[PATCH v4 3/3] ARM: imx6: Fix procedure to switch the parent of LDB_DI_CLK

[p.zabel@pengutronix.de (Philipp Zabel)]

Hi Fabio,

Am Montag, den 28.03.2016, 16:33 -0300 schrieb Fabio Estevam:
> Hi Akshay,
> 
> On Mon, Mar 28, 2016 at 4:26 PM, Akshay Bhat <akshay.bhat@timesys.com> wrote:
> 
> > The 2 discrepancies I see with this patch with regards to the sequence from
> > NXP 3.10 kernel are:
> >
> > 1. NXP 3.10 kernel switches clock source as follows:
> > current_parent -> 7 -> 4 -> new_parent
> 
> Yes, we need to follow this requirement.

Thanks. Does anybody know why?
(Does this work for current_parent == 4 and new_parent != 0)?

> > Ref:
> > +       /*
> > +        * Set the ldb_di0_clk and ldb_di1_clk to 111b.
> > +        */
> > +       reg = readl_relaxed(ccm_base + 0x2c);
> > +       reg |= ((7 << 9) | (7 << 12));
> > +       writel_relaxed(reg, ccm_base + 0x2c);
> > +
> > +       /*
> > +        * Set the ldb_di0_clk and ldb_di1_clk to 100b.
> > +        */
> > +       reg = readl_relaxed(ccm_base + 0x2c);
> > +       reg &= ~((7 << 9) | (7 << 12));
> > +       reg |= ((4 << 9) | (4 << 12));
> > +       writel_relaxed(reg, ccm_base + 0x2c);
> > +
> > +       /*
> > +        * Perform the LDB parent clock switch.
> > +        */
> > +       clk_set_parent(clk[ldb_di0_sel], clk[new_parent]);
> > +       clk_set_parent(clk[ldb_di1_sel], clk[new_parent]);
> >
> >
> > If I am reading this patch correctly, then this patch does:
> > current_parent -> (current_parent | 4) -> (new_parent | 4) -> new_parent

Yes. Switch to bottom mux, manipulate 4-port mux, and switch back to top
mux again was the intention.

> > Ref:
> > +       /* First switch to the bottom mux */
> > +       sel[i][1] = sel[i][0] | 4;
> > +
> > +       /* Then configure the top mux before switching back to it */
> > +       sel[i][2] = sel[i][3] | 4;
> > +
> > +       pr_debug("ccm: switching ldb_di%d_sel: %d->%d->%d->%d\n", i,
> > +                        sel[i][0], sel[i][1], sel[i][2], sel[i][3]);
> >
> >
> > 2. The NXP 3.10 kernel does not have any restriction on what the new parent
> > is.
> 
> That's correct. There is no restriction on what the new parent will be.

Is this also true for starting values other than 3?

best regards
Philipp