Re: [PATCH 4/4] drm/rcar-du: dsi: Implement DSI command support

[Marek Vasut <marek.vasut@mailbox.org>]

On 8/18/25 9:20 AM, Tomi Valkeinen wrote:

Hello Tomi,

>>> If yes, then it might take
>>> much longer until the command can be transferred.
>>
>> Do you know the upper limit , is that one or two frame times ?
> 
> If using DSI video mode and the stream is on, the DSI TX has to
> interleave the commands either to the line blanking or frame blanking.
> Usually this is configurable in the DSI TX (if the chip can do
> interleaving).
> 
> A read command will be the most difficult to interleave, as it takes
> more time with the BTA and reply.
> 
> I think the worst case is one frame delay (next vblank).

I set the timeouts to 50ms, which at 60 Hz refresh should be a bit over 
three frame delays, which should be safe.

>>> If not maybe the
>>> function should return an error if the video stream is enabled.
>>
>> I haven't seen any drivers special casing that, I'd prefer to increase
>> the timeouts. For V3, I'll update the timeout to 50ms , which is about 3
>> frame times.
> 
> If the docs have no word about interleaving the commands and there are
> no related registers, I would guess that it's not supported. If this
> can't be tested, I suggest just returning an error if a command is sent
> while the video is on.
> 
> You should be able to test this, though. E.g. just add a debugfs/sysfs
> file to the panel or dsi tx driver, which does a DSI command, possibly a
> read. See what happens when the video is enabled.

I patched the ili9881c panel and triggered DCS read of its ID via sysfs 
attribute , I could always get valid ID, so I think we are safe here.

[...]

>>>> +    /* Wait a bit between commands */
>>>> +    usleep_range(1000, 2000);
>>>
>>> Why wait and wait a bit between what?
>> The consecutive command transmission was unreliable unless there was a
>> slight delay between the consecutive commands. Hence this delay.
> 
> This sounds like there's a bug in the driver, or the TX or RX hardware.
> Please document the sleep clearly in the comment.
Done.