Re: [PATCH 07/89] clk: bcm: rpi: Allow the driver to be probed by DT

[Nicolas Saenz Julienne]

Hi Maxime,

On Mon, 2020-02-24 at 10:06 +0100, Maxime Ripard wrote:
> The current firmware clock driver for the RaspberryPi can only be probed by
> manually registering an associated platform_device.
> 
> While this works fine for cpufreq where the device gets attached a clkdev
> lookup, it would be tedious to maintain a table of all the devices using
> one of the clocks exposed by the firmware.
> 
> Since the DT on the other hand is the perfect place to store those
> associations, make the firmware clocks driver probe-able through the device
> tree so that we can represent it as a node.

I'm not convinced this is the right approach, and if we decide to go this way,
there are more changes to take into account.

For one, if we create a dt node for this driver, we'd have to delete the
platform device creation in firmware/raspberrypi.c and then we'd be even able
to bypass raspberrypi-cpufreq altogether by creating opp tables in dt. But
there are reasons we didn't go that way at the time.

We've made an effort to avoid using dt for firmware interfaces whenever
possible as, on one hand, it's arguable they don't fit device-tree's hardware
description paradigm and, on the other, the lack of flexibility they impose
once the binding is defined. VC4's firmware interfaces are not set in stone,
nor standardized like SCMI, so the more flexible we are to future changes the
better.

Another thing I'm not all that happy about it's how dynamic clock registering
is handled in patch #22 (but I'll keep it here as relevant to the discussion):

- Some of those fw managed clocks you're creating have their mmio counterpart
  being registered by clk-bcm238. IMO either register one or the other, giving
  precedence to the mmio counterpart. Note that for pllb, we deleted the
  relevant code from clk-bcm2385.

- The same way we were able to map the fw CPU clock into the clk tree
  (pllb/pllb_arm) there are no reasons we shouldn't be able to do the same for
  the VPU clocks. It's way nicer and less opaque to users (this being a
  learning platform adds to the argument).

- On top of that, having a special case for the CPU clock registration is
  nasty. Lets settle for one solution and make everyone follow it.

- I don't see what's so bad about creating clock lookups. IIUC there are only
  two clocks that need this special handling CPU & HDMI, It's manageable. You
  don't even have to mess with the consumer driver, if there was ever to be a
  dt provided mmio option to this clock.

>  drivers/clk/bcm/clk-raspberrypi.c | 11 ++++++++---
>  1 file changed, 8 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/clk/bcm/clk-raspberrypi.c b/drivers/clk/bcm/clk-
> raspberrypi.c
> index 1654fd0eedc9..94870234824c 100644
> --- a/drivers/clk/bcm/clk-raspberrypi.c
> +++ b/drivers/clk/bcm/clk-raspberrypi.c
> @@ -255,15 +255,13 @@ static int raspberrypi_clk_probe(struct platform_device
> *pdev)
>  	struct raspberrypi_clk *rpi;
>  	int ret;
>  
> -	firmware_node = of_find_compatible_node(NULL, NULL,
> -					"raspberrypi,bcm2835-firmware");
> +	firmware_node = of_parse_phandle(dev->of_node, "raspberrypi,firmware",
> 0);

There is no such phandle in the upstream device tree. Maybe this was aimed at
the downstream dt?

Regards,
Nicolas

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