usb 3.0 thumb drive speed limit

[Warner Losh]

On Mon, Jan 2, 2017 at 11:42 AM, Gary Palmer <gpalmer at freebsd.org> wrote:
> On Mon, Jan 02, 2017 at 11:27:49AM +0300, Marat N.Afanasyev wrote:
>> I wonder is there a speed limit on usb 3.0? I've bought
>>
>> ugen0.4: <SanDisk> at usbus0
>> umass2 on uhub7
>> umass2: <SanDisk Extreme, class 0/0, rev 2.10/0.10, addr 4> on usbus0
>> da2 at umass-sim2 bus 2 scbus9 target 0 lun 0
>> da2: <SanDisk Extreme 0001> Removable Direct Access SPC-4 SCSI device
>> da2: Serial Number AA010808161609220143
>> da2: 40.000MB/s transfers
>> da2: 59840MB (122552320 512 byte sectors)
>> da2: quirks=0x2<NO_6_BYTE>
>>
>> that claims 'Up to 245 MBytes/sec read speed'
>
> I don't think the speed reported by the SCSI layer (CAM) is correct
> for USB, although it seems from an experiment here that if you
> plug a USB3 drive into a USB2 port it reports "40.000MB/s transfers"
> and on a USB3 port it reports "400.000MB/s transfers".  SCSI doesn't
> really have any direct mapping to the USB speeds so I suspect the
> USB stack uses the closest value.  The tests I did were with 10.3.
> Other releases may behave differently.
>
> Check with usbconfig what the negotiated USB speed is (the "spd=" value
> with the Mbps or Gbps value in brackets afterwards)
>
> FULL = USB1
> HIGH = USB2
> SUPER = USB3

40MB/s is just a number that's reported. It's not a speed limit.

Our USB stack reports that we understand SCSI protocol rev 2, but we
use the XPORT_USB for the transport. This means the code that prints
those numbers in scsi_xpt.c will use base_transfer_speed from the
XPT_PATH_INQ command. umass.c sets this to either 40MB/s or 400MB/s in
a decision that boils down to if that code thinks the transfer is over
USB 2.0 or 3.0. The fact that we're reporting 40MB/s rather than
400MB/s suggests to me that it was attached on a USB 2.0 hub rather
than a USB 3.0 hub, or it was attached with HIGH speed to a 3.0 hub
because the signals needed to do the SUPER speed were missing. On many
computers, only the "blue" USB ports are 3.0 speed, and only then when
wired correctly (as I discovered the hard way with a recent mobo I put
into service).

For reference, for USB connected umass devices, the driver returns
20kB/s for devices claiming to be floppies, 1MB/s for FULL speed
transfers, 40MB/s for HIGH speed and 400MB/s for SUPER speed.

Looking at the latest code, it sure looks like USB speeds are reported
correctly, or nearly correctly, up to the CAM layer as a good first
order estimate for how fast the drive can code. They do assume a 33%
overhead on USB (which is typical), so that's consistent with getting
just over 40MB/s from dd for a HIGH speed device.

Warner