Re: [PATCH v2 3/5] xhci: Add rmb() between reading event validity & event data access.

2011-03-29 Thread Benjamin Herrenschmidt 
On Tue, 2011-03-29 at 11:56 -0700, Dmitry Torokhov wrote:
> > + /*
> > +  * Barrier between reading the TRB_CYCLE (valid) flag above
> and any
> > +  * speculative reads of the event's flags/data below.
> > +  */
> > + rmb();
> >   /* FIXME: Handle more event types. */
> >   switch ((le32_to_cpu(event->event_cmd.flags) &
> TRB_TYPE_BITMASK)) {
> 
> Isn't it the same memory that is being read the first time around? How
> reordering could happen here? 

We don't try to enforce ordering specifically with the subsequent read
of the flags that is visible in the snipped above (indeed that's the
same address and so hopefully should be in program order), but all
subsequent reads which could have been performed speculatively such as
the rest of the event and including whatever other in memory information
the chip might have updated prior to sending the event.

Cheers,
Ben.


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Re: [PATCH v2 3/5] xhci: Add rmb() between reading event validity & event data access.

2011-03-29 Thread Dmitry Torokhov 
On Sunday, March 27, 2011 09:52:57 pm Matt Evans wrote:
> On weakly-ordered systems, the reading of an event's content must occur
> after reading the event's validity.
> 
> Signed-off-by: Matt Evans 
> ---
> Segher, thanks for the comment; explanation added.
> 
>  drivers/usb/host/xhci-ring.c |5 +
>  1 files changed, 5 insertions(+), 0 deletions(-)
> 
> diff --git a/drivers/usb/host/xhci-ring.c b/drivers/usb/host/xhci-ring.c
> index 45f3b77..d6aa880 100644
> --- a/drivers/usb/host/xhci-ring.c
> +++ b/drivers/usb/host/xhci-ring.c
> @@ -2152,6 +2152,11 @@ static void xhci_handle_event(struct xhci_hcd
> *xhci) }
>   xhci_dbg(xhci, "%s - OS owns TRB\n", __func__);
> 
> + /*
> +  * Barrier between reading the TRB_CYCLE (valid) flag above and any
> +  * speculative reads of the event's flags/data below.
> +  */
> + rmb();
>   /* FIXME: Handle more event types. */
>   switch ((le32_to_cpu(event->event_cmd.flags) & TRB_TYPE_BITMASK)) {

Isn't it the same memory that is being read the first time around? How 
reordering could happen here?

Thanks.


-- 
Dmitry
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Re: [PATCH v2 3/5] xhci: Add rmb() between reading event validity & event data access.

2011-03-28 Thread Sarah Sharp 
This patch looks fine, thanks!

Sarah Sharp

On Mon, Mar 28, 2011 at 03:52:57PM +1100, Matt Evans wrote:
> On weakly-ordered systems, the reading of an event's content must occur
> after reading the event's validity.
> 
> Signed-off-by: Matt Evans 
> ---
> Segher, thanks for the comment; explanation added.
> 
>  drivers/usb/host/xhci-ring.c |5 +
>  1 files changed, 5 insertions(+), 0 deletions(-)
> 
> diff --git a/drivers/usb/host/xhci-ring.c b/drivers/usb/host/xhci-ring.c
> index 45f3b77..d6aa880 100644
> --- a/drivers/usb/host/xhci-ring.c
> +++ b/drivers/usb/host/xhci-ring.c
> @@ -2152,6 +2152,11 @@ static void xhci_handle_event(struct xhci_hcd *xhci)
>   }
>   xhci_dbg(xhci, "%s - OS owns TRB\n", __func__);
>  
> + /*
> +  * Barrier between reading the TRB_CYCLE (valid) flag above and any
> +  * speculative reads of the event's flags/data below.
> +  */
> + rmb();
>   /* FIXME: Handle more event types. */
>   switch ((le32_to_cpu(event->event_cmd.flags) & TRB_TYPE_BITMASK)) {
>   case TRB_TYPE(TRB_COMPLETION):
> -- 
> 1.7.0.4
> 
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[PATCH v2 3/5] xhci: Add rmb() between reading event validity & event data access.

2011-03-27 Thread Matt Evans 
On weakly-ordered systems, the reading of an event's content must occur
after reading the event's validity.

Signed-off-by: Matt Evans 
---
Segher, thanks for the comment; explanation added.

 drivers/usb/host/xhci-ring.c |5 +
 1 files changed, 5 insertions(+), 0 deletions(-)

diff --git a/drivers/usb/host/xhci-ring.c b/drivers/usb/host/xhci-ring.c
index 45f3b77..d6aa880 100644
--- a/drivers/usb/host/xhci-ring.c
+++ b/drivers/usb/host/xhci-ring.c
@@ -2152,6 +2152,11 @@ static void xhci_handle_event(struct xhci_hcd *xhci)
}
xhci_dbg(xhci, "%s - OS owns TRB\n", __func__);
 
+   /*
+* Barrier between reading the TRB_CYCLE (valid) flag above and any
+* speculative reads of the event's flags/data below.
+*/
+   rmb();
/* FIXME: Handle more event types. */
switch ((le32_to_cpu(event->event_cmd.flags) & TRB_TYPE_BITMASK)) {
case TRB_TYPE(TRB_COMPLETION):
-- 
1.7.0.4

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