czw., 14 maj 2020 o 18:19 Arnd Bergmann <arnd@xxxxxxxx> napisał(a):
>
> On Thu, May 14, 2020 at 10:00 AM Bartosz Golaszewski <brgl@xxxxxxxx> wrote:
> >
> > From: Bartosz Golaszewski <bgolaszewski@xxxxxxxxxxxx>
> >
> > This adds the driver for the MediaTek Ethernet MAC used on the MT8* SoC
> > family. For now we only support full-duplex.
> >
> > Signed-off-by: Bartosz Golaszewski <bgolaszewski@xxxxxxxxxxxx>
>
> Looks very nice overall. Just a few things I noticed, and some ideas
> that may or may not make sense:
>
> > +/* This is defined to 0 on arm64 in arch/arm64/include/asm/processor.h but
> > + * this IP doesn't work without this alignment being equal to 2.
> > + */
> > +#ifdef NET_IP_ALIGN
> > +#undef NET_IP_ALIGN
> > +#endif
> > +#define NET_IP_ALIGN 2
>
> Maybe you should just define your own macro instead of replacing
> the normal one then?
>
I did in an earlier version and was told to use NET_IP_ALIGN but then
found out its value on arm64 doesn't work for me so I did the thing
that won't make anybody happy - redefine the existing constant. :)
> > +static void mtk_mac_lock(struct mtk_mac_priv *priv)
> > +{
> > + spin_lock_irqsave(&priv->lock, priv->lock_flags);
> > +}
> > +
> > +static void mtk_mac_unlock(struct mtk_mac_priv *priv)
> > +{
> > + spin_unlock_irqrestore(&priv->lock, priv->lock_flags);
> > +}
>
> This looks wrong: you should not have shared 'flags' passed into
> spin_lock_irqsave(), and I don't even see a need to use the
> irqsave variant of the lock in the first place.
>
> Maybe start by open-coding the lock and remove the wrappers
> above.
>
> Then see if you can use a cheaper spin_lock_bh() or plain spin_lock()
> instead of irqsave.
>
This is from an earlier version where I did a lot more in hard irq
context. Now that almost all of the processing happens in soft-irq
context I guess you're right - I can go with a regular spin_lock().
> Finally, see if this can be done in a lockless way by relying on
> appropriate barriers and separating the writers into separate
> cache lines. From a brief look at the driver I think it can be done
> without too much trouble.
>
Unfortunately I do need some locking. Accessing RX and TX descriptors
at the same time seems to upset the controller. I experimented a lot
with barriers but it turned out that I got a lot of weird bugs at high
throughput.
> > +static unsigned int mtk_mac_intr_read_and_clear(struct mtk_mac_priv *priv)
> > +{
> > + unsigned int val;
> > +
> > + regmap_read(priv->regs, MTK_MAC_REG_INT_STS, &val);
> > + regmap_write(priv->regs, MTK_MAC_REG_INT_STS, val);
> > +
> > + return val;
> > +}
>
> Do you actually need to read the register? That is usually a relatively
> expensive operation, so if possible try to use clear the bits when
> you don't care which bits were set.
>
I do care, I'm afraid. The returned value is being used in the napi
poll callback to see which ring to process.
> > +/* All processing for TX and RX happens in the napi poll callback. */
> > +static irqreturn_t mtk_mac_handle_irq(int irq, void *data)
> > +{
> > + struct mtk_mac_priv *priv;
> > + struct net_device *ndev;
> > +
> > + ndev = data;
> > + priv = netdev_priv(ndev);
> > +
> > + if (netif_running(ndev)) {
> > + mtk_mac_intr_mask_all(priv);
> > + napi_schedule(&priv->napi);
> > + }
> > +
> > + return IRQ_HANDLED;
>
>
> > +static int mtk_mac_netdev_start_xmit(struct sk_buff *skb,
> > + struct net_device *ndev)
> > +{
> > + struct mtk_mac_priv *priv = netdev_priv(ndev);
> > + struct mtk_mac_ring *ring = &priv->tx_ring;
> > + struct device *dev = mtk_mac_get_dev(priv);
> > + struct mtk_mac_ring_desc_data desc_data;
> > +
> > + desc_data.dma_addr = mtk_mac_dma_map_tx(priv, skb);
> > + if (dma_mapping_error(dev, desc_data.dma_addr))
> > + goto err_drop_packet;
> > +
> > + desc_data.skb = skb;
> > + desc_data.len = skb->len;
> > +
> > + mtk_mac_lock(priv);
> > + mtk_mac_ring_push_head_tx(ring, &desc_data);
> > +
> > + if (mtk_mac_ring_full(ring))
> > + netif_stop_queue(ndev);
> > + mtk_mac_unlock(priv);
> > +
> > + mtk_mac_dma_resume_tx(priv);
> > +
> > + return NETDEV_TX_OK;
> > +
> > +err_drop_packet:
> > + dev_kfree_skb(skb);
> > + ndev->stats.tx_dropped++;
> > + return NETDEV_TX_BUSY;
> > +}
>
> I would always add BQL flow control in new drivers, using
> netdev_sent_queue here...
