Re: [PATCH v5] drm/pl111: Initial drm/kms driver for pl111

[Neil Armstrong <narmstrong@baylibre.com>]

On 04/12/2017 09:40 AM, Linus Walleij wrote:
> On Wed, Apr 12, 2017 at 12:13 AM, Eric Anholt <eric@anholt.net> wrote:
> 
>> Oh, one last thing I think we need to figure out: I'm using TIM2_CLKSEL,
>> which seems to be necessary on this platform.  My understanding is that
>> this means that the pixel clock is divided from clcdclk instead of
>> apb_pclk.  Do you agree?
> 
> Yes the pixed clock is always derived from clcdclk.
> 
> In most older ARM reference designs this is a VCO so that
> is why there is a clk_set_rate() on this in the fbdev code.
> (On some platforms that even has no effect I guess.)
> 
>> The fbdev driver is using
>> clk_get(&fb->dev->dev, NULL) and not TIM2_CLKSEL, which I'm surprised by
>> because I would have thought that would give us the first clock from the
>> DT node (also clcdclk).
> 
> So that thing is a 1-bit line that can select one of two clocks
> to be muxed into the PL111/CLCD.
> 
> I guess that up until now all platforms just left that line dangling in
> the silicon. Congratulations, you came here first ;)
> 
> Though when I look at the Nomadik it seems that it might be muxing
> the clock between 48 and 72 MHz, and I've been using 48MHz
> all along ooopsie.
> 
> The current assumption in the bindings is that we have only
> one clock and TIM2_CLKSEL is N/A.
> 
> If we want proper clcdclk handling with CLKSEL you should
> probably add some code to implement a real mux clock for
> this using <linux/clock-provider.h> and drivers/clk/clk-mux.c
> with select COMMON_CLK
> so that the driver still only sees clcdclk but that in turn is a
> mux that can select one of two sources and will react to
> the clk_set_rate() call by selecting the clock which is
> closest in frequency to what you want.
> 
> This needs a small patch to alter the bindings too I guess.
> A small clock node inside the CLCD, just like PCI bridges have
> irqchips inside them etc:
> 
> clcd@10120000 {
>         compatible = "arm,pl110", "arm,primecell";
>         reg = <0x10120000 0x1000>;
>         (...)
>         clocks = <&clcdclk>, <&foo>;
>         clock-names = "clcdclk", "apb_pclk";
> 
>         clcdclk: clock-controller@0 {
>                 compatible = "arm,pl11x-clock-mux";
>                 clocks = <&source_a>, <&source_b>;
>         };
> };
> 
> This can be set up easily in the OF probe path since that
> is what we're doing: just look for this subnode, if it is there
> create the clock controller.
> 
> I do not think the clk maintainers would mind a small mux
> clock controller inside the CLCD driver to handle this mux
> if we need it.

Hi Linus,

Indeed it's the way of handling this use case, but no need to add
a clk node here, you can copy what we did in pwm-meson, meson-gx-mmc.c,
or dwmac-meson8b.c (we went further by also adding clk dividers).

In the probe code, simply add a clock mux provider with the two parents
clock names (these can and should be platform specific) and only call
a clk_set_parent() with the clock specified in the node clocks cell.

> 
> It would *maybe* also be possible to add a second "clcdclk2"
> to the block and make an educated decision on which clock
> to use in the driver but that is not as elegant as using the
> clock framework mux clock I think.
> 
> Yours,
> Linus Walleij

Neil