Re: [PATCH 2/2] dt-bindings: rockchip-dw-mshc: add rockchip,default-drv-phase

[Shawn Lin]

On 2016/5/7 1:26, Doug Anderson wrote:
> Shawn,
>
> On Fri, May 6, 2016 at 2:41 AM, Shawn Lin <shawn.lin@xxxxxxxxxxxxxx> wrote:
> > rockchip,default-drv-phase is used to set the default drv degrees.
> > drv phases will decide the write timing of mmc controller.
>
> Device tree bindings should probably have been patch 1/2, then the
> code patch 2/2.
>
>
> > Signed-off-by: Shawn Lin <shawn.lin@xxxxxxxxxxxxxx>
> > ---
> >
> >  Documentation/devicetree/bindings/mmc/rockchip-dw-mshc.txt | 3 +++
> >  1 file changed, 3 insertions(+)
> >
> > diff --git a/Documentation/devicetree/bindings/mmc/rockchip-dw-mshc.txt b/Documentation/devicetree/bindings/mmc/rockchip-dw-mshc.txt
> > index ea5614b..c48dba6 100644
> > --- a/Documentation/devicetree/bindings/mmc/rockchip-dw-mshc.txt
> > +++ b/Documentation/devicetree/bindings/mmc/rockchip-dw-mshc.txt
> > @@ -29,6 +29,9 @@ Optional Properties:
> >    probing, low speeds or in case where all phases work at tuning time.
> >    If not specified 0 deg will be used.
> >
> > +* rockchip,default-drv-phase: The default phase to set ciu_drv at probing
> > +  for host to write data to devices. If not specified 180 deg will be used.
>
> This is probably not right for a few reasons.
>
> 1. Specifying a single number for this property in terms of "degrees"
> is probably not right.  The whole point of setting the "drive phase"
> is to meet hold times, which are specified in the spec in terms of ns
> in the spec and also specified differently for different SD/MMC speed
> modes.  Note also that "phase" translates to very different delays (in
> terms of ns) depending on the clock rate:
>
> At 400 kHz, period is 2.5 us, so 90 degree phase offset is a delay of 625 ns
> At 25 MHz, period is 40 ns, so a 90 degree phase offset represents a
> delay of 10 ns.
> At 50 MHz, period is 20 ns, so a 90 degree phase offset represents a
> delay of 5 ns.
> At 200 MHz, period is period is 5 ns, so a 90 degree phase offset
> represents a delay of 1.25 ns.

yes, if we use degrees only(0/90/180/270), the timing is always right.
But considering the delay number, we need to do some crazy calculation
in the set_ios callback.

> 2. As I understand it, the value needed for the drive phase is not
> board specific unless you've got super crazy layout on a board (where
> the clock line takes a very different path than everything else).
> It's also not even terribly SoC-specific unless you've got some very
> strange incarnation of dw_mmc that has very different internal delays
> than everyone else.  Said another way, until we see an instance of an
> SoC/board that really needs to do things special I'd say that we
> should just implement this all in code (no device tree bindings).

I'm prone to think it should be Soc specific if making sure the layout
for data lines is in equal length.

> 3. If this property was actually board specific and actually needed to
> be tuned board-by-board, you'd have a bug because your new device tree
> bindings are not backward compatible and you'd probably be breaking
> old boards.  Specifically you're changing the definition of what
> happens when "rockchip,default-drv-phase" is not specified.  Old
> behavior was to leave the value that was setup by the firmware (or
> perhaps the hardware default if the firmware didn't touch this).

drv_phase is for all the data lines instead of tuning the lines
one-by-one. So this patch can't save the terrible board layout.
But I agree that it will break the compatibility backward if firware
touch this value.

> ---
>
> OK, so what should we do?
>
> We could certainly do lots of crazy math to come up with the ideal
> hold time for all different speed modes and all different types of
> cards.  With my reading of the Designware Databook this would mean
> that somewhere we'd want to specify which delay method we're using
> (phase shift vs. delay line) and how long all the delays timings all
> are on your particular SoC.  That all sounds quite difficult, though.

delay line is diff from chip to chip, soc-to-soc, board-to-board. For 
sample-phase we have tuning process and re-tune, but not for drv-phase.
So We bascially should avoid to use it for drv-phase. Another
consideration is the temperature drift of delay line.

Maybe we should do some tricky limitation on clk-mmc-phase to only
support fixed degrees?

> Probably you could just add a simple function that looked at the clock
> and speed mode and always chose an offset of 90 or 180 degrees.  At
> least on Rockchip devices you can be certain that you can make 90 and
> 180 degrees using phase shifts and thus the timings should be
> consistent.  By default you could just always choose 180.  The
> Designware databook has some examples where it picked 90 degrees
> (SDR50, DDR50, SDR25, MMC High Speed), but I'm not enough of an MMC
> expert to know if there is some benefit to choosing 90.  Would we
> violate any specs if we just chose 180 degrees all the time everywhere
> on all Rockchip devices?

It needs more waveform test to see how things going. But most of
rockchip platforms in the pass years didn't touch drv-phase stuff not
only in kernel but also in firmware, then we still cannot see the
violation against the spec when using defalut reset value, namely 180, 
for drv-phase.

> -Doug


--
Best Regards
Shawn Lin