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. 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). 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). --- 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. 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? -Doug -- To unsubscribe from this list: send the line "unsubscribe linux-mmc" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
