Re: [PATCH] char: xillybus: Add driver for XillyUSB (Xillybus variant for USB)

[Eli Billauer]

Hello Greg,

Merging XillyUSB's driver into xillybus_core.c was of course the initial 
idea. Practically, it turned out that this doesn't reduce the number of 
code lines nor makes the code easier to understand: The XillyUSB driver 
is a completely different deal internally, in almost every aspect of it.

Indeed, the two drivers do basically the same thing: They create a 
pipe-like API using a hardware interface that is based upon buffers. 
This is what most of the code in both drivers is about. As this 
underlying hardware interface is so fundamentally different, there is 
little in common between the drivers.

The existing xillybus_core.c driver is based upon direct memory register 
+ DMA interaction with the hardware. XillyUSB relies on the USB 
framework for all communication. I'll try to demonstrate the practical 
differences with two examples.

(1) Sending commands to the hardware: The existing Xillybus driver just 
writes to registers in memory space. Its XillyUSB counterpart calls 
xillyusb_send_opcode() to prepare a little packet for transmission over 
USB, and may possibly sleep if there's a (temporary) lack of resources 
to complete that task.

(2) Data handling: The existing Xillybus driver just copies user data to 
and from DMA buffers. Its main business is to maintain and juggle these 
buffers with the hardware. The XillyUSB driver, on the other hand, 
manages a pool of URBs to efficiently shuffle the data to and from the 
hardware. The main challenge is to keep the data flowing at 400 MB/s.

This goes on for every single aspect of the two drivers: They do the 
same things essentially, but the actual actions are completely 
different, as they have different means to do get the job done. And 
completely different challenges.

The only sensible code reuse I can see, is to merge the new "xillyusb" 
device class into the existing "xillybus" class in a new common module 
file, say xillybus_class.c. Maybe also move the code snippets in the 
open() methods which look for a major/minor match to that new file. I 
don't expect that to reduce the total amount of code lines, nor make it 
friendlier to read. But this will reuse the existing device class name 
instead of creating a new one. Does this sound like a good idea?

Thanks and regards,
   Eli

On 05/01/21 15:47, Greg KH wrote:
> On Sun, Dec 13, 2020 at 07:05:03PM +0200,eli.billauer@xxxxxxxxx  wrote:
>    
> > From: Eli Billauer<eli.billauer@xxxxxxxxx>
> >
> > The XillyUSB driver is the USB variant for the Xillybus FPGA IP core.
> > Even though it presents a nearly identical API on the FPGA and host,
> > it's almost a complete rewrite of the driver: The framework for exchanging
> > data on a USB bus is fundamentally different from doing the same with a
> > PCIe interface, which leaves very little in common between the existing
> > driver and the new one for XillyUSB.
> >      
> But in this one you are talking to userspace directly through a char
> node, why not use the same interface that the xillybus_core.c code uses?
> Creating yet-another-class-device feels odd when you already have one
> that is in use.
>
> Try to merge them together to use the same framework, or document the
> heck out of why this is somehow different, yet looks the same...
>
> thanks,
>
> greg k-h
>
>