Re: [PATCH] USB: OHCI: ohci-sm501: complete URBs in BH context

[Guenter Roeck]

On 08/05/2018 11:31 AM, Alan Stern wrote:
> On Sat, 4 Aug 2018, Guenter Roeck wrote:
>
> > On Sat, Aug 04, 2018 at 10:50:15AM -0400, Alan Stern wrote:
> > > On Fri, 3 Aug 2018, Guenter Roeck wrote:
> > >
> > > > Testing an USB drive connected to ohci-sm501 results in a large number
> > > > of runtime warnings.
> > > >
> > > > WARNING: CPU: 0 PID: 0 at ./include/linux/dma-mapping.h:541
> > > > hcd_buffer_free+0x148/0x178
> > > > Modules linked in:
> > > >
> > > > CPU: 0 PID: 0 Comm: swapper Not tainted 4.18.0-rc7-00014-g7ec386e4c991-dirty
> > > > PC is at hcd_buffer_free+0x148/0x178
> > > > PR is at hcd_buffer_free+0x66/0x178
> > > > PC  : 8c26cbb0 SP  : 8c481da8 SR  : 400080f1
> > > > TEA : c00c8fe0
> > > > R0  : 000000f0 R1  : 000000f0 R2  : 8f9bb890 R3  : 00000000
> > > > R4  : 8f9c8800 R5  : 00001004 R6  : b07c6000 R7  : 007c6000
> > > > R8  : 00001004 R9  : 8f9bb814 R10 : 8c388104 R11 : 007c6000
> > > > R12 : b07c6000 R13 : 8f875680 R14 : 00000000
> > > > MACH: 000002fe MACL: 0000017c GBR : 00000000 PR  : 8c26cace
> > > >
> > > > Call trace:
> > > >   [<(ptrval)>] usb_hcd_unmap_urb_for_dma+0xf4/0x13c
> > > >   [<(ptrval)>] arch_local_save_flags+0x0/0x8
> > > >   [<(ptrval)>] __usb_hcd_giveback_urb+0x2e/0xdc
> > > >   [<(ptrval)>] arch_local_save_flags+0x0/0x8
> > > >   [<(ptrval)>] finish_urb+0x8a/0x164
> > > >   [<(ptrval)>] arch_local_save_flags+0x0/0x8
> > > >   [<(ptrval)>] printk+0x0/0x48
> > > >   [<(ptrval)>] ohci_work.part.11+0x150/0x41c
> > > >   [<(ptrval)>] td_done.isra.4+0x0/0x11c
> > > >   [<(ptrval)>] vprintk_default+0x14/0x20
> > > >   [<(ptrval)>] arch_local_save_flags+0x0/0x8
> > > >   [<(ptrval)>] ohci_irq+0x20c/0x314
> > > >   [<(ptrval)>] usb_hcd_irq+0x16/0x28
> > > >
> > > > Code analysis shows that interrupts are indeed disabled in ohci_irq().
> > > > Handle the situation by setting the HCD_BH flag in the ohci-sm501 driver.
> > > > With this flag set, urbs are released in a tasklet and not by the
> > > > interrupt handler.
> > > >
> > > > Fixes: f54aab6ebcecd ("usb: ohci-sm501 driver")
> > > > Signed-off-by: Guenter Roeck <linux@xxxxxxxxxxxx>
> > >
> > > Unfortunately, one must not simply turn on this flag.  Doing so will
> > > violate some of the documented requirements for scheduling of periodic
> > > transfers.  Significantly deeper changes to the OHCI driver are
> > > necessary before the flag is set.
> >
> > Well, it was worth a try. Note that I did try to figure out if there are
> > any pitfalls when setting HCD_BH, but I didn't find anything. Reminds me
> > of Hitchhiker through the Galaxy for some reason.
>
> The interactions are pretty subtle, and the descriptions are hidden in
> the kerneldoc for usb_submit_urb and usb_unlink_urb.  ehci-hcd, for
> example, required a number of non-obvious changes when it was converted
> to use HCD_BH.  If anyone wants to write something similar for
> ohci-hcd I'll be happy to review it, but I don't want to write it
> myself.  (Besides, speeding up the time spent in interrupt handling

Not me. I stumbled over the problem while enhancing my qemu boot tests,
after attaching a USB drive to the sm501-ohci controller on a virtual sh4
system. Who knows if that system is still used in the real world and,
if it does, if it runs a recent kernel. Worth a quick fix, but not a major
code overhaul - even more so without access to real hardware.

> seems less urgent for OHCI, which runs much slower than EHCI and is
> not getting used very much in new hardware.)
>
> Another way to attack this problem would be to allow DMA unmapping in
> interrupt context.  I have never understood why DMA mapping is okay
> with this but unmapping isn't.

AFAICS it used to be interrupt tolerant for all but x86 up to commit
6894258eda ("dma-mapping: consolidate dma_{alloc,free}_{attrs,coherent}").
A quick test shows that the warning is indeed not seen if I run my test
on v3.18.y.

You would have to ask Christoph why it is now interrupt-intolerant for all
architectures.

Guenter