linux-kernel - Re: [PATCH 2/9] clk: Introduce get_parent

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Date:   Wed, 13 Feb 2019 10:16:18 +0100
From:   Jerome Brunet <jbrunet@...libre.com>
To:     Stephen Boyd <sboyd@...nel.org>,
        Michael Turquette <mturquette@...libre.com>
Cc:     linux-kernel@...r.kernel.org, linux-clk@...r.kernel.org,
        Miquel Raynal <miquel.raynal@...tlin.com>,
        Russell King <linux@...linux.org.uk>
Subject: Re: [PATCH 2/9] clk: Introduce get_parent_hw clk op

On Tue, 2019-02-05 at 16:01 -0800, Stephen Boyd wrote:
> Quoting Jerome Brunet (2019-01-31 10:40:07)
> > On Wed, 2019-01-30 at 13:30 -0800, Stephen Boyd wrote:
> > > > With this quirk, CCF is making an assumption that might be wrong.
> > > > 
> > > > The quirk is very easy put in the get_parent() callback of the said
> > > > driver, or
> > > > even better, don't provide the callback if it should not be called.
> > > > 
> > > > I understand the need for a cautious approach. It seems I'm only one
> > > > with
> > > > that
> > > > issue right now and since I have a work around, there is no rush. But
> > > > we
> > > > must
> > > > have plan to make it right.
> > > > 
> > > > To be clear, I'm not against your new API but I don't think it should
> > > > be a
> > > > reason to keep a broken behavior the framework.
> > > > 
> > > 
> > > So do you think you can use this new clk_op and ignore the problems with
> > > the .get_parent clk op? Putting effort into fixing the .get_parent
> > > design isn't very useful from my perspective. There's more than just the
> > > problem that we don't call it when .num_parents is 1. There's the
> > > inability to return errors without doing weird things to return an index
> > > out of range and there isn't any way for us to really know if the clk is
> > > an orphan or not. If we can migrate all drivers to use the new clk op
> > > then we can fix these problems too, and deprecate and eventually remove
> > > the broken by design .get_parent clk op API.
> > 
> > Stephen, I have nothing against your new API, I'm sure it will solve many
> > issues
> > 
> > I'm also quite sure that, like round_rate() and determine_rate(),
> > migrating to
> > the new API won't happen overnight. We are likely to still see
> > get_parent()
> > for a while. I don't understand why we would keep something wrong when it
> > is
> > that easy to fix.
> > 
> > I have spent quite sometime debugging this weird behavior of CCF, I'd
> > prefer
> > if it can avoided for others.
> > 
> > Yes, fixing the case I reported does not solves all the problem you have
> > mentionned. Keeping this bug does not help either, AFAICT.
> > 
> > The fact is that get_parent() already return out of bound values on some
> > occasion, and we already have to deal with this when converting the index
> > to
> > parent clk_hw pointer. Doing it in the same way when num_parent == 1 does
> > not
> > change anything.
> > 
> > I really don't understand why you insist on keeping this special case for
> > num_parent == 1, when we know it is not coherent.
> > 
> > Considering, that I already proposed the fix, what is the effort here ?
> > If it is fixing the driver that rely this weird thing, I'd be happy to do
> > it.
> > 
> > 
> 
> Ok. I'm happy to merge your patch to always call the .get_parent clk op
> when num_parents > 0, but please fix all the drivers and analyze all the
> implementations of .get_parent to make sure that they aren't broken by
> the change in behavior. Furthermore, please add a debug/warning message
> into the code when .get_parent returns a number outside of the range of
> [0, num_parents) so that they can be converted to use .get_parent_hw
> instead.

Fair enough.

>  Ideally there wouldn't be anything returning a parent index
> outside the range of possible parents from .get_parent because this
> analysis of drivers would find those implementations and migrate them to
> .get_parent_hw instead.
> 
> In parallel, I'd like to convert all drivers to use .get_parent_hw
> instead of .get_parent and then remove the .get_parent clk op right
> away. 

Fine by me. Of course step #1 is not required if you get this is in before.
As long as things are coherent, I'm happy :)

> I'll start a sweep of the users of clk_hw_get_parent_by_index() (I
> see 50 calls in the tree right now) and see if I can convert them to
> handle errors returned from that API, probably by just continuing and
> ignoring errors. I'll start doing the same conversion for .round_rate
> and .determine_rate so that we can get rid of that duplicate clk op as
> well. Hopefully that's a mostly mechanical conversion.

This would be nice !

> 
> For now I'll move this patch to the end of this series so that it
> doesn't hold things up otherwise.

It could even be separate series ? with the migration you mentionned above ?