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Date:   Sat, 4 Aug 2018 10:50:15 -0400 (EDT)
From:   Alan Stern <stern@rowland.harvard.edu>
To:     Guenter Roeck <linux@roeck-us.net>
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
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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.

Alan Stern

> ---
>  drivers/usb/host/ohci-sm501.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/usb/host/ohci-sm501.c b/drivers/usb/host/ohci-sm501.c
> index c9233cddf9a2..eeb5b3137cf2 100644
> --- a/drivers/usb/host/ohci-sm501.c
> +++ b/drivers/usb/host/ohci-sm501.c
> @@ -49,7 +49,7 @@ static const struct hc_driver ohci_sm501_hc_driver = {
>  	 * generic hardware linkage
>  	 */
>  	.irq =			ohci_irq,
> -	.flags =		HCD_USB11 | HCD_MEMORY | HCD_LOCAL_MEM,
> +	.flags =		HCD_USB11 | HCD_MEMORY | HCD_LOCAL_MEM | HCD_BH,
>  
>  	/*
>  	 * basic lifecycle operations
>