Re: [PATCH 3/6] mailbox: Add support for ST's Mailbox IP

Jassi Brar
2015-07-24T17:34:54Z

On Fri, Jul 24, 2015 at 3:06 PM, Lee Jones <lee.jones@linaro.org> wrote:
> On Thu, 23 Jul 2015, Jassi Brar wrote:
>
>> >> > +static void sti_mbox_enable_channel(struct mbox_chan *chan)
>> >> > +{
>> >> > + struct sti_channel *chan_info = chan->con_priv;
>> >> > + struct sti_mbox_device *mdev = chan_info->mdev;
>> >> > + struct sti_mbox_pdata *pdata = dev_get_platdata(mdev->dev);
>> >> > + unsigned int instance = chan_info->instance;
>> >> > + unsigned int channel = chan_info->channel;
>> >> > + unsigned long flags;
>> >> > + void __iomem *base;
>> >> > +
>> >> > + base = mdev->base + (instance * sizeof(u32));
>> >> > +
>> >> Maybe have something simpler like MBOX_BASE(instance)? Or some inline
>> >> function to avoid this 5-lines ritual?
>> >
>> > I've checked and we can't do this, as the we need most (all?) of the
>> > intermediary variables too. No ritual just to get the final variable
>> > for instance.
>> >
>> OK. How about ?
>> #define MBOX_BASE(m, n) ((m)->base + (n) * 4)
>> void __iomem *base = MBOX_BASE(mdev, instance);
>
> Oh, those 5 lines. I thought you meant:
>
> struct sti_channel *chan_info = chan->con_priv;
> struct sti_mbox_device *mdev = chan_info->mdev;
> unsigned int instance = chan_info->instance;
> base = mdev->base + (instance * sizeof(u32));
>
> ... which is why I said that the intermediary variables are required.
>
> Well, I 'can' do that, but it seems to be unnecessarily obfuscating
> what's going on and doesn't actually save any lines.
>
> It's not a point that I consider arguing over though, so if you want
> me to do it, I will. You have the final say here.
>
The macro seems tidier. Just a nit.

>> >> > + spin_lock_irqsave(&sti_mbox_chan_lock, flags);
>> >> > + mdev->enabled[instance] |= BIT(channel);
>> >> > + writel_relaxed(BIT(channel), base + pdata->ena_set);
>> >> > + spin_unlock_irqrestore(&sti_mbox_chan_lock, flags);
>> >> >
>> >> You don't need locking for SET/CLR type registers which are meant for
>> >> when they could be accessed by processors that can not share a lock.
>> >> So maybe drop the lock here and elsewhere.
>> >
>> > From what I can gather, I think we need this locking. What happens if
>> > we get scheduled between setting the enabled bit in our cache and
>> > actually setting the ena_set bit? We would be out of sync.
>> >
>> IIU what you mean... can't that still happen because of the _relaxed()?
>
> Not sure what you mean. The _relaxed variant merely omit some IO
> barriers.
>
By the time you exit the spinlock the write may still haven't been
effected. Maybe use writel() there.

>> And maybe embed sti_mbox_chan_lock inside sti_mbox_device.
>
> Not sure this is required. I can find >600 instances of others using
> spinlocks as static globals.
>
And there should be >600 instances of *static* globals that are
protected by some static spinlock ;)

Here the static sti_mbox_chan_lock protects sti_mbox_device which is
allocated during probe. I hope you agree that the standard practice is
to make the lock a member of the same structure that it protects.
Otherwise it gives the wrong impression that the same lock will be
used for any number of allocated mailbox instances.

cheers.