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Date:   Sat, 4 Aug 2018 12:19:56 -0700
From:   Guenter Roeck <linux@roeck-us.net>
To:     Alan Stern <stern@rowland.harvard.edu>
Cc:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        linux-usb@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] USB: OHCI: ohci-sm501: complete URBs in BH context
Message-ID: <20180804191956.GA5639@roeck-us.net>
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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@roeck-us.net>
> 
> 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.

Cheers,
Guenter