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Date:   Fri, 08 Jan 2021 12:59:14 +0200
From:   Eli Billauer <>
To:     Greg KH <>
Subject: Re: [PATCH] char: xillybus: Add driver for XillyUSB (Xillybus variant
 for USB)

Hello, Greg.

On 07/01/21 13:39, Greg KH wrote:
> My point is, do NOT have different file names.  Userspace should not
> care about the backing transport layer of a device.
Regarding sound cards and such -- we agree perfectly. For a driver like 
XillyUSB, it's not necessarily clear what is correct formally. Either 
way, for XillyUSB the choice on this matter was practical.

First thing, this is what users expect. Xillybus is an FPGA project, and 
its users are hardware oriented. They've spent time designing the FPGA 
logic, and the connection with the host has been part of the effort. As 
the connection via PCIe or USB is part of the design, they definitely 
see these as different things, and don't expect them to appear to be the 

Also, the application software is virtually always written specially for 
the project. Even if a specific hardware application is designed with 
both connection options possible (USB or PCIe), it will be very much 
desired to distinguish between them. For example, because odds are that 
the PCIe option will allow a much higher bandwidth capacity.

Another thing is that it's quite possible that some users will connect 
an FPGA board to a host through USB and PCIe simultaneously. For 
example, when developing an FPGA to work with the USB variant, but at 
the same time using the PCIe variant for passing debug data. In this 
case, using the same namespace for both variants will cause confusion.

So yes, calling the new XillyUSB device files xillyusb_* instead of 
xillybus_* may appear awkward. But I'm quite sure that no single user 
will ever thank me or anyone else for merging them. Maybe the formality 
is still important enough to do this anyhow...?

Thanks and regards,

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