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usb: ehci: remove the 1st wmb in qh_append_tds

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According to ehci spec 4.10.2, Advance Queue

	If the fetched qTD has its Active bit set to a zero, the
	host controller aborts the queue advance and follows the
	queue head's horizontal pointer to the next schedule data
	structure.

the 'qtd' will be linked into qh hardware queue after the line
below

	*dummy = *qtd;

is executed and observed by EHCI HC, but EHCI HC won't have chance to
fetch the qtd descriptor pointed by 'qtd' in qh_append_tds until the
line below

	dummy->hw_token = token;	#set Active bit here

is executed by CPU and observed by EHCI HC.

There is already one 'wmb' to order writing to 'dummy'/'qtd' descriptors
and writing 'token' to 'dummy' descriptor(set Active bit), so the 1st
wmb is not needed and can be removed.

Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Ming Lei <tom.leiming@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>
This commit is contained in:
Ming Lei 2011-09-05 21:05:58 +08:00 committed by Greg Kroah-Hartman
parent fcda37cb42
commit 41f05dedea
1 changed files with 1 additions and 1 deletions
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2
drivers/usb/host/ehci-q.c
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@ -1056,7 +1056,7 @@ static struct ehci_qh *qh_append_tds (
*/
token = qtd->hw_token;
qtd->hw_token = HALT_BIT(ehci);
wmb ();
dummy = qh->dummy;
dma = dummy->qtd_dma;