Re: [PATCH net-next 06/19] net: usb: aqc111: Introduce link management

On Mon, Oct 08, 2018 at 09:29:26AM +0000, Igor Russkikh wrote:

Hi Andrew,

quoted

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 	aqc111_read_fw_version(dev, aqc111_data);
+	aqc111_data->autoneg = AUTONEG_ENABLE;
+	aqc111_data->advertised_speed = (usb_speed == USB_SPEED_SUPER) ?
+					 SPEED_5000 : SPEED_1000;

quoted

USB 3 has a raw bandwidth of 5Gbps. But it is a shared bus. So you
have no guaranteed you are actually going to get the needed bandwidth
to support line rate.

USB 2.0 only gives you 480Mbps. So it won't even give you the full
1G. So using the same reasoning for USB3, maybe you should limit it to
100Mbps?

I personally would not apply restrictions on the PHY depending on what
USB is being used.

First argument here is to reduce power consumption on USB2.
2.5G/5G uses OCSGMII/XFI serdes which consumes more power.
Of course in normal conditions usb2 is capable to feed that, but
the risk still exists on legacy usb2 hardware.

O.K, that sounds like a sensible argument. Please add a comment. I
hope the Marketing Department also understand this. It should probably
explain this on the product packaging.

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This becomes more important when using SFPs. If i have an SFP peer
which is expecting 2500Base-X, but because the device is plugged into
USB 2 port it is forced to use 1000Base-X, it is not going to get
link.

Do you mean here 2500Base-T? This particular device is an integrated
mac+phy, thus we can't easily link it with -X SFP endpoint.

I only went to find the product brief after finishing the
review. Without an external SERDES interface, SFP is not possible. But
from your comment above, i does sound like internally it has such a
SERDES. So it is not out of the question a follow up device is
produced which could connect to an SFP. I actually have a no-name USB
based SFP dongle...

      Andrew