Re: [PATCH RFC 01/11] drm: bridge/dw_hdmi: clean up hdmi_set_clk_regenerator()

[Russell King - ARM Linux <linux@arm.linux.org.uk>]

On Tue, Mar 31, 2015 at 02:55:53AM -0400, Yang Kuankuan wrote:
> Hi Russell,
> 
> I'm okay with this change, and also I am preparing that collect N/CTS
> setting to an array, like this :
> 
>     struct n_cts {
>         unsigned int cts;
>         unsigned int n;
>     };
> 
>     struct tmds_n_cts {
>         unsigned long tmds;
>         /* 1 entry each for 32KHz, 44.1KHz, and 48KHz */
>         struct n_cts n_cts[3];
>     };
> 
>     static const struct tmds_n_cts n_cts_table[] = {
>         { 25175000, {{ 28125,  4576}, { 31250,  7007}, { 25175, 6144} } },
>     }

I think this is something which should be a common helper rather than
being specific to the driver.  I believe these are the standard values
for coherent audio/video clocks which can be found in the HDMI
specifications.  Such a helper should document that it is only for
use when the audio and video clocks are coherent.

(The HDMI spec gives alternative N values for use with auto-CTS value
generation for non-coherent clocks.)

I'd also prefer the lookup to use the same units as the drm_display_mode
video clock specification, which is kHz, so we can eventually kill the
needless *1000 in dw_hdmi.

One of the issues with using a common helper is that the common helper
would include the 1.001 clocks, which are allegedly unsupported by the
FSL iMX6 implementation, and, if proven that they don't work, we would
need some way to disable audio for those devices.

> But I am confused by the "hdmi->ratio", this variable was modify to
> 100 in bind funciton, then nowhere would change it again. In this
> case "hdmi->ratio" seems an unused variable, can we remove it ?

I guess the FSL sources should be checked to find out what the intended
use for that was, and we then need to decide whether to keep it (and
implement it) or remove it.

-- 
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