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

[Marcus Weseloh]

Hi,

2016-01-10 19:14 GMT+01:00 Maxime Ripard <maxime.ripard at free-electrons.com>:
> On Mon, Dec 28, 2015 at 12:29:16AM +0100, Marcus Weseloh wrote:
>> 2015-12-27 22:09 GMT+01:00 Maxime Ripard <maxime.ripard at free-electrons.com>:
>> > On Sat, Dec 26, 2015 at 04:53:05PM +0100, Marcus Weseloh wrote:

>> >> This patch fixes multiple problems with the current clock calculations:
[...]
>> > These 3 should probably be separate patches (and be Cc'd to stable
>>
>> Will do. But I have the feeling that at least 1. and 2. should be in
>> the same patch as they touch the same lines of code. Do you think that
>> would be ok?
>
> It can also be two subsequent patches that are part of the same serie.

OK, will prepare three separate patches in a series for the fixes.

>> And before CC'ing stable, I would love to have someone with access to
>> A10 hardware and a scope (or even a bus pirate) check that the A10 SPI
>> controller does indeed have the same "bug". I strongly think so, but
>> would sleep better if it could be confirmed.
>
> We never noticed any significant difference between the two. By now,
> if there was any, we probably would be aware of it. And if there's
> any, we can always send a subsequent patch.

That's good to know and makes life much easier, thanks!

>> >> -     /* 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.

>> [...]
>> >> -     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).

Cheers,

  Marcus