Re: clock driver

[York Sun]

On 05/27/2015 11:11 PM, Michael Turquette wrote:

<snip>

> 
> Hi Andrey,
> 
> I think this is a cool problem to solve. As far as frontier devices go I
> really can't say. Other companies create similar clock generators that
> are programmed at run-time over i2c. And we already have support merged
> for Silicon Labs 5351 and 570 devices:
> 
> 	drivers/clk/clk-si5351.c
> 	drivers/clk/clk-si5351.h
> 	drivers/clk/clk-si570.c
> 
> I'm not sure that we need to group such devices into a new "class". The
> Linux common clock framework (ccf) has two classes: clock providers and
> clock consumers. We haven't needed any more classification than that
> thus far.
> 
> I took a look at your github code and it looks like you expose registers
> individually as sysfs files. That is a start, but a better abstraction
> is to consider the clock input signals that your pcie/fpga device will
> take as input. With a proper clock driver for the silabs part your
> pcie/fpga driver could hopefully just call clk_prepare_enable and
> clk_set_rate and clk_set_parent as needed on those clocks to configure
> them for consumption by the downstream fpga.
> 
> According to the data sheet[0] it looks like there are not many clock
> outputs (CLK0{A,B}, CLK1{A,B}, CLK2{A,B}, and CLK3{A,B}).
> 
> At what point do you know how the clocks should be configured? I am
> guessing that your fpga driver (e.g. the clock consumer) figures that
> out as bytestream blobs are loaded? So that seems like the right call
> site to start enabling clocks and configuring rates, instead of stuffing
> that into the silabs driver (e.g. the clock provider).

I think it works for my case. I will explore this direction.

> 
> York,
> 
> There is already a way to clock drivers to extend the debugfs interfaces
> for per-driver stuff. The Nvidia Tegra guys do this already in their
> out-of-tree test modules. Do you think such an interface might be
> helpful to you? See:
> 
> clk_debugfs_add_file in drivers/clk/clk.c
> and,
> http://lkml.kernel.org/r/<1403794853-16928-1-git-send-email-pdeschrijver@xxxxxxxxxx>
> 
> So your silabs clock generator driver could export some custom knobs
> which help out in the early phases of development.
> 
> Ideally this interface would not be a register-level interface, but an
> output signal type interface. Here is an example of the files you will
> have by default:
> 
> $ ls /sys/kernel/debug/clk/clk0a/
> clk_accuracy      clk_flags           clk_phase          clk_rate
> clk_enable_count  clk_notifier_count  clk_prepare_count
> 
> With the custom debugfs interface you might add a "clk_set_rate" file
> where user space can write to it and change the frequency of that clock
> signal. You can do the same to change mux settings and clock gates as
> well.
> 
> A userspace tool might even be able to take the clockbuilder data to do
> this, if someone is willing to write it.
> 
> [0] https://www.silabs.com/Support%20Documents/TechnicalDocs/Si5338.pdf

Thanks for the suggestion. I think I can limit the features of this clock chip
and fit it into CCF. Earlier I thought too much about exposing all features for
general use, which shouldn't be the case for the clocks. I will see if I can
remove those features or reserve them for debug use.

York



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