[PATCH v2] drm/panel: Set max rate for Ilitek ILI9881C

[Andrzej Hajda]

On 20.11.2018 13:45, Linus Walleij wrote:
> On Tue, Nov 20, 2018 at 12:02 PM Andrzej Hajda <a.hajda at samsung.com> wrote:
>
>>> +     /*
>>> +      * The datasheet (table 39) specifies "limited clock channel speed"
>>> +      * for 4 lanes as 550 Mbps for RGB888. As this is 4 bits at the time,
>>> +      * the maximum HS frequency is 550/4 = 137.5 MHz.
>>> +      */
>> Are you sure it is OK?
>>
>> 550Mbps is per lane, so since DSI is DoubleDataRate clock should be the
>> half of the speed - 275MHz.
> That is a bit confusing indeed.
>
>> Anyway more I think about it more doubts I have. hs_rate can be helpful
>> for command mode panels - panel refresh rate (provided by timings)
>> imposes only lower limit on the speed, max hs rate will impose upper limit.
> Yeah. I added it for the command mode-only Samsung s6d16d0.
>
>> In case of video mode, timings determines exactly value of the clock, so
>> max hs rate is unnecessary, am I right?
> As far as I can tell, yes, provided that the video mode timings are
> <= HS rate. (Here it would mainly be a safeguard/sanity check.)
>
> I wonder what happens with a video mode panel in LP mode
> really. It would be good to get feedback from some experts
> here.
>
> I assume that in practice in LP mode the frequency is set
> down and with the rest of the frame properties remaining
> the same, the frame rate will go down (as expected).


Nope, LP is used mainly to send control (ie. no data) commands, so there
are no special video timings for it.

In some rare cases it could be used to send video data - partial
updates, very low-res/low-rate screens.

LP is used mainly to configuration (for example tc358764 DSI-LVDS
bridge), or to read from device - since HS is only unidirectional.

It can be used only if there is no HS transmission - display off, or in
vblank. In latter case DSI controller has often some logic preventing
sending LP commands in non-vblank periods.


>
> So the relationship between mode clock, HS and LP in
> practice is a bit of mystery to me. I haven't seen anything
> in the upstream kernel making it possible for panels to
> enter LP mode, so I assume that is only done by vendor
> out-of-tree code as of today.


Every read command from panel is in LP for example.


>
> It would be great if some people who worked with this in
> practice could provide some input.
>
> My naive idea is that the panel should provide two
> "resolutions", one for HS and one for LP, and they only
> really differ in frame rate. So then userspace choose the
> resolution with the lower frame rate to save power.


As I said earlier, LP is for non-video transmissions.


Regards

Andrzej


>
> Yours,
> Linus Walleij
>
>