On 5.12.2018 14:37, claudiu.bez...@microchip.com wrote:
>
> On 30.11.2018 20:21, Anssi Hannula wrote:
>> When reading buffer descriptors on RX or on TX completion, an
>> RX_USED/TX_USED bit is checked first to ensure that the descriptor has
>> been populated. However, there are no memory barriers to ensure that the
>> data protected by the RX_USED/TX_USED bit is up-to-date with respect to
>> that bit.
>>
>> Fix that by adding DMA read memory barriers on those paths.
>>
>> I did not observe any actual issues caused by these being missing,
>> though.
>>
>> Tested on a ZynqMP based system.
>>
>> Signed-off-by: Anssi Hannula <anssi.hann...@bitwise.fi>
>> Fixes: 89e5785fc8a6 ("[PATCH] Atmel MACB ethernet driver")
>> Cc: Nicolas Ferre <nicolas.fe...@microchip.com>
>> ---
>> drivers/net/ethernet/cadence/macb_main.c | 20 ++++++++++++++++----
>> 1 file changed, 16 insertions(+), 4 deletions(-)
>>
>> diff --git a/drivers/net/ethernet/cadence/macb_main.c
>> b/drivers/net/ethernet/cadence/macb_main.c
>> index 430b7a0f5436..c93baa8621d5 100644
>> --- a/drivers/net/ethernet/cadence/macb_main.c
>> +++ b/drivers/net/ethernet/cadence/macb_main.c
>> @@ -861,6 +861,11 @@ static void macb_tx_interrupt(struct macb_queue *queue)
>>
>> /* First, update TX stats if needed */
>> if (skb) {
>> + /* Ensure all of desc is at least as up-to-date
>> + * as ctrl (TX_USED bit)
>> + */
>> + dma_rmb();
>> +
> Is this necessary? Wouldn't previous rmb() take care of this? At this time
> data specific to this descriptor was completed. The TX descriptors for next
> data to be send is updated under a locked spinlock.
The previous rmb() is before the TX_USED check, so my understanding is
that the following could happen in theory:
1. rmb().
2. Reads are reordered so that TX timestamp is read first - no barriers
are crossed.
3. HW writes timestamp and sets TX_USED (or they become visible).
4. Code checks TX_USED.
5. Code operates on timestamp that is actually garbage.
I'm not 100% sure that there isn't some lighter/cleaner way to do this
than dma_rmb(), though.
>> if (gem_ptp_do_txstamp(queue, skb, desc) == 0) {
>> /* skb now belongs to timestamp buffer
>> * and will be removed later
>> @@ -1000,12 +1005,16 @@ static int gem_rx(struct macb_queue *queue, int
>> budget)
>> rmb();
>>
>> rxused = (desc->addr & MACB_BIT(RX_USED)) ? true : false;
>> - addr = macb_get_addr(bp, desc);
>> - ctrl = desc->ctrl;
>>
>> if (!rxused)
>> break;
>>
>> + /* Ensure other data is at least as up-to-date as rxused */
>> + dma_rmb();
> Same here, wouldn't previous rmb() should do this job?
The scenario I'm concerned about here (and in the last hunk) is:
1. rmb().
2. ctrl is read (i.e. ctrl read reordered before addr read).
3. HW updates to ctrl and addr become visible.
4. RX_USED check.
5. code operates on garbage ctrl.
I think it may be OK to move the earlier rmb() outside the loop so that
there is an rmb() only before and after the RX loop, as I don't at least
immediately see any hard requirement to do it on each loop pass (unlike
the added dma_rmb()). But my intent was to fix issues instead of
optimization so I didn't look too closely into that.
>> +
>> + addr = macb_get_addr(bp, desc);
>> + ctrl = desc->ctrl;
>> +
>> queue->rx_tail++;
>> count++;
>>
>> @@ -1180,11 +1189,14 @@ static int macb_rx(struct macb_queue *queue, int
>> budget)
>> /* Make hw descriptor updates visible to CPU */
>> rmb();
>>
>> - ctrl = desc->ctrl;
>> -
>> if (!(desc->addr & MACB_BIT(RX_USED)))
>> break;
>>
>> + /* Ensure other data is at least as up-to-date as addr */
>> + dma_rmb();
> Ditto
>
>> +
>> + ctrl = desc->ctrl;
>> +
>> if (ctrl & MACB_BIT(RX_SOF)) {
>> if (first_frag != -1)
>> discard_partial_frame(queue, first_frag, tail);
>>
--
Anssi Hannula / Bitwise Oy
+358 503803997
