[Bug 220491] usb_storage connected SD card disconnects/reconnects on resume from suspend

[bugzilla-daemon@kernel.org]

https://bugzilla.kernel.org/show_bug.cgi?id=220491

--- Comment #29 from Paul Ausbeck (paula@alumni.cse.ucsc.edu) ---
I think it somewhat important to say why I reported the Samsung problem
initially when I think that the emb-qm77 board may be a better canonical
example. The emb-qm77 board appears canonical to me only in that the problem is
reliably repeatable. The Samsung laptop is actually canonical in the relative
worth of a kernel fix. The emb-qm77 machine has 5 SATA ports, so I would not
dream of actually using its USB 3.0 ports for mounting storage devices that
need to survive suspend/resume.

On the other hand, the Samsung laptop has only one m.2 SATA slot and one SD
card slot. Right now, the ativ9 contains its original Samsung 256GB m.2 SATA
drive and a Samsung 512GB U1/A2 SD card. I use the SD card for
larger/downloaded files: video, images, audio, pdfs, downloaded web pages,
compressed tarballs, and the m.2 SATA drive for everything else. The ativ9 SD
card reader limits the SD card to ~20MB/s, rather less than its 100MB/s U1
rating. Even at 20MB/s the USB 3.0 connection can theoretically support 40K
512B transactions per second. This is more than the few thousand specified for
the SD card's A2 rating. Knowing USB, though, the number of transactions
actually available may likely be substantially less than 40k/s. Still, in my
experience, USB 3.0 can support a few thousand transactions/s.

I really like these Samsung U1/A2 cards. Even in a compromised reader like in
the ativ9, it is very possible to build a linux kernel on the SD card. The
linux kernel build is so CPU limited that it barely notices that it's on an SD
card. These U1/A2 cards generally behave quite similarly to mmc connected
soldered flash as in typical phone. I also trust these cards degrade properly
in that write problems start surfacing while it is still possible to recover
previously written data from the card. I don't want to imply that write
problems surface that often because they don't. Again, I really like these
Samsung U1/A2 cards.

I really like the Samsung ativ9 laptop as well. I paid $500 for it ten years
ago. It is small, light and physically robust. It has a lot of bright display
pixels. It is efficient and thus far the battery works as if new. It has an
Atheros QCA6174 radio which actually does 2x2 MIMO. The wifi throughput can be
25MB/s on a 40MHz 2.4GHz connection, and 50MB/s on a 80MHz 5GHz connection. The
QCA6174 firmware crashes on most suspend/resume cycles but the system recovers
without any user intervention. I would not trade the known limitations of the
ativ9 for any newer system with unknown limitations.

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