mar 2022 at 12:25, Dave Stevenson
>>> <dave.stevenson at raspberrypi.com>  wrote:
>>>> On Fri, 4 Mar 2022 at 15:18, Dave Stevenson
>>>> <dave.stevenson at raspberrypi.com>  wrote:
>>>>> Hi All
>>>> A gentle ping on this series. Any comments on the approach?
>>>> Thanks.
>>> I realise the merge window has just closed and therefore folks have
>>> been busy, but no responses on this after a month?
>>>
>>> Do I give up and submit a patch to document that DSI is broken and no one cares?
>>
>> Thanks for pointing this patchset in the 'drm: bridge: Add Samsung MIPI 
>> DSIM bridge' thread, otherwise I would miss it since I'm not involved 
>> much in the DRM development.
>>
>> This resolves most of the issues in the Exynos DSI and its recent 
>> conversion to the drm bridge framework. I've added the needed 
>> prepare_upstream_first flags to the panels and everything works fine 
>> without the bridge chain order hacks.
>>
>> Feel free to add:
>>
>> Tested-by: Marek Szyprowski <m.szyprowski at samsung.com>
>>
>>
>> The only remaining thing to resolve is the moment of enabling DSI host. 
>> The proper sequence is:
>>
>> 1. host power on, 2. device power on, 3. host init, 4. device init, 5. 
>> video enable.
>>
>> #1 is done in dsi's pre_enable, #2 is done in panel's prepare. #3 was so 
>> far done in the first host transfer call, which usually happens in 
>> panel's prepare, then the #4 happens. Then video enable is done in the 
>> enable callbacks.
>>
>> Jagan wants to move it to the dsi host pre_enable() to let it work with 
>> DSI bridges controlled over different interfaces 
>> (https://lore.kernel.org/all/20220504114021.33265-6-jagan@amarulasolutions.com/ 
>> ). This however fails on Exynos with DSI panels, because when dsi's 
>> pre_enable is called, the dsi device is not yet powered. I've discussed 
>> this with Andrzej Hajda and we came to the conclusion that this can be 
>> resolved by adding the init() callback to the struct mipi_dsi_host_ops. 
>> Then DSI client (next bridge or panel) would call it after powering self 
>> on, but before sending any DSI commands in its pre_enable/prepare functions.
>>
>> I've prepared a prototype of such solution. This approach finally 
>> resolved all the initialization issues! The bridge chain finally matches 
>> the hardware, no hack are needed, and everything is controlled by the 
>> DRM core. This prototype also includes the Jagan's patches, which add 
>> IMX support to Samsung DSIM. If one is interested, here is my git repo 
>> with all the PoC patches:
>>
>> https://github.com/mszyprow/linux/tree/v5.18-next-20220511-dsi-rework
> 
> While this needs rework on the bridge chip side, I fear that we need
> something like that to allow the bridge to control the sequencing of
> the DSI host init. While most bridges that aren't controlled via the
> DSI channel might be fine with just initializing the host right before
> a video signal is driven, there are some that need a different
> sequencing.
> 
> The chip I'm currently looking at is a TC368767, where the datasheet
> states that the DSI lanes must be in LP-11 before the reset is
> released. While the datasheet doesn't specify what happens if that
> sequence is violated, Marek Vasut found that the chip enters a test
> mode if the lanes are not in LP-11 at that point and I can confirm this
> observation.

The SN65DSI84 also has this requirement according to the datasheet.

> Now with the TC358767 being a DSI to (e)DP converter, we need to
> release the chip from reset pretty early to establish the DP AUX
> connection to communicate with the display, in order to find out which
> video modes we can drive. As the chip is controlled via i2c in my case,
> initializing the DSI host on first DSI command transaction is out and
> doing so before the bridge pre_enable is way too late.
> 
> What I would need for this chip to work properly is an explicit call,
> like the mipi_dsi_host_init() added in the PoC above, to allow the
> bridge driver to kick the DSI host initialization before releasing the
> chip from reset state.

So to sum up on the missing parts:

1. This series needs more reviews, but is generally agreed on.
2. Jagan will integrate Marek's mipi_dsi_host_init() PoC when respinning
his series "drm: bridge: Add Samsung MIPI DSIM bridge".
3. Bridge drivers need to be adjusted to call mipi_dsi_host_init() if
required.

Did I get anything wrong here, or is there some point that I'm missing?
Jagan, do you have any plan to pick up the threads?

Thanks
Frieder