RFC: DSI panel lane frequency

[Andrzej Hajda]

On 16.10.2018 16:06, Linus Walleij wrote:
> Hi folks,
>
> I just randomly add some people that committed code to the
> DSI core so I can get some reasonable feedback.
>
> I started looking at some DSI drivers I'm adding and it seems
> this platform (Ux500 MCDE) can control the bus frequency
> of the DSI interface. It can be controlled independently for
> command and video mode, and there is an LP (low power)
> frequency and a HS (high speed) frequency for the lane.
>
> The MIPI specification seems to say "The maximum Lane
> frequency shall be documented by the DSI device manufacturer."
> Then it goes on to specify tolerance for the HS and LP
> frequency.
>
> So apparently those are not standard frequencies.
>
> I was thinking to add something like this:
>
> diff --git a/include/drm/drm_mipi_dsi.h b/include/drm/drm_mipi_dsi.h
> index 4fef19064b0f..9c78eb78b027 100644
> --- a/include/drm/drm_mipi_dsi.h
> +++ b/include/drm/drm_mipi_dsi.h
> @@ -168,6 +168,8 @@ struct mipi_dsi_device_info {
>   * @format: pixel format for video mode
>   * @lanes: number of active data lanes
>   * @mode_flags: DSI operation mode related flags
> + * @hs_frequency: Maximum frequency for high speed operation
> + * @lp_frequency: Maximum frequency for low power operation

Yes, it is missing part, please add units (Hz ?) and  some optional
propositions:
1. maybe hs_speed instead of hs_frequency - shorter.
2. s/operation/mode/

>   */
>  struct mipi_dsi_device {
>         struct mipi_dsi_host *host;
> @@ -178,6 +180,8 @@ struct mipi_dsi_device {
>         unsigned int lanes;
>         enum mipi_dsi_pixel_format format;
>         unsigned long mode_flags;
> +       unsigned long hs_frequency;
> +       unsigned long lp_frequency;

I hope in case of Hz it will not reach MAX_ULONG.

>  };
>
> Is this what we should do to make DSI panels expose their max
> LS/HS frequency? (If zero, we could assume some default
> I guess.)

I guess one can assume sth based on display timings, but I am not sure
if it is proper way, temporary hosts should work as before if 0 is
specified, but new devices should specify HS speed explicitly IMO.
Regarding LP as I remember spec says sth about some fraction of HS, but
I wouldn't be against requiring it as well.

Regards
Andrzej

>
> Yours,
> Linus Walleij
>
>