Subject: RFC: A better clock rounding API?

From: [email-masked] (Saravana Kannan)
Date: Tue, 17 Aug 2010 22:39:31 -0700

I'm mostly familiar with ARM.  So I will limit the discussion to ARM 
boards/machines.  But I think the points raised in this email would 
apply for most architectures.

I'm sending this to the ARM mailing list first to see if I can get a 
consensus within the ARM community before I propose this to the wider 
audience in LKML.

The meaning of clk_round_rate() is very ambiguous since the nature of 
the rounding is architecture and platform specific.  In the case of ARM, 
it's up to each ARM mach's clock driver to determine how it wants to 
round the rate.

To quote Russel King from an email about a month ago:
"clk_round_rate() returns the clock rate which will be set if you ask
clk_set_rate() to set that rate.  It provides a way to query from the 
implementation exactly what rate you'll get if you use clk_set_rate() 
with that same argument."

So when someone writes a device driver for a device that's external to 
the SoC or is integrated into more than one SoC, they currently have the 
following options to deal with clock rates differences:

1. Use clk_round_rate() to get clock rates and test their driver on the 
one/few board(s) they have at hand and hope it works on boards using 
different SoCs.

2. Add each and every needed clock rate (low power rate, high 
performance rate, handshake rate, etc) as fields to their platform data 
and have it populated in every board file that uses the device.

3. Do a search of the frequency space of the clock by making several 
clk_round_rate() calls at various intervals between their minimum and 
maximum acceptable rates and hope to find one of the supported rates. 
If clk_round_rate() does a +/- N percentage rounding and the interval is 
larger, even this searching might not find an existing rate that's 
supported between the driver's min and max acceptable rates.

IMHO, each of these options have short comings that could be alleviated 
by adding a more definitive "rounding" API.  Also, considering that it's 
the consumer of each clock that knows best what amount of rounding and 
in which direction is acceptable, IMHO the current approach of hiding 
the rounding inside the clock drivers seems counter intuitive.

I would like to propose the addition of either:

long clk_find_rate(struct clk *clk,
			unsigned long min_rate,
			long max_rate);

or

long clk_find_rate(struct clk *clk,
			unsigned long start_rate,
			long end_rate);

The advantage of the 2nd suggestion is that it allows a driver to 
specify which end of a frequency range it prefers.  But I'm not sure how 
important of an advantage that is.  So, proposing both and having the 
community decide which one, if any, is acceptable.

If the clk_find_rate() API is available, the driver developer wouldn't 
have to worry about figuring out a way for the clk_set_rate() to work on 
different platforms/machs/SoCs. If a platform/mach/SoC can provide a 
clock rate that's acceptable to their hardware and software 
requirements, then they can be assured to find it without having to jump 
through hoops or having a driver not work when it could have.

Does the addition of one of the suggested APIs sound reasonable? If not, 
can someone explain what the right/better solution is? If the addition 
of a new API is reasonable, what's the community preference between the 
two suggestion? If I submit a patch that will add one of the APIs is it 
likely to be accepted?

Thanks for your time.

-Sarav

-- 
Sent by an employee of the Qualcomm Innovation Center, Inc.
The Qualcomm Innovation Center, Inc. is a member of the Code Aurora Forum.