[PATCH 22/22] usb: document that URB transfer_buffer should be aligned

[Mauro Carvalho Chehab]

Em Thu, 30 Mar 2017 12:34:32 +0300
Laurent Pinchart <laurent.pinchart at ideasonboard.com> escreveu:

> Hi Oliver,
> 
> On Thursday 30 Mar 2017 10:11:31 Oliver Neukum wrote:
> > Am Donnerstag, den 30.03.2017, 01:15 +0300 schrieb Laurent Pinchart:  
> > > > +   may also override PAD bytes at the end of the ``transfer_buffer``,
> > > > up to the
> > > > +   size of the CPU word.  
> > > 
> > > "May" is quite weak here. If some host controller drivers require buffers
> > > to be aligned, then it's an API requirement, and all buffers must be
> > > aligned. I'm not even sure I would mention that some host drivers require
> > > it, I think we should just state that the API requires buffers to be
> > > aligned.  
> > 
> > That effectively changes the API. Many network drivers are written with
> > the assumption that any contiguous buffer is valid. In fact you could
> > argue that those drivers are buggy and must use bounce buffers in those
> > cases.

Blaming the dwc2 driver was my first approach, but such patch got nacked ;)

Btw, the dwc2 driver has a routine that creates a temporary buffer if the
buffer pointer is not DWORD aligned. My first approach were to add
a logic there to also use the temporary buffer if the buffer size is
not DWORD aligned:
	https://patchwork.linuxtv.org/patch/40093/

While debugging this issue, I saw *a lot* of network-generated URB
traffic from RPi3 Ethernet port drivers that were using non-aligned 
buffers and were subject to the temporary buffer conversion.

My understanding here is that having a temporary bounce buffer sucks,
as the performance and latency are affected. So, I see the value of
adding this constraint to the API, pushing the task of getting 
aligned buffers to the USB drivers, but you're right: that means a lot
of work, as all USB drivers should be reviewed.

Btw, I'm a lot more concerned about USB storage drivers. When I was
discussing about this issue at the #raspberrypi-devel IRC channel,
someone complained that, after switching from the RPi downstream Kernel
to upstream, his USB data storage got corrupted. Well, if the USB
storage drivers also assume that the buffer can be continuous,
that can corrupt data.

That's why I think that being verbose here is a good idea.

I'll rework on this patch to put more emphasis about this issue.

> > 
> > So we need to include the full story here.  
> 
> I personally don't care much about whose side is responsible for handling the 
> alignment constraints, but I want it to be documented before "fixing" any USB 
> driver.
> 



Thanks,
Mauro