linux-kernel - Re: [PATCH net 1/2] r8152: fix the sw rx checksum is unavailable

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Date:   Thu, 24 Nov 2016 14:10:36 -0500
From:   Mark Lord <mlord@...ox.com>
To:     Greg KH <greg@...ah.com>
Cc:     David Miller <davem@...emloft.net>, hayeswang@...ltek.com,
        netdev@...r.kernel.org, nic_swsd@...ltek.com,
        linux-kernel@...r.kernel.org, linux-usb@...r.kernel.org
Subject: Re: [PATCH net 1/2] r8152: fix the sw rx checksum is unavailable

On 16-11-24 02:00 PM, Greg KH wrote:
> On Thu, Nov 24, 2016 at 01:34:08PM -0500, Mark Lord wrote:
>> One thought:  bulk data streams are byte streams, not packets.
>> Scheduling on the USB bus can break up larger transfers across
>> multiple in-kernel buffers.  A "real" URB buffer on USB2 is max 512 bytes.
>> The driver is providing 16384-byte buffers, and assumes that data will
>> never spill over from one such buffer to the next.
>> Yet the observations here consistently show otherwise.
> 
> Wait, how do you know that data will not spill over?  What is making
> that guarantee?  Will the USB device send a "zero packet" in order to
> show that all of the "logical" data is now sent for this specific
> endpoint?  Is there some sort of "framing" that the device does with the
> USB data so that the driver "knows" where the end of packet is?

Exactly my point.

> Check the zero-packet stuff for this device, that's tripped up many a
> USB driver writer over the years, myself included.

I haven't tripped over it myself, but only because we were careful
to allow for such in the USB drivers I have worked on.

The r8152 driver just assumes it never happens.