linux-kernel - Re: [PATCH 08/18] soc: qcom: ipa: the generic software interface

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Date:   Wed, 15 May 2019 12:47:58 +0200
From:   Arnd Bergmann <arnd@...db.de>
To:     Alex Elder <elder@...aro.org>
Cc:     David Miller <davem@...emloft.net>,
        Bjorn Andersson <bjorn.andersson@...aro.org>,
        Ilias Apalodimas <ilias.apalodimas@...aro.org>,
        syadagir@...eaurora.org, mjavid@...eaurora.org,
        evgreen@...omium.org, Ben Chan <benchan@...gle.com>,
        Eric Caruso <ejcaruso@...gle.com>, abhishek.esse@...il.com,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 08/18] soc: qcom: ipa: the generic software interface

On Sun, May 12, 2019 at 3:25 AM Alex Elder <elder@...aro.org> wrote:

The per-event interrupt handling seems to be more complex than
necessary:

> +/* Enable or disable an event interrupt */
> +static void
> +_gsi_irq_control_event(struct gsi *gsi, u32 evt_ring_id, bool enable)
> +{
> +       u32 mask = BIT(evt_ring_id);
> +       u32 val;
> +
> +       if (enable)
> +               gsi->event_enable_bitmap |= mask;
> +       else
> +               gsi->event_enable_bitmap &= ~mask;
> +
> +       val = gsi->event_enable_bitmap;
> +       iowrite32(val, gsi->virt + GSI_CNTXT_SRC_IEOB_IRQ_MSK_OFFSET);
> +}
> +
> +static void gsi_irq_enable_event(struct gsi *gsi, u32 evt_ring_id)
> +{
> +       _gsi_irq_control_event(gsi, evt_ring_id, true);

You maintain a bitmap here of the enabled-state, and have
to use a spinlock to ensure that the two are in sync.

> +/* Returns true if the interrupt state (enabled or not) changed */
> +static bool gsi_channel_intr(struct gsi_channel *channel, bool enable)
> +{
> +       u32 evt_ring_id = channel->evt_ring_id;
> +       struct gsi *gsi = channel->gsi;
> +       u32 mask = BIT(evt_ring_id);
> +       unsigned long flags;
> +       bool different;
> +       u32 enabled;
> +
> +       spin_lock_irqsave(&gsi->spinlock, flags);
> +
> +       enabled = gsi->event_enable_bitmap & mask;
> +       different = enable == !enabled;
> +
> +       if (different) {
> +               if (enabled)
> +                       gsi_irq_disable_event(channel->gsi, evt_ring_id);
> +               else
> +                       gsi_irq_enable_event(channel->gsi, evt_ring_id);
> +       }
> +
> +       spin_unlock_irqrestore(&gsi->spinlock, flags);
> +
> +       return different;
> +}

This gets called for each active channel, so you repeatedly
have to get the spinlock and read the irq-enabled state for it.

> +static void gsi_isr_ieob(struct gsi *gsi)
> +{
> +       u32 evt_mask;
> +
> +       evt_mask = ioread32(gsi->virt + GSI_CNTXT_SRC_IEOB_IRQ_OFFSET);
> +       evt_mask &= ioread32(gsi->virt + GSI_CNTXT_SRC_IEOB_IRQ_MSK_OFFSET);
> +       iowrite32(evt_mask, gsi->virt + GSI_CNTXT_SRC_IEOB_IRQ_CLR_OFFSET);
> +
> +       while (evt_mask) {
> +               u32 evt_ring_id = __ffs(evt_mask);
> +
> +               evt_mask ^= BIT(evt_ring_id);
> +
> +               gsi_event_handle(gsi, evt_ring_id);
> +       }
> +}

However, you start out by clearing all bits here.

Why not skip the clearing and and leave the interrupts enabled,
while moving the GSI_CNTXT_SRC_IEOB_IRQ_CLR_OFFSET
write (for a single channel that was completed) to the end of
gsi_channel_poll()?

Something like

static void gsi_isr_ieob(struct gsi *gsi)
{
      u32 evt_mask;

      evt_mask = ioread32(gsi->virt + GSI_CNTXT_SRC_IEOB_IRQ_OFFSET);
      while (evt_mask) {
               u32 evt_ring_id = __ffs(evt_mask);
               evt_mask ^= BIT(evt_ring_id);

               napi_schedule(gsi->evt_ring[evt_ring_id].channel.napi);
      }
}

I also removed the GSI_CNTXT_SRC_IEOB_IRQ_MSK_OFFSET
read here, as that is probably more expensive than calling napi_schedule()
for a channel that is already scheduled. Most of the time, I'd expect the
interrupt to only signal a single channel anyway.

        Arnd