Re: [PATCH v3 1/2] spi: support inter-word delay requirement for devices

[Geert Uytterhoeven <geert@xxxxxxxxxxxxxx>]

Hi Jonas, Baolin,

On Tue, Jan 29, 2019 at 10:50 AM Jonas Bonn <jonas@xxxxxxxxxxx> wrote:
> On 29/01/2019 10:35, Baolin Wang wrote:
> > On Tue, 29 Jan 2019 at 17:14, Jonas Bonn <jonas@xxxxxxxxxxx> wrote:
> >> On 29/01/2019 10:04, Baolin Wang wrote:
> >>> On Tue, 29 Jan 2019 at 05:28, Jonas Bonn <jonas@xxxxxxxxxxx> wrote:
> >>>> On 28/01/2019 19:10, Mark Brown wrote:
> >>>>> On Sat, Jan 26, 2019 at 05:32:19PM +0100, Jonas Bonn wrote:
> >>>>>
> >>>>>> @@ -164,6 +166,7 @@ struct spi_device {
> >>>>>>        char                    modalias[SPI_NAME_SIZE];
> >>>>>>        const char              *driver_override;
> >>>>>>        int                     cs_gpio;        /* chip select gpio */
> >>>>>> +    uint16_t                word_delay;     /* inter-word delay (us) */
> >>>>>
> >>>>> This needs some code in the core joining it up with the per-transfer
> >>>>> word delay similar to what we have for speed_hz and bits_per_word in
> >>>>> __spi_validate().  Then the controller drivers can just look at the
> >>>>> per-transfer value and support both without having to duplicate logic.
> >>>>>
> >>>>
> >>>> So spi_transfer already has a field word_delay and it's defined as
> >>>> _clock cycles_ to delay between words.  I defined word_delay in
> >>>> spi_device as _microseconds_ to delay along the lines of delay_usecs.
> >>>>
> >>>> Given that the inter-word delay is a function of the slave device speed
> >>>> and not of the SPI bus speed, I'm inclined to say that a time-based
> >>>> delay is what we want (to be independent of bus speed).  As such, I want
> >>>> to know if I should add word_delay_usecs to _both_ spi_transfer and
> >>>> spi_device?
> >>>>
> >>>> There's only one user of word_delay from spi_transfer.  Just looking at
> >>>> it quickly, it looks like it wants the word_delay in
> >>>> SPI-controller-clock cycles and not SCK cycles which seems pretty broken
> >>>> to me.  Adding Baolin and Lanqing to CC: for comment.  Could we rework
> >>>> that to be microseconds and do the calculation in the driver?
> >>>
> >>> The Spreadtrum SPI controller's word delay unit is clock cycle of the
> >>> SPI clock, since the SPI source clock can be changed, we can not force
> >>> user to know the real microseconds. But can we change it to a union
> >>> structure? not sure if this is a good way.
> >>
> >> OK, so it is the SPI clock.  That's good.  There's a comment in the
> >> driver that makes it look like it should be the source clock.
> >
> > Sorry for my unclear description, what I mean is that it is the SPI
> > source clock cycles.
> >
> >> The problem with a delay in clock cycles is that the faster the clock,
> >> the shorter the delay.  The delay is a property of the slave and the
> >> slave has a fixed internal clock.  This means that if we increase SCK we
> >> also need to increase the word_delay (in cycles) in order to give the
> >> slave the same amount of breathing room.
> >
> > Sorry for my confusing description, our case requires source clock
> > cycles for word delay.
>
> OK.  So the user (perhaps in userspace using spidev) has to know the
> rate of the IO clock that the SPI controller sits behind and then has to
> match this to the required delay of the slave device...  Doesn't sound
> very portable.

I can see the value of having both:
On some slaves, the delay may depend on a fixed internal or
external clock[1] on the SPI slave, so it should be specified in time units.
Some slaves may be clocked by the SPI clock[2], so the delay should be
specified in SPI clock cycles.

[1] For an external clock, the SPI slave driver may need to obtain a clock
    reference from DT, get its rate, and calculate the needed delay.
[2] I've seen hardware designs where the SPI clock had to be kept running all
    the time because of this.

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@xxxxxxxxxxxxxx

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds