Re: [PATCH] EHCI: Update qTD next pointer in QH overlay region during unlink

[Pavan Kondeti <pkondeti@codeaurora.org>]

On 9/7/2012 9:22 PM, Alan Stern wrote:
> On Fri, 7 Sep 2012, Pavankumar Kondeti wrote:
> 
>> There is a possibility of QH overlay region having reference to a stale
>> qTD pointer during unlink.
>>
>> Consider an endpoint having two pending qTD before unlink process begins.
>> The endpoint's QH queue looks like this.
>>
>> qTD1 --> qTD2 --> Dummy
>>
>> To unlink qTD2, QH is removed from asynchronous list and Asynchronous
>> Advance Doorbell is programmed.  The qTD1's next qTD pointer is set to
>> qTD2'2 next qTD pointer and qTD2 is retired upon controller's doorbell
>> interrupt.  If QH's current qTD pointer points to qTD1, transfer overlay
>> region still have reference to qTD2. But qtD2 is just unlinked and freed.
>> This may cause EHCI system error.  Fix this by updating qTD next pointer
>> in QH overlay region with the qTD next pointer of the current qTD.
>>
>> Signed-off-by: Pavankumar Kondeti <pkondeti@codeaurora.org>
>> ---
>>  drivers/usb/host/ehci-q.c |   12 ++++++++++--
>>  1 files changed, 10 insertions(+), 2 deletions(-)
>>
>> diff --git a/drivers/usb/host/ehci-q.c b/drivers/usb/host/ehci-q.c
>> index 9bc39ca..4b66374 100644
>> --- a/drivers/usb/host/ehci-q.c
>> +++ b/drivers/usb/host/ehci-q.c
>> @@ -128,9 +128,17 @@ qh_refresh (struct ehci_hcd *ehci, struct ehci_qh *qh)
>>  	else {
>>  		qtd = list_entry (qh->qtd_list.next,
>>  				struct ehci_qtd, qtd_list);
>> -		/* first qtd may already be partially processed */
>> -		if (cpu_to_hc32(ehci, qtd->qtd_dma) == qh->hw->hw_current)
>> +		/*
>> +		 * first qtd may already be partially processed.
>> +		 * If we come here during unlink, the QH overlay region
>> +		 * might have reference to the just unlinked qtd. The
>> +		 * qtd is updated in qh_completions(). Update the QH
>> +		 * overlay here.
>> +		 */
>> +		if (cpu_to_hc32(ehci, qtd->qtd_dma) == qh->hw->hw_current) {
>> +			qh->hw->hw_qtd_next = qtd->hw_next;
>>  			qtd = NULL;
>> +		}
>>  	}
>>  
>>  	if (qtd)
> 
> Acked-by: Alan Stern <stern@rowland.harvard.edu>
> 

Thanks Alan for reviewing the patch.

> Have you been able to determine that this eliminates your host system
> errors?

Yes. We are able to determine that this patch is fixing the EHCI system
error.

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