netdev - RE: [PATCH net 2/2] be2net: Need a delay before processing CQE after 2nd mbox register write

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Date:	Wed, 8 Jan 2014 09:32:18 +0000
From:	David Laight <David.Laight@...LAB.COM>
To:	'Somnath Kotur' <somnath.kotur@...lex.com>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>
CC:	"davem@...emloft.net" <davem@...emloft.net>,
	Kalesh AP <kalesh.purayil@...lex.com>
Subject: RE: [PATCH net 2/2] be2net: Need a delay before processing CQE
 after 2nd mbox register write

> From: Somnath Kotur
> Due to Host platform synchronization issues between the mbox RDY bit polled
> status and the completion of the DMA for the CQE, it is preferable that the
> Host always wait for the RDY bit to transition to 1 after the 2nd mbox register
> write and always follow that with a short wait for the valid bit in the CQE,
> before processing the CQE.

While I don't doubt that a delay(1) fixes the problem it doesn't
seem an ideal solution.
I've not looked at what the code is doing (or how often it does it)
but either delay(1) is far, far longer than is necessary or it might
not be long enough and some kind of retry loop is required.

It might even be that the driver is just missing a memory barrier.

	David

> Signed-off-by: Kalesh AP <kalesh.purayil@...lex.com>
> Signed-off-by: Somnath Kotur <somnath.kotur@...lex.com>
> ---
>  drivers/net/ethernet/emulex/benet/be_cmds.c |    3 +++
>  1 files changed, 3 insertions(+), 0 deletions(-)
> 
> diff --git a/drivers/net/ethernet/emulex/benet/be_cmds.c b/drivers/net/ethernet/emulex/benet/be_cmds.c
> index 94c35c8..78560f2 100644
> --- a/drivers/net/ethernet/emulex/benet/be_cmds.c
> +++ b/drivers/net/ethernet/emulex/benet/be_cmds.c
> @@ -502,6 +502,9 @@ static int be_mbox_notify_wait(struct be_adapter *adapter)
>  	if (status != 0)
>  		return status;
> 
> +	/* Need a delay before processing CQE after 2nd mbox register write */
> +	udelay(1);
> +
>  	/* A cq entry has been made now */
>  	if (be_mcc_compl_is_new(compl)) {
>  		status = be_mcc_compl_process(adapter, &mbox->compl);
> --
> 1.6.0.2


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