Re: [PATCH] mmc: enable UHS voltage switch for SDSC if supported

On Thu, 6 May 2021 at 10:34, Christian Löhle [off-list ref] wrote:

Well I am definitely for keeping this bug as a feature
and utilizing UHS with this patch ;)
I have tried all SDSC cards I could get my hands on and
have not seen any 'accidentally' reporting UHS support.

That sounds promising.

I tested the patch with own custom cards.
(Hyperstone, my employer, develops controllers for flash
storage.)

I see, thanks for sharing!

There you would have to make the same choice, disabling
UHS support reporting for all SDSC cards, but as some
hosts accept UHS (including voltage switch) without checking
CCS, this would throttle performance.
(One such host is e.g.
8564:4000 Transcend Information, Inc. microSD/SD/CF UHS-II Card Reader)

It would be nice to have this in linux-mmc as arguably the most
important SD native host.
I'm sure the other manufacturers either do not disable UHS
support for SDSC cards, or have at least been playing with the idea.

Assuming I don't find any issues when I am testing some of my older SD
cards, I am definitely open to change into what you propose in
$subject patch.

Although, I think it's important that we also state in the code with a
comment somewhere - that we deliberately have decided to deviate from
the SD spec for the reasons you have pointed out in our discussions.

Can you perhaps prepare and post a new version and possibly also work
a little bit on the commit message to make things a bit more clear?

In the meantime, I will test some SD cards at my side...

Kind regards
Uffe

Regards,
Christian

From: Ulf Hansson <redacted>
Sent: Wednesday, May 5, 2021 4:20 PM
To: Christian Löhle <redacted>
Cc: linux-kernel@vger.kernel.org <redacted>; linux-mmc@vger.kernel.org <redacted>; pali@kernel.org <pali@kernel.org>; huyue2@yulong.com <redacted>; tiantao6@hisilicon.com <tiantao6@hisilicon.com>
Subject: Re: [PATCH] mmc: enable UHS voltage switch for SDSC if supported

On Wed, 5 May 2021 at 14:13, Christian Löhle [off-list ref] wrote:

quoted

So two cards that advertise UHS:
https://www.euric.de/de/flash-speicher/sd-microsd/sd/
Model SDI790
and
https://na.industrial.panasonic.com/products/storage-media/industrial-sd-cards/lineup/industrial-grade-2d-nand/series/71695

Thanks for the links!

Are these the ones that you have been using for testing?

quoted

Hopefully that suffices as a couple, otherwise I would buy some
that do not advertise it and actually check if they support it anyway.
But 2GB SLC listings are kind of rare these days, so it's not as
easy to find many that are still sold and available.

Right. I have a whole bunch of old SD cards that I can test. I will
let you know when I have got some time to test some of them.

The main worry (but I may be wrong) I have with the patch is that it
could cause problems for other older SDSC cards, which happens to
incorrectly set the S18A bit. I mean, before UHS-I was introduced in
the SD spec, this bit was just being reserved and potentially it could
have *any* value by the card manufacturer. As I said, let me do some
tests of my older SD cards.

quoted

My point about deviating from the standard was that these cards
are already initialized as UHS, they just don't do the voltage switch.
So anything reserved for UHS-I (e.g. CMD23 )is already being done
by the host with the current linux-mmc,
so the standard deviation is already there.
Just the voltage switch is missing.

Ah, yes you are right!

We call mmc_sd_init_uhs_card() no matter whether the CCS bit is set.
Certainly this is a bug. Both the card and host aren't designed to
work under these conditions/timings.

quoted

Regards,
Christian

Kind regards
Uffe

quoted

From: Ulf Hansson <redacted>
Sent: Wednesday, May 5, 2021 12:09 PM
To: Christian Löhle <redacted>
Subject: Re: [PATCH] mmc: enable UHS voltage switch for SDSC if supported

On Thu, 22 Apr 2021 at 15:18, Christian Löhle [off-list ref] wrote:

quoted

Ignore the reported capacity if the card otherwise reports UHS support.

Currently SDSC cards reporting UHS support except for the CCS do not run
through the voltage switch.
While strictly speaking a SDSC card cannot support UHS in compliance
with the standard, there is no good reason to throttle them that way.

Maybe not, but I think the code just tries to conform to the SD spec.

quoted

Especially for pSLCs in practice such cards benefit greatly by this patch,
as they can be new and UHS supporting, but must not lie about their CCS.

Can you provide some concrete examples of SDSC cards that support UHS-I?

quoted

The behaviour of linux-mmc for SDSC is deviating from the standard anyway
in such a case, as the card is treated as UHS card not supporting the
voltage switch in general.
Such a card will come up as
mmc0: new ultra high speed SDR25 SD card at address 0001.
Thus the subsystem will support CMD23 and others to the card.
But if we deviate from the standard anyway, then we might as well
not throttle SDSC to 25MB/s.

Not sure I understand this correctly? Can you elaborate a bit more, so
I understand how we deviate from the SD spec here?

As far as I understand the spec, SDSC cards shouldn't support UHS-I.
If we decide to deviate from the spec to support this anyway, at least
we need to know about a couple of cards that actually supports this.

Kind regards
Uffe

quoted

Signed-off-by: Christian Loehle <redacted>
---
 drivers/mmc/core/sd.c | 7 +++----
 1 file changed, 3 insertions(+), 4 deletions(-)

diff --git a/drivers/mmc/core/sd.c b/drivers/mmc/core/sd.c
index 6fa51a6ed058..281ca2da8e0b 100644
--- a/drivers/mmc/core/sd.c
+++ b/drivers/mmc/core/sd.c

@@ -841,11 +841,10 @@ int mmc_sd_get_cid(struct mmc_host *host, u32 ocr, u32 *cid, u32 *rocr)
                return err;

        /*
-        * In case CCS and S18A in the response is set, start Signal Voltage
-        * Switch procedure. SPI mode doesn't support CMD11.
+        * In case S18A in the response is set, start Signal Voltage Switch
+        * procedure. SPI mode doesn't support CMD11.
         */
-       if (!mmc_host_is_spi(host) && rocr &&
-          ((*rocr & 0x41000000) == 0x41000000)) {
+       if (!mmc_host_is_spi(host) && rocr && (*rocr & 0x01000000)) {
                err = mmc_set_uhs_voltage(host, pocr);
                if (err == -EAGAIN) {
                        retries--;
--

2.31.1

Hyperstone GmbH | Line-Eid-Strasse 3 | 78467 Konstanz
Managing Directors: Dr. Jan Peter Berns.
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Hyperstone GmbH | Line-Eid-Strasse 3 | 78467 Konstanz
Managing Directors: Dr. Jan Peter Berns.
Commercial register of local courts: Freiburg HRB381782

Hyperstone GmbH | Line-Eid-Strasse 3 | 78467 Konstanz
Managing Directors: Dr. Jan Peter Berns.
Commercial register of local courts: Freiburg HRB381782