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SubjectRe: [PATCH v6 2/3] spi: sun4i: Fix clock calculations to be predictable and never exceed the requested rate
From
Hi,

2016-01-17 19:51 GMT+01:00 Maxime Ripard <maxime.ripard@free-electrons.com>:
> On Sun, Jan 10, 2016 at 10:11:11PM +0100, Marcus Weseloh wrote:
>> >> >> - /* Ensure that we have a parent clock fast enough */
>> >> >> + /*
>> >> >> + * Ensure that the parent clock is set to twice the max speed
>> >> >> + * of the spi device (possibly rounded up by the clk driver)
>> >> >> + */
>> >> >> mclk_rate = clk_get_rate(sspi->mclk);
>> >> >> - if (mclk_rate < (2 * tfr->speed_hz)) {
>> >> >> - clk_set_rate(sspi->mclk, 2 * tfr->speed_hz);
>> >> >> + if (spi->max_speed_hz != sspi->cur_max_speed ||
>> >> >> + mclk_rate != sspi->cur_mclk) {
>> >> >
>> >> > Do you need to cache the values? As far as I'm aware, you end up doing
>> >> > the computation all the time anyway.
>> >>
>> >> By caching the values we optimize the case when a single SPI slave
>> >> device (or even multiple slave devices with the same max_speed) are
>> >> used multiple times in a row. In that case, we're saving two calls:
>> >> clk_set_rate and clk_get_rate. I was unsure about how expensive the
>> >> clk_* calls were, so I thought it would be safer use caching. But
>> >> maybe it's not worth the extra code?
>> >
>> > Unless you can point that there's a significant performance
>> > difference, I'm not sure it's worth it.
>>
>> I did actually notice a significant transfer latency when a new mod0
>> clock frequency is set, probably due to the __delay in
>> drivers/clk/sunxi/clk-factors.c (line 147). So my feeling is that the
>> caching is worth it... at least for the case when there are two slave
>> devices with different transfer speeds accessing the same SPI module.
>
> I'm not sure we even need that delay in the first place, at least not
> for all the clocks using factor.
>
> AFAIK, the only case where it's useful is for PLL, and all of them
> have a bit you can busy-wait on that will let you know when the clock
> has stabilized.

OK, I didn't know that. Does that mean you would like me to drop the
caching from this patch and prepare a patch to remove the __delay from
clk-factors?

>> >> [...]
>> >> >> - div = mclk_rate / (2 * tfr->speed_hz);
>> >> >> - if (div <= (SUN4I_CLK_CTL_CDR2_MASK + 1)) {
>> >> >> - if (div > 0)
>> >> >> - div--;
>> >> >> -
>> >> >> + div = DIV_ROUND_UP(mclk_rate, 2 * tfr->speed_hz) - 1;
>> >> >
>> >> > Isn't it exactly the same thing as mclk_rate / (2 * tfr->speed_hz) ?
>> >>
>> >> It is quite often, but not in all cases. The plain division rounds to
>> >> the nearest integer, so it rounds down sometimes. Consider the
>> >> following case: we have a slow SPI device with a spi-max-frequency of
>> >> 50kHz. Our clock driver can't find a clock as slow as 100kHz, so it
>> >> sets mclk to 214,285Hz.
>> >>
>> >> Using the old calculation we get: 214,285 / (2 * 50,000) = 2, so div =
>> >> 1 as the old code subtracts 1 two lines further down
>> >> The new calculation results in: DIV_ROUND_UP(214,285, 2 * 50,000) =
>> >> 3, so div = 2 if we add the -1
>> >
>> > Except that you have that extra - 1 after your DIV_ROUND_UP
>> > calculation in the line you add. so you have to remove 1 from that
>> > line above, and then 1 again when we set the register, which ends up
>> > being the exact same thing, or am I missing something?
>>
>> The -1 after the DIV_ROUND_UP is already the -1 that is needed to set
>> the register. There's no need for another -1 after that (and there
>> isn't one in the code).
>
> I was probably hallucinating :) My bad.
>
> That being said, I still have a hard time figuring out what would not
> be solved simply by removing the div--, which seems to match what your
> commit log says.

I'm probably not doing a good job explaining the change. Let me try it
with a few examples. Consider the following three approaches:

A (original, unpatched version):
========================
div = mclk_rate / (2 * tfr->speed_hz);
if (div <= (SUN4I_CLK_CTL_CDR2_MASK + 1)) {
if (div > 0)
div--;
} else {
...
}

B (original version, but with div-- removed):
=================================
div = mclk_rate / (2 * tfr->speed_hz);
if (div <= (SUN4I_CLK_CTL_CDR2_MASK + 1)) {
...
} else {
...
}

C (version after this patch):
=====================
div = DIV_ROUND_UP(mclk_rate, 2 * tfr->speed_hz) - 1;
if (div <= SUN4I_CLK_CTL_CDR2_MASK) {
...
} else {
...
}

And now the following values for mclk, tfr->speed and the resulting
div and SPI_CLK

tfr->speed_hz = 50,000
mclk = 214,285
A: div = 1, SPI_CLK = 53,571(!)
B: div = 2, SPI_CLK = 35,714
C: div = 2, SPI_CLK = 35,714

tfr->speed_hz = 50,000
mclk = 200,000
A: div = 1, SPI_CLK = 50,000
B: div = 2, SPI_CLK = 33,333(!)
C: div = 1, SPI_CLK = 50,000

tfr->speed_hz = 650,000
mclk = 1,6000,000
A: div = 11, SPI_CLK = 666,667(!)
B: div = 12, SPI_CLK = 615,385
C: div = 12, SPI_CLK = 615,385

tfr->speed_hz = 1,000,000
mclk = 2,000,000
A: div = 0, SPI_CLK = 1,000,000
B: div = 1, SPI_CLK = 500,000(!)
C: div = 0, SPI_CLK = 1,000,000

I hope that makes it a little bit easier to understand the change. Of
course, the change only makes sense if you agree that the acutal SPI
transfer speed should never exceed the requested speed. Which I think
is the right approach... but maybe you think otherwise?

Thanks for taking the time to look at this so carefully!

Marcus

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