Re: [RFC PATCH] CLK: Allow parent clock and rate to be configured in DT.

[Tomasz Figa <tomasz.figa@gmail.com>]

CCing Sylwester and Mike (correctly)

On Saturday 06 of April 2013 15:21:19 Tomasz Figa wrote:
> Hi,
> 
> [RANT]
> 
> I tend to disagree about this whole hype about device tree usage for
> other things than pure hardware description. I don't think device tree
> should be a way to force some kind of new world order, but rather a
> more convenient and more maintainable (than board files) way of support
> hardware platforms in Linux kernel.
> 
> On Friday 05 of April 2013 20:07:09 Matt Sealey wrote:
> > On Thu, Apr 4, 2013 at 6:08 PM, Fabio Estevam <festevam@gmail.com>
> 
> wrote:
> > > This could be useful for removing the imx6q_sabrelite_cko1_setup()
> > > function from arch/arm/mach-imx/mach-imx6q.c.
> > > 
> > > I would like to add audio support for another board and would like
> > > to
> > > avoid to do the same as imx6q_sabrelite_cko1_setup()  for setting up
> > > the CLKO, if possible.
> > 
> > You know, we have the same problem on one of our designs here (CLKO is
> > used on MX53 for audio codec and camera device, shared) - the current
> > solution is hack it into platform support in a BSP kernel.
> > 
> > If we move to device tree, we know and you know the solution already;
> > all this clock setup HAS to be done in the bootloader.
> 
> Assuming that you can change the bootloader... Isn't bootloader this
> piece of code that you shouldn't touch if it's working (especially in
> production environments)?
> 
> > The device tree really, really is only a way to describe the
> > configuration as it exists or to describe alternatives - for instance,
> > a clock with 10 parents may be described as having 10 parents, and the
> > binding (in complicated cases) or driver (if it is simple as a value
> > from 0 to 7 and the field width is 3 bits for example) will determine
> > how that alternative can be selected (and by consequence, what the
> > current setting is).
> > 
> > The device tree concept is NOT a place to dump configuration items for
> > your board as hardcoded values to try and force something you could
> > have done elsewhere. On i.MX53 you cannot boot a kernel verbatim - you
> > at least have to run through the Boot ROM and supply a DCD table or
> > plugins first. This is where you figure things like this out.
> 
> Why not? For Linux and most of ARM-based platforms device tree is just a
> replacement for board files. They are not going to be used with any
> other imaginary kernel supporting device tree. On many of them even the
> device tree blob will be distributed along with kernel image, if not
> simply appended to the zImage.
> 
> I think it should be up to the board/platform maintainer whether they
> want to limit device tree on particular boards/platforms just to
> hardware description or extend it to handle everything that was
> originally handled by board files.
> 
> > In a case where you have, say, an audio codec and it has a dynamic
> > input clock that you want to change on the fly, first of all you would
> > not hardcode a frequency into a device tree, second of all there is
> > nothing stopping you from doing that right now anyway. If the clock is
> > static and fixed frequency and can only be turned on and off, there
> > are clock bindings for this already. If it is static and can never be
> > turned off, then there are clock bindings for this too, and in this
> > case the proviso is that the clock is ALREADY turned on and ALREADY
> > stable before you enter the kernel otherwise the description is by
> > it's very definition invalid.
> > 
> > If the clock frequency in hardware is not what you wanted when the
> > driver comes up, and you only have an on/off switch, it is not up to
> > the device tree binding to describe how to go about configuring the
> > system to make sure. You cannot in good conscience put a clock
> > frequency "to be set later" in the device tree along with a clock
> > phandle, simply because that means the device tree no longer describes
> > the hardware accurately - the clock phandle is a valid clock, the
> > hardware will tell you a frequency from registers in the clock driver,
> > the device tree will disagree. How do you know which one is the
> > correct value, or if the frequency in the tree is a suggestion rather
> > than a description?
> 
> Sure. This is (and has always been) something to account for when
> bringing up new platforms from the ground up.
> 
> But you can't always have control over the bootloader. What's wrong in
> letting the kernel configure the board itself? It must configure most of
> the hardware anyway, based on platform data (either located in board
> files or parsed from device tree).
> 
> Why not to make the kernel independent from the bootloader at all? Then
> the bootloader could just do some minimal initialization needed to load
> a kernel image from flash memory and launch it (+ some code to allow
> flashing of new images).
> 
> > I am totally against this (sorry Sascha..). Let's put some effort into
> > fixing the bootloaders rather than trying to use the device tree to
> > enforce the ridiculous assumption that "Linux kernel does not trust
> > the bootloader". Once the bindings and trees are out of the kernel
> > source, you're going to HAVE to trust what the bootloader passes, be
> > it pregenerated compiled blob (which needs to be written to match the
> > hardware state the bootloader finishes up at) or dynamically generated
> > at runtime during the boot process (which can describe to the bit what
> > the hardware is doing). If you are testing this on Beagle XM you can
> > fix your U-Boot easily. New boards will have to be designed *with the
> > idea of device trees and working configurations in mind* - that is a
> > fact of life, in fact this is how it should be in reality these days
> > anyway (most HW designers will do initial bringup of the board - at
> > least a minimal working configuration, in a restricted environment
> > where they can use test pads to measure clocks, voltage outputs and
> > levels of devices to ensure both production was successful and that
> > the design is in itself electrically sound. This code 99% of the time
> > ends up in the bootloader... sometimes not, when the board was
> > designed by one group and the software written by the community, but
> > in this case the community needs to learn board bringup and proper
> > behavior...)
> 
> Well, so you are basically suggesting to throw away Linux support for
> boards/platforms designed without device tree in mind and on which
> replacing the bootloader is simply infeasible or sometimes even
> impossible.
> 
> I don't think that ARM Linux is adopting device tree just to have device
> tree. There are many problems that can be solved using device tree,
> like multiplatform support, separation of board support from kernel
> code, binding consumers with providers (PHYs, GPIOs, clocks, etc.). Why
> do we have to cover all these advantages behind a curtain of new
> problems?
> 
> [/RANT]
> 
> Best regards,
> Tomasz