Re: [PATCH v1 2/2] drm/tegra: dc: rgb: Allow changing PLLD rate on Tegra30+

[Dmitry Osipenko <digetx@gmail.com>]

06.10.2021 21:13, Thierry Reding пишет:
> On Thu, Sep 30, 2021 at 01:28:05AM +0300, Dmitry Osipenko wrote:
>> Asus Transformer TF700T is a Tegra30 tablet device which uses RGB->DSI
>> bridge that requires a precise clock rate in order to operate properly.
>> Tegra30 has a dedicated PLL for each display controller, hence the PLL
>> rate can be changed freely. Allow PLL rate changes on Tegra30+ for RGB
>> output. Configure the clock rate before display controller is enabled
>> since DC itself may be running off this PLL and it's not okay to change
>> the rate of the active PLL that doesn't support dynamic frequency
>> switching since hardware will hang.
>>
>> Tested-by: Maxim Schwalm <maxim.schwalm@gmail.com> #TF700T
>> Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
>> ---
>>  drivers/gpu/drm/tegra/dc.c  | 27 ++++++++++++--------
>>  drivers/gpu/drm/tegra/dc.h  |  1 +
>>  drivers/gpu/drm/tegra/rgb.c | 49 +++++++++++++++++++++++++++++++++++--
>>  3 files changed, 65 insertions(+), 12 deletions(-)
> 
> This seems overly complicated. I especially don't like the way how
> clocks are looked up with clk_get_sys() and then used in the comparison.
> Could this not be achieved by using assigned-clocks and friends
> properties in DT?

Assigned-clocks have nothing to do with this patch. We need to check
whether PLLD *is* already pre-assigned as the parent.

Adding properties for describing the clk parents is overly complicated,
clk_get_sys() is a much simpler solution that doesn't involve extra DT
changes.

BTW, assigned-clocks can't be used for display controller  because
controller is usually turned on during boot and reparenting of active DC
will hang machine.