linux-kernel - RE: [PATCH v2 1/2] clk: divider: prepare for minimum divider

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Date:	Thu, 24 Jan 2013 11:29:15 +0000
From:	"Mohammed, Afzal" <afzal@...com>
To:	Mike Turquette <mturquette@...aro.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"linux-omap@...r.kernel.org" <linux-omap@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
CC:	Stephen Boyd <sboyd@...eaurora.org>
Subject: RE: [PATCH v2 1/2] clk: divider: prepare for minimum divider

Hi Mike,

On Thu, Jan 24, 2013 at 03:10:53, Mike Turquette wrote:
> Quoting Afzal Mohammed (2013-01-23 03:38:52)

> > Some of clocks can have a limit on minimum divider value that can be
> > programmed, prepare for such a support.

> > Add a new field min_div for the basic divider clock and a new dynamic
> > clock divider registration function where minimum divider value can
> > be specified. Keep behaviour of existing divider clock registration
> > functions, static initialization helpers as was earlier.

> My first question is whether the minimum divider you plan to use is an
> actual constraint of the hardware (e.g. the clock controller ip only
> lets program two bits which divide by 4, 5, 6 or 7, where 4 is the
> minimum divider) or if this is a functional constraint (e.g. the clock
> hardware can divide by a lower value, but you never want that since it
> results in non-functional video/audio/whatever).  If this is more of a
> functional constraint then perhaps a new api like clk_set_min_rate makes
> more sense.

It is a functional constraint: divider has 8 bits and it can have
all possible values (0 to 255) and divider value corresponds to
value set in the 8 bits. But depending on the modes the minimum
value that can be configured (to get display working) varies.
Eg. For raster mode (which the driver is presently supporting), it
can take a minimum value of 2, while in LIDD (LCD interface display
driver) mode it can take a min value of 1.

Here min rate is not a constraint w.r.t divider in LCDC IP, but
rather min divider.

As it is the case, you prefer a clk_divider_set_min_div()?

> 
> Secondly, have you looked into using the rate-table option provided by
> the basic divider clock?  Can you explain how this is not a good fit for
> your needs?  Perhaps there are too many divisor values so the table
> would be large?

Divider values can range from 2-255 (254 possible values), so I believe
it is not a suitable candidate here (also divider to values have 1-to-1
mapping)

Regards
Afzal