>
Ok, will do.
> > +static int mtk_mac_tx_complete_one(struct mtk_mac_priv *priv)
> > +{
> > + struct mtk_mac_ring *ring = &priv->tx_ring;
> > + struct mtk_mac_ring_desc_data desc_data;
> > + int ret;
> > +
> > + ret = mtk_mac_ring_pop_tail(ring, &desc_data);
> > + if (ret)
> > + return ret;
> > +
> > + mtk_mac_dma_unmap_tx(priv, &desc_data);
> > + dev_kfree_skb_irq(desc_data.skb);
> > +
> > + return 0;
> > +}
>
> ... and netdev_completed_queue() here.
>
Same here.
> > +static void mtk_mac_tx_complete_all(struct mtk_mac_priv *priv)
> > +{
> > + struct mtk_mac_ring *ring = &priv->tx_ring;
> > + struct net_device *ndev = priv->ndev;
> > + int ret;
> > +
> > + for (;;) {
> > + mtk_mac_lock(priv);
> > +
> > + if (!mtk_mac_ring_descs_available(ring)) {
> > + mtk_mac_unlock(priv);
> > + break;
> > + }
> > +
> > + ret = mtk_mac_tx_complete_one(priv);
> > + if (ret) {
> > + mtk_mac_unlock(priv);
> > + break;
> > + }
> > +
> > + if (netif_queue_stopped(ndev))
> > + netif_wake_queue(ndev);
> > +
> > + mtk_mac_unlock(priv);
> > + }
> > +}
>
> It looks like most of the stuff inside of the loop can be pulled out
> and only done once here.
>
I did that in one of the previous submissions but it was pointed out
to me that a parallel TX path may fill up the queue before I wake it.
> > +static int mtk_mac_poll(struct napi_struct *napi, int budget)
> > +{
> > + struct mtk_mac_priv *priv;
> > + unsigned int status;
> > + int received = 0;
> > +
> > + priv = container_of(napi, struct mtk_mac_priv, napi);
> > +
> > + status = mtk_mac_intr_read_and_clear(priv);
> > +
> > + /* Clean up TX */
> > + if (status & MTK_MAC_BIT_INT_STS_TNTC)
> > + mtk_mac_tx_complete_all(priv);
> > +
> > + /* Receive up to $budget packets */
> > + if (status & MTK_MAC_BIT_INT_STS_FNRC)
> > + received = mtk_mac_process_rx(priv, budget);
> > +
> > + /* One of the counter reached 0x8000000 - update stats and reset all
> > + * counters.
> > + */
> > + if (status & MTK_MAC_REG_INT_STS_MIB_CNT_TH) {
> > + mtk_mac_update_stats(priv);
> > + mtk_mac_reset_counters(priv);
> > + }
> > +
> > + if (received < budget)
> > + napi_complete_done(napi, received);
> > +
> > + mtk_mac_intr_unmask_all(priv);
> > +
> > + return received;
> > +}
>
> I think you want to leave (at least some of) the interrupts masked
> if your budget is exhausted, to avoid generating unnecessary
> irqs.
>
The networking stack shouldn't queue any new TX packets if the queue
is stopped - is this really worth complicating the code? Looks like
premature optimization IMO.
> It may also be faster to not mask/unmask at all but just
> clear the interrupts that you have finished processing
>
Bart
