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

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

On Wed, Jun 22, 2022 at 07:34:57PM +0900, Vincent MAILHOL wrote:
> 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.

And is the memcpy the actual issue here?  Even tiny cpus can do large
and small memcopy very very very fast.

> Unfortunately I did not do any benchmark myself so I won't be able to
> back my explanation with numbers.

That might be the simplest solution here :)

thanks,

greg k-h