[Bug 89511] USB-storage mount error

[bugzilla-daemon@bugzilla.kernel.org]

https://bugzilla.kernel.org/show_bug.cgi?id=89511

--- Comment #51 from Alan Stern <stern@rowland.harvard.edu> ---
For those new to this bug report, the interesting part starts here:

    https://bugzilla.kernel.org/show_bug.cgi?id=89511#c19

In short, these are USB drive adapters that doesn't support the
SYNCHRONIZE CACHE command but claim to be write-back.  (In at least 
one case, it initially claims to be write-through but then changes to 
write-back following a reset, or something like that.)

On Thu, 4 Jun 2015 bugzilla-daemon@bugzilla.kernel.org wrote:

> --- Comment #50 from Markus Rathgeb <maggu2810@gmail.com> ---
> I ran in the same trouble with an old USB case for 2.5'' IDE hard discs.
> 
> The drive cache_type is set to "write back" if it plugged in.
> The drive is working using if I set the cache type to "temporary write
> through".
> 
> So, I think this is the exact same problem discussed here.
> 
> At the moment I tested the workaround using v3.18.2 (gentoo-sources).
> Without the workaround the problem also exists using the Kernel v4.0.4 (used
> gentoo-sources, but I think that should not be related to the problem at all).
> 
> Is there anything I / we can do, to solve this problem at all in the kernel
> space (e.g. if SYNCHRONIZE CACHE command is not working, fallback to write
> through)?

This is up to the SCSI developers.

The real question is what should happen when a drive really does have a
write cache but doesn't recognize the SYNCHRONIZE CACHE command.  I
don't know what the kernel can do in this case -- maybe just log a
one-time error message and turn off sdkp->WCE?

Alan Stern

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