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

[Marcus Weseloh]

Hi,

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:
>>
>> 1. The A10/A20 datasheet contains the formula AHB_CLK / (2^(n+1)) to
>> calculate SPI_CLK from CDR1, but this formula is wrong. The actual
>> formula - determined by analyzing the actual waveforms - is
>> AHB_CLK / (2^n).
>>
>> 2. The divisor calculations for CDR1 and CDR2 both rounded to the
>> nearest integer. This could lead to a transfer speed that is higher than
>> the requested speed. This patch changes both calculations to always
>> round down.
>>
>> 3. The mclk frequency was only ever increased, never decreased. This could
>> lead to unpredictable transfer speeds, depending on the order in which
>> transfers with different speeds where serviced by the SPI driver.
>
> 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?
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.

[...]
>> -     /* 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?

Oh, and I just noticed a mistake in the comment: the clock driver
rounds up _or_ down, so I should drop the "up".

[...]
>> -     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
We end up with a SPI_CLK of 53,571Hz using the old calculation and
35,714Hz using the new one. The old SPI_CLK is obviously closer to the
requested speed, but nevertheless it exceeds the requested limit and
should not have been chosen.

Thanks for the review!

Cheers,

   Marcus