Re: [PATCH 1/3] spi/bcm63xx-hsspi: allow providing clock rate through a second clock

[Jonas Gorski <jonas.gorski-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>]

On 22 February 2017 at 19:26, Mark Brown <broonie-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org> wrote:
> On Wed, Feb 22, 2017 at 02:19:38PM +0100, Jonas Gorski wrote:
>> Instead of requiring the hsspi clock to have a rate, allow using a second
>> clock for providing the Hz rate, which is probably more correct anyway.
>
> I'm sorry but I just can't follow the logic here at all - why would we
> use "a second clock" to get the rate of a clock and why would that be
> more correct?  There's a few steps in the reasoning missing here I
> think!

The PLL rate is hardcoded in the original broadcom code (with
#if-deffery for different SoCs); when I added support for the HSSPI
controller in upstream linux, I thought "hey, let's put it as the rate
of the (gateable) HSSPI clock, then I don't need to pass it as
platform data". Part of the issue is also that bcm63xx has its own clk
code, so couldn't use clk lookups even if I wanted.

The register for the gated clock control is calld blkEnables (so might
be just power gating?), and the #define for the rate is called
HS_SPI_PLL_FREQ, which at least gives me the impression these could be
two different things/clocks. Whether it's a separate PLL from the
gated clock or not someone with actual hardware knowledge would have
to tell.

To (eventually) get rid of bcm63xx's own clk code and replace it with
a proper clock driver with lookups and stuff, it would make it much
easier to just move the PLL_FREQ into a separate fixed-rate clock, as
it is the only clock in the bcm63xx code that has a rate attached and
is gated. So I wouldn't need to create composite clocks to and could
just directly make it a simple gated-clock driver.

Hope that explanation helps.


Regards
Jonas
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