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@xxxxxxxxxxxxxxxxxxx> 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@xxxxxxxxxxxxxxxxxxx> 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
