[PATCH v2] mmc: dw_mmc: rockchip: Set the drive phase properly

[Doug Anderson]

Hi,

On Wed, May 11, 2016 at 2:40 PM, Douglas Anderson <dianders at chromium.org> wrote:
> Historically for Rockchip devices we've relied on the power-on
> default (or perhaps the firmware setting) to get the correct drive
> phase for dw_mmc devices.  This worked OK for the most part, but:
>
> * Relying on the setting just "being right" is a bit fragile.
>
> * As soon as there is an instance where the power on default is wrong or
>   where the firmware didn't configure this properly then we'll get a
>   mysterious failure.
>
> Let's explicitly set this phase in the kernel.
>
> The comments inside this patch try to explain the situation quite
> throughly, but the high level overview of this is:
>
> Before this patch on rk3288 devices tested:
> * eMMC: 180 degrees
> * SDMMC/SDIO0/SDIO1: 90 degrees
>
> After this patch:
> * Use 90 degree phase offset usually.
> * Use 180 degree phase offset for MMC_DDR52, SDR104, HS200.
>
> That means we are _changing_ behavior for those devices in this way:
>
> * If we have HS200 eMMC or DDR52 eMMC, we'll run ID mode at 90
>   degrees (vs 180) but otherwise have no change.
>
> * For any non-HS200 / non-DDR52 eMMC devices we'll now _always_ run at
>   90 degrees (vs 180).  It seems fairly unlikely that building modern
>   hardware is using an eMMC that isn't using DDR52 or HS200, of course.
>
> * For SDR104 cards we'll now run with 180 degree phase offset (vs 90).
>   It's expected that 90 degree phase offset would have worked OK, but
>   this gives us extra margin.
>
> I have tested this by inserting my collection of uSD cards (mostly UHS,
> though a few not) into a veyron_minnie and confirmed that they still
> seem to enumerate properly.  For a subset of them I tried putting a
> filesystem on them and also tried running mmc_test.
>
> Signed-off-by: Douglas Anderson <dianders at chromium.org>
> ---
> Changes in v2:
> - Now use 90 degrees for some modes; updated comments to say why.
>
>  drivers/mmc/host/dw_mmc-rockchip.c | 64 ++++++++++++++++++++++++++++++++++++++
>  1 file changed, 64 insertions(+)

Just a note that I have since found out that recent patches in the
kernel in fact _do_ init the drive phase and apparently do it
improperly.  That means that $SUBJECT patch actually is expected to
fix real problems on real devices.

See <https://patchwork.kernel.org/patch/9085311/> for some details.

-Doug