[PATCH v4 61/80] drm/omap: dsi: send nop instead of page & column

[Tomi Valkeinen]

On 30/11/2020 11:58, Laurent Pinchart wrote:
> Hi Tomi,
> 
> Thank you for the patch.
> 
> On Tue, Nov 24, 2020 at 02:45:19PM +0200, Tomi Valkeinen wrote:
>> The OMAP DSI command mode panel driver used to send page & column
>> address before each frame update, and this code was moved into the DSI
>> host driver when converting it to the DRM bridge model.
>>
>> However, it's not really required to send the page & column address
>> before each frame. It's also something that doesn't really belong to the
>> DSI host driver, so we should drop the code.
>>
>> That said, frame updates break if we don't send _something_ between the
>> frames. A NOP command does the trick.
>>
>> It is not clear if this behavior is as expected from a DSI command mode
>> frame transfer, or is it a feature/issue with OMAP DSI driver, or a
>> feature/issue in the command mode panel used. So this probably needs to
>> be revisited later.
> 
> This is important information, could you capture it in a comment in the
> code ?

I think this is related to the following (from DSI spec):

"To enable PHY synchronization the host processor should  periodically end HS transmission and drive
the Data Lanes to the LP state. This transition should take place at least once per frame"

And in OMAP TRM:

"Special care must be taken in the case of the last line of the frame. The LPS transition is
required when the link is in HS mode for the whole frame."

I'm not 100% sure about that, but I'll change the commit description and add a comment to reflect
the above text.

 Tomi

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