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

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Message-ID: <CAMz4kuLOSx9mQ-RG9qqy9Qbor97Nj5oYdOgLg_dNfxFeT+XK5g@mail.gmail.com>
Date:   Tue, 29 Jan 2019 17:35:16 +0800
From:   Baolin Wang <baolin.wang@...aro.org>
To:     Jonas Bonn <jonas@...rbonn.se>
Cc:     Mark Brown <broonie@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-spi <linux-spi@...r.kernel.org>,
        Rob Herring <robh+dt@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Lanqing Liu <lanqing.liu@...eadtrum.com>
Subject: Re: [PATCH v3 1/2] spi: support inter-word delay requirement for devices

On Tue, 29 Jan 2019 at 17:14, Jonas Bonn <jonas@...rbonn.se> wrote:
>
>
>
> On 29/01/2019 10:04, Baolin Wang wrote:
> > Hi Jonas,
> > On Tue, 29 Jan 2019 at 05:28, Jonas Bonn <jonas@...rbonn.se> wrote:
> >>
> >> Hi,
> >>
> >> 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.

>
> >
> > union {
> >       int word_delay_us;
> >       int word_delay_cycle;
> > } w;
> >
>
> I don't think that's a practical solution.
>
> The register setting in the spi-sprd driver is what... SCK cycles?  So
> you'd want word_delay_us * max_speed_hz?
>
> The register setting on my Atmel board is in SPI-clock cycles
> (effectively).  So I want word_delay_us*clk_get_rate(spi-clk).

Okay, so your case is different with Spreadtrum SPI.

-- 
Baolin Wang
Best Regards