From: Felix Fietkau <nbd@openwrt•org>
To: "Rafał Miłecki" <zajec5@gmail•com>
Cc: Network Development <netdev@vger•kernel.org>,
Hauke Mehrtens <hauke@hauke-m•de>
Subject: Re: [PATCH v4 1/9] bgmac: simplify tx ring index handling
Date: Mon, 13 Apr 2015 17:01:02 +0200 [thread overview]
Message-ID: <552BDA2E.3060909@openwrt.org> (raw)
In-Reply-To: <CACna6rxeBNuSq8OJTBH1Yr75o79q9CDqc2JTW4=kEk+HQt5JRg@mail.gmail.com>
On 2015-04-13 16:21, Rafał Miłecki wrote:
>> @@ -158,13 +157,7 @@ static netdev_tx_t bgmac_dma_tx_add(struct bgmac *bgmac,
>> skb_checksum_help(skb);
>>
>> nr_frags = skb_shinfo(skb)->nr_frags;
>> -
>> - if (ring->start <= ring->end)
>> - free_slots = ring->start - ring->end + BGMAC_TX_RING_SLOTS;
>> - else
>> - free_slots = ring->start - ring->end;
>> -
>> - if (free_slots <= nr_frags + 1) {
>> + if (ring->end - ring->start + nr_frags + 1 >= BGMAC_TX_RING_SLOTS) {
>> bgmac_err(bgmac, "TX ring is full, queue should be stopped!\n");
>> netif_stop_queue(net_dev);
>> return NETDEV_TX_BUSY;
>
> How is this going to work with ring->end lower than ring->start? Let's
> say you have 2 empty slots at the end of ring and 2 empty slots at the
> beginning. In total 4 free slots. Doing ring->end - ring->start will
> give you some big negative number (depending on the ring size), won't
> it?
It won't, because the resulting data type is limited to 32 bit. I'm
pretty sure there are other places in the kernel that rely on the same
behavior.
>> @@ -284,10 +276,8 @@ static void bgmac_dma_tx_free(struct bgmac *bgmac, struct bgmac_dma_ring *ring)
>> slot->skb = NULL;
>> }
>>
>> -next:
>> slot->dma_addr = 0;
>> - if (++ring->start >= BGMAC_TX_RING_SLOTS)
>> - ring->start = 0;
>> + ring->start++;
>> freed = true;
>> }
>>
>
> Do I understand correctly you're using u32 overflow here? Is this
> OK/allowed in kernel to knowingly use overflows?
I think so.
>> diff --git a/drivers/net/ethernet/broadcom/bgmac.h b/drivers/net/ethernet/broadcom/bgmac.h
>> index 3ad965f..5a198d5 100644
>> --- a/drivers/net/ethernet/broadcom/bgmac.h
>> +++ b/drivers/net/ethernet/broadcom/bgmac.h
>> @@ -414,10 +414,10 @@ enum bgmac_dma_ring_type {
>> * empty.
>> */
>> struct bgmac_dma_ring {
>> - u16 num_slots;
>> - u16 start;
>> - u16 end;
>> + u32 start;
>> + u32 end;
>>
>> + u16 num_slots;
>> u16 mmio_base;
>> struct bgmac_dma_desc *cpu_base;
>> dma_addr_t dma_base;
>
> Any reason for u32 instead of u16?
To avoid writes touching other fields close to it.
- Felix
next prev parent reply other threads:[~2015-04-13 15:01 UTC|newest]
Thread overview: 19+ messages / expand[flat|nested] mbox.gz Atom feed top
2015-04-13 13:52 [PATCH v4 1/9] bgmac: simplify tx ring index handling Felix Fietkau
2015-04-13 13:52 ` [PATCH v4 2/9] bgmac: leave interrupts disabled as long as there is work to do Felix Fietkau
2015-04-13 14:34 ` Rafał Miłecki
2015-04-13 15:03 ` Felix Fietkau
2015-04-13 15:06 ` Rafał Miłecki
2015-04-13 15:44 ` Felix Fietkau
2015-04-13 16:35 ` Rafał Miłecki
2015-04-13 13:52 ` [PATCH v4 3/9] bgmac: set received skb headroom to NET_SKB_PAD Felix Fietkau
2015-04-13 13:52 ` [PATCH v4 4/9] bgmac: simplify rx DMA error handling Felix Fietkau
2015-04-13 14:53 ` Rafał Miłecki
2015-04-13 15:05 ` Felix Fietkau
2015-04-13 13:52 ` [PATCH v4 5/9] bgmac: add check for oversized packets Felix Fietkau
2015-04-13 13:52 ` [PATCH v4 6/9] bgmac: increase rx ring size from 511 to 512 Felix Fietkau
2015-04-13 13:52 ` [PATCH v4 7/9] bgmac: simplify dma init/cleanup Felix Fietkau
2015-04-13 13:52 ` [PATCH v4 8/9] bgmac: fix DMA rx corruption Felix Fietkau
2015-04-13 13:52 ` [PATCH v4 9/9] bgmac: drop ring->num_slots Felix Fietkau
2015-04-13 14:21 ` [PATCH v4 1/9] bgmac: simplify tx ring index handling Rafał Miłecki
2015-04-13 15:01 ` Felix Fietkau [this message]
2015-04-13 18:05 ` Rafał Miłecki
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