[U-Boot] [PATCH v2 00/10] clk: imx: Add i.MX6 CLK support

[Lukasz Majewski]

Hi Jagan,

> On Thu, Apr 4, 2019 at 3:31 PM Lukasz Majewski <lukma at denx.de> wrote:
> >
> > On Thu, 4 Apr 2019 14:56:36 +0530
> > Jagan Teki <jagan at amarulasolutions.com> wrote:
> >  
> > > On Thu, Apr 4, 2019 at 2:31 PM Lukasz Majewski <lukma at denx.de>
> > > wrote:  
> > > >
> > > > On Tue,  2 Apr 2019 16:58:33 +0530
> > > > Jagan Teki <jagan at amarulasolutions.com> wrote:
> > > >  
> > > > > This is revised version of previous i.MX6 clock management
> > > > > [1].
> > > > >
> > > > > The main difference between previous version is
> > > > > - Group the i.MX6 ccm clocks into gates and tree instead of
> > > > > handling the clocks in simple way using case statement.
> > > > > - use gate clocks for enable/disable management.
> > > > > - use tree clocks for get/set rate or parent traverse
> > > > > management.
> > > > > - parent clock handling via clock type.
> > > > > - traverse the parent clock using recursive functionlaity.
> > > > >
> > > > > The main motive behind this tree framework is to make the
> > > > > clock tree management simple and useful for U-Boot
> > > > > requirements instead of garbing Linux clock management code.
> > > > >
> > > > > We are trying to manage the Allwinner clocks with similar
> > > > > kind, so having this would really help i.MX6 as well.
> > > > >
> > > > > Added simple names for clock macros, but will update it in
> > > > > future version.
> > > > >
> > > > > I have skipped ENET clocks from previous series, will add it
> > > > > in future patches.
> > > > >
> > > > > Changes for v2:
> > > > > - changed framework patches.
> > > > > - add support for imx6qdl and imx6ul boards
> > > > > - add clock gates, tree.
> > > > >
> > > > > [1] https://patchwork.ozlabs.org/cover/950964/
> > > > >
> > > > > Any inputs?  
> > > >
> > > > Hmm.... It looks like we are doing some development in parallel.
> > > >
> > > > Please look into following commit [1]:
> > > > https://patchwork.ozlabs.org/patch/1034051/
> > > >
> > > > It ports from Linux 5.0 the CCF framework for iMX6Q, which IMHO
> > > > in the long term is a better approach.
> > > > The code is kept simple and resembles the code from Barebox.
> > > >
> > > > Please correct me if I'm wrong, but the code from your work is
> > > > not modeling muxes, gates and other components from Linux CCF.  
> > >
> > > The U-Boot implementation of CLK would require as minimal and
> > > simple as possible due to requirement of U-Boot itself. Hope you
> > > agree this point?  
> >
> > Now i.MX6 is using clock.c CLK implementation. If we decide to
> > replace it - we shall do it in a way, which would allow us to follow
> > Linux kernel. (the barebox implementation is a stripped CCF from
> > Linux, the same is in patch [1]).
> >  
> > > if yes having CCF stack code to handle all clock with
> > > respective separate drivers management is may not require as of
> > > now, IMHO.  
> >
> > I do have a gut feeling, that we will end up with the need to have
> > the CCF framework ported anyway. As for example imx7/8 can re-use
> > muxes, gates code.  
> 
> As per my experience the main the over-ahead to handle clocks in
> U-Boot if we go with separate clock drivers is for Video and Ethernet
> peripherals. these are key IP's which use more clocks from U-Boot
> point-of-view, others can be handle pretty straight-forward unless if
> they don't have too much tree chain.
> 
> On this series, the tree management is already supported ENET in
> i.MX6, and Allwinner platforms.
> 
> As of now, I'm thinking I can handle reset of the clocks with similar
> way.

But this code also supports ENET and ESDHCI clocks on i.MX6Q (as
supporting those was the motivator for this work).

One important thing to be aware of - the problem with SPL's footprint.
The implementation with clock.c is small and simple, but doesn't scale
well.

> 
> >
> > However, those are only my "feelings" after a glimpse look - I will
> > look into your code more thoroughly and provide feedback.  
> 
> Please have a look, if possible check even the code size by adding
> USDHC clocks.

Yes, code size (especially in SPL) is an _important_ factor here.

> 
> >  
> > >
> > > This series is using recursive calls for handling parenting stuff
> > > to handle get or set rates, which is fine for handling clock tree
> > > management as far as U-Boot point-of-view. We have faced similar
> > > situation as I explained in commit message about Allwinner clocks
> > > [2] and we ended up going this way.  
> >
> > I'm not Allwinner expert - but if I may ask - how far away is this
> > implementation from mainline Linux kernel?
> >
> > How difficult is it to port the new code (or update it)?  
> 
> Allwinner clocks also has similar gates, muxs, and with other platform
> stuff which has too much scope in Linux to use CCM.

For example the barebox managed to get subset of Linux CCF ported,
without loosing the CCF similarity.


Important factors/requirements for the i.MX clock code:

1. Easy maintenance in long-term

2. Reusing the code in SPL (with a very important factor of
_code_size_).

3. Reuse the code for other i.MX SoCs (imx7, imx8)

4. Effort needed to use DM with this code 



Best regards,

Lukasz Majewski

--

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