[Nut-upsdev] blazer_usb: compatibility with Ippon BackPro UPSes (Phoenix Power Co., Ltd usb-to-serial controller)

[Arjen de Korte]

Citeren Alexey Loukianov <mooroon2 op mail.ru>:

> Well, that is true and false in the same time. The trouble is that  
> usb-to-serial
> implementations that are used in UPSes are totally messed up when it comes to
> the compliance with USB standards regarding to HID.

That is *very* true. There are only a few vendors that took the effort  
to make their USB to serial converters in the UPS compliant with  
usbserial (PowerCOM being one of them).

> So it might be either true
> or not that the real MAX_READ_WRITE for device equals to reported 8 bytes.

That's the reason why we stick to 8 bytes. Devices that support  
reading 64 bytes at a time will also allow to read in 8 chunks of 8  
bytes (but not the other way around).

> Winpower software on linux does reads in a chunks larger than 8  
> bytes and it is proven to work.

On this particular UPS. But it is also proven *not* to work on devices  
with the same VID:PID combination. I tried that before and got burned  
(see the archives).

> In case of libusb-0.1.12 reading data in 8-bytes chunks works normally with
> Ippon UPSes (unsure about other brands - it should be tested before doing any
> commits to SVN),

Not easily. We don't have access to all these devices. Most of the  
reports about drivers being broken are from users from the field, from  
whom we usually never hear again until something breaks. I'm not  
willing to take the risk to break this driver, if there are good  
alternatives around.

> but when using libusb-0.1.8 that is the stock version for RHEL4
> reading in 8-bytes chunks makes phoenix usb-to-serial device in Ippon UPSes
> hang.

In that case, we don't support this device with libusb-0.1.8. I don't  
think we should consider patches that might break other devices that  
are supported now, to add support for a library that is almost six  
years old. The fact that even libusb-0.1.12 is almost four years old  
tells me that we should probably up our minimum libusb requirements to  
that instead.

Best regards, Arjen
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