linux-kernel - Re: [PATCH v5 1/3] clk: qcom: Allow constant ratio freq tables for rcg

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Message-Id: <20191108064436.E868F21882@mail.kernel.org>
Date:   Thu, 07 Nov 2019 22:44:36 -0800
From:   Stephen Boyd <sboyd@...nel.org>
To:     Jeffrey Hugo <jeffrey.l.hugo@...il.com>
Cc:     Bjorn Andersson <bjorn.andersson@...aro.org>,
        Michael Turquette <mturquette@...libre.com>,
        Andy Gross <agross@...nel.org>,
        Marc Gonzalez <marc.w.gonzalez@...e.fr>,
        MSM <linux-arm-msm@...r.kernel.org>, linux-clk@...r.kernel.org,
        lkml <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v5 1/3] clk: qcom: Allow constant ratio freq tables for rcg

Quoting Jeffrey Hugo (2019-11-07 14:12:09)
> On Thu, Nov 7, 2019 at 2:43 PM Stephen Boyd <sboyd@...nel.org> wrote:
> >
> > Quoting Jeffrey Hugo (2019-10-31 11:57:15)
> > > Some RCGs (the gfx_3d_src_clk in msm8998 for example) are basically just
> > > some constant ratio from the input across the entire frequency range.  It
> > > would be great if we could specify the frequency table as a single entry
> > > constant ratio instead of a long list, ie:
> > >
> > >         { .src = P_GPUPLL0_OUT_EVEN, .pre_div = 3 },
> > >         { }
> > >
> > > So, lets support that.
> > >
> > > We need to fix a corner case in qcom_find_freq() where if the freq table
> > > is non-null, but has no frequencies, we end up returning an "entry" before
> > > the table array, which is bad.  Then, we need ignore the freq from the
> > > table, and instead base everything on the requested freq.
> > >
> > > Suggested-by: Stephen Boyd <sboyd@...nel.org>
> > > Signed-off-by: Jeffrey Hugo <jeffrey.l.hugo@...il.com>
> > > ---
> >
> > Applied to clk-next and fixed the space thing. I guess ceil/floor
> > rounding isn't a problem?
> >
> 
> Thanks for fixing the nit.
> 
> Hmm.  Looking back at it, floor is only used with the rcg_floor_ops.
> Right now, you can't use a constant ratio table with rcg_floor_ops -
> looks like you'd probably hit a null pointer dereference.  I'm having
> trouble seeing how the floor operation would work with this constant
> ratio idea in a way that would be different than the normal rcg_ops.
> I think I would say that either you have a good reason for using the
> constant ratio table, in which case you should be using the normal
> rcg_ops, or you have a good reason for using floor which is then
> incompatible with a constant ratio table.  If you think that the
> constant ratio table concept should be applied to floor ops, can you
> please detail what you expect the behavior to be?

I don't think floor ops make sense. I just wanted to make sure that the
floor and ceiling stuff in here isn't going to cause problems. Looking
again after reading your response I think we're going to be fine.