IMX6 ESDHC mmc with UHS-I occasional failure with DDR50 cards

[Tim Harvey <tharvey@gateworks.com>]

Greetings,

I've been seeing mmc failures with Linux 4.10 on some IMX6 boards with
UHS-I support when used with DDR50 cards (I've got a couple from
different manufacturers). I do not see the same issues when using
SDR104 cards. I can also see the issue on the latest linux-mmc repo.

The issue is easily replicated by ejecting and inserting the DDR50 card

A successful init looks like this:
[  150.514018] mmc0: host does not support reading read-only switch,
assuming write-enable
[  150.578204] mmc0: new ultra high speed DDR50 SDHC card at address aaaa
[  150.592140] mmcblk0: mmc0:aaaa SL08G 7.40 GiB
[  150.611512]  mmcblk0: p1

A failing init (about 1 of every 5 trys) looks like this:
[   49.514082] mmc0: host does not support reading read-only switch,
assuming write-enable
[   49.676112] mmc0: tuning execution failed: -5
[   49.680510] mmc0: ddr50 tuning failed
[   49.684206] mmc0: new ultra high speed DDR50 SDHC card at address aaaa
[   49.696456] mmcblk0: mmc0:aaaa SL08G 7.40 GiB
^^^^ failed to read the partition table and at this point all access fails:
# dd if=/dev/mmcblk0 of=/dev/null bs=1k count=1
[   89.417210] mmcblk0: error -84 transferring data, sector 0, nr 32,
cmd response 0x900, card status 0xb00
1+0 records in
1+0 records out
1024 bytes (1.0 kB, 1.0 KiB) copied, 0.1227 s, 8.3 kB/s

I know the issue started occurring in Linux v4.4 but I also have heard
there are some tuning patches in the queue so before I bisected
between v4.4 and v4.3. Note that v4.3 is missing DDR50 tuning but when
I backport that I found that when ddr tuning failed there it did not
cause the card to not be accessible.

Regards,

Tim