Re: [PATCH v3 1/2] drivers: usb/core/urb: Add URB_FREE_COHERENT

[Vincent MAILHOL <mailhol.vincent@wanadoo.fr>]

On Wed. 22 Jun 2022 at 18:44, Greg Kroah-Hartman
<gregkh@linuxfoundation.org> wrote:
> On Wed, Jun 22, 2022 at 09:22:12AM +0000, David Laight wrote:
> > From: Vincent MAILHOL
> > > Sent: 21 June 2022 16:56
> > >
> > > On Wed. 22 Jun 2022 at 00:13, Alan Stern <stern@rowland.harvard.edu> wrote:
> > > > On Tue, Jun 21, 2022 at 11:59:16PM +0900, Vincent MAILHOL wrote:
> > > > > I (probably wrongly) assumed that urb::transfer_buffer_length was the
> > > > > allocated length and urb::actual_length was what was actually being
> > > > > transferred. Right now, I am just confused. Seems that I need to study
> > > > > a bit more and understand the real purpose of
> > > > > urb::transfer_buffer_length because I still fail to understand in
> > > > > which situation this can be different from the allocated length.
> > > >
> > > > urb->transfer_buffer_length is the amount of data that the driver wants
> > > > to send or expects to receive.  urb->actual_length is the amount of data
> > > > that was actually sent or actually received.
> > > >
> > > > Neither of these values has to be the same as the size of the buffer --
> > > > but they better not be bigger!
> > >
> > > Thanks. Now things are a bit clearer.
> > > I guess that for the outcoming URB what I proposed made no sense. For
> > > incoming URB, I guess that most of the drivers want to set
> > > urb::transfer_buffer once for all with the allocated size and never
> > > touch it again.
> > >
> > > Maybe the patch only makes sense of the incoming URB. Would it make
> > > sense to keep it but with an additional check to trigger a dmesg
> > > warning if this is used on an outcoming endpoint and with additional
> > > comment that the URB_FREE_COHERENT requires urb::transfer_buffer to be
> > > the allocated size?
> >
> > IIRC urb are pretty big.
>
> What exactly do you mean by "pretty big" here?  And what is wrong with
> that, I have never seen any issues with the current size of that
> structure in any benchmark or performance results.  All USB bottlenecks
> that I know of are either in the hardware layer, or in the protocol
> layer itself (i.e. usb-storage protocol).
>
> > You'd be unlucky if adding an extra field to hold the allocated
> > size would ever need more memory.
> > So it might just be worth saving the allocated size.
>
> Maybe, yes, then we could transition to allocating the urb and buffer at
> the same time like we do partially for iso streams in an urb.  But that
> still might be overkill for just this one driver.

Well, I wouldn't have proposed the patch if it only applied to a
single driver. If we add a urb::allocated_transfer_size as suggested
by David, I believe that the majority of the drivers using DMA memory
will be able to rely on that URB_FREE_COHERENT flag for the garbage
collection.

The caveat, as you pointed before, is that the developper still needs
to be aware of the limitations of DMA and that it should not be freed
in an IRQ context. e.g. no call to usb_kill_anchored_urbs() or other
functions that would lead to urb_destroy().

> I'm curious as to why
> a slow and tiny protocol like CAN needs to use usb_alloc_coherent() for
> its buffers in the first place.

The CAN protocol, in its latest revision, allows for transfer speed up
to ~5Mbits. For low performance CPUs, this starts to be a significant
load. Also, the CAN PDU being small (0 to 64 bytes), many small
transfers occur.
Unfortunately I did not do any benchmark myself so I won't be able to
back my explanation with numbers.


Yours sincerely,
Vincent Mailhol