Re: [RFC 1/2] sdhci: Add device tree property sd-broken-highspeed

From: Shawn Lin
Date: Wed Oct 05 2016 - 21:34:24 EST

On 2016/10/6 5:22, Julia Cartwright wrote:
On Wed, Oct 05, 2016 at 03:03:44PM -0500, Rob Herring wrote:
On Wed, Oct 5, 2016 at 1:33 PM, Ulf Hansson <ulf.hansson@xxxxxxxxxx> wrote:
On 23 September 2016 at 22:01, Zach Brown <zach.brown@xxxxxx> wrote:
Certain board configurations can make highspeed malfunction due to
timing issues. In these cases a way is needed to force the controller
and card into standard speed even if they otherwise appear to be capable
of highspeed.

The sd-broken-highspeed property will let the sdhci driver know that
highspeed will not work.

Signed-off-by: Zach Brown <zach.brown@xxxxxx>
Documentation/devicetree/bindings/mmc/mmc.txt | 2 ++
1 file changed, 2 insertions(+)

diff --git a/Documentation/devicetree/bindings/mmc/mmc.txt b/Documentation/devicetree/bindings/mmc/mmc.txt
index 8a37782..59332ea 100644
--- a/Documentation/devicetree/bindings/mmc/mmc.txt
+++ b/Documentation/devicetree/bindings/mmc/mmc.txt
@@ -52,6 +52,8 @@ Optional properties:
- no-sdio: controller is limited to send sdio cmd during initialization
- no-sd: controller is limited to send sd cmd during initialization
- no-mmc: controller is limited to send mmc cmd during initialization
+- sd-broken-highspeed: Highspeed is broken, even if the controller and card
+ themselves claim they support highspeed.

Regarding a broken card, that is managed via the card quirks and not in DT.

If this is about a controller limitation, we already have the option
to describe what it supports, so we don't need an option to tell what
it *not* supports.

For example "cap-sd-highspeed" tells whether the controller supports
SD high-speed, please use that instead.

If a controller has a capability register and it lies (perhaps the
board has limitations that the SoC does not), then you may need to
disable a feature.

That's precisely the case here. This is a board-level problem, not a
card or controller problem. As Zach mentioned in the cover letter, the
trace length between controller and card on some of our boards is too
long to meet high-speed timings, even though both card and controller
advertise it.

IIRC, I saw the same problem while using sdhci to bring up a
industrial board for vehicle. The trace length is so long that
I have to limit the max-frequency to make it works properly when
running at hishspeed.

So could you try to limit the max-frequency in your DT to see
if it could work for you? I guess it should work as once reducing
the frequency, the timing per cycle will be large enough to meet
the spec. At least that helped me solve my problem.

For further consideration, I deployed a mechanism called "tuning for
non UHS or non-hs200/400" for my donwstream tree at that time. The basic
concept is to ask devices to send ext_csd(or send status for SD case) *repeatedly*. Some hosts, i.e. sdhci-of-arasan or dw_mmc-rockchip, can
manually set rx delay via some clock unit registers to capture the
working sample window and select the middle point of the longest good
phase region. The same for retune. But it is a little complicated, and
could only be applicable to the hosts who could adjust the rx delay
manually when claiming the caps of MMC_CAPS_CAN_TUNING_FOR_HS, whatever
the name is...

I don't know if it is worth to add this, and I don't know if it's
a legit way. Anyway, I just share my thought(experience) for you and
linux-mmc to think more about how to deal with the case you meet rather
than sacrificing the performence by removing highspeed or reducing

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Best Regards
Shawn Lin