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Date:   Thu, 26 Nov 2020 14:31:43 +0100
From:   Wilken Gottwalt <wilken.gottwalt@...teo.net>
To:     Bjorn Andersson <bjorn.andersson@...aro.org>
Cc:     linux-kernel@...r.kernel.org, Ohad Ben-Cohen <ohad@...ery.com>,
        Baolin Wang <baolin.wang7@...il.com>,
        Rob Herring <robh+dt@...nel.org>,
        Maxime Ripard <mripard@...nel.org>,
        Chen-Yu Tsai <wens@...e.org>,
        Jernej Skrabec <jernej.skrabec@...l.net>
Subject: Re: [PATCH 2/2] hwspinlock: add sunxi hardware spinlock support

On Tue, 24 Nov 2020 08:54:25 -0600
Bjorn Andersson <bjorn.andersson@...aro.org> wrote:

> On Mon 23 Nov 12:17 CST 2020, Wilken Gottwalt wrote:
> 
> > On Sat, 21 Nov 2020 23:19:00 -0600
> > Bjorn Andersson <bjorn.andersson@...aro.org> wrote:
> > > > +static int hwlocks_inuse_show(struct seq_file *seqf, void *unused)
> > > > +{
> > > > +	struct sunxi_hwspinlock_data *priv = seqf->private;
> > > > +	int inuse;
> > > > +
> > > > +	/* getting the status of only the main 32 spinlocks is supported */
> > > > +	inuse = hweight32(readl(priv->io_base + SPINLOCK_STATUS_REG));
> > > 
> > > So this returns how many of the locks are taken? How is that useful?
> > 
> > It is a way to see if locks were taken from linux or the arisc core without
> > touching the actual hwspinlock abi or the locks. So it is a nice way to debug
> > hwspinlocks, hence it is part of debugfs.
> > 
> 
> So in a scenario where two remote processors ping-pong the lock between
> them, this will always read 1 and you won't know why?

I know it is not perfect. I will change it to actually report which locks are
taken. And currently the crust firmware does not use the locks and on the
Linux side there are only a handful of driver/components using hwspinlocks,
and none of them are active in a kernel compiled for a H5. So it really is a
nice way to check/debug the hwspinlocks. I already have a simple test running
where crust sets a spinlock and I can see it on Linux without touching the
actuall locks thanks to this status register.

> > > > +	seq_printf(seqf, "%d\n", inuse);
> [..]
> > > > +
> > > > +	return 0;
> > > > +}
> > > > +
> > > > +static const struct of_device_id sunxi_hwspinlock_ids[] = {
> > > > +	{ .compatible = "allwinner,sun8i-hwspinlock", },
> > > > +	{ .compatible = "allwinner,sun50i-hwspinlock", },
> > > > +	{},
> > > > +};
> > > > +MODULE_DEVICE_TABLE(of, sunxi_hwspinlock_ids);
> > > > +
> > > > +static struct platform_driver sunxi_hwspinlock_driver = {
> > > > +	.probe	= sunxi_hwspinlock_probe,
> > > > +	.remove	= sunxi_hwspinlock_remove,
> > > > +	.driver	= {
> > > > +		.name		= DRIVER_NAME,
> > > > +		.of_match_table	= of_match_ptr(sunxi_hwspinlock_ids),
> > > 
> > > Please avoid of_match_ptr, as this will cause warnings about unused
> > > variables when COMPILE_TEST without OF.
> > 
> > So did you mean to leave it out completely?
> > 
> 
> Yes, "worst case" is that you include the reference to
> sunxi_hwspinlock_ids on a build without CONFIG_OF and wasting a little
> bit of memory.
> 
> Using of_match_ptr() with CONFIG_OF=n will result in NULL and as such
> we'll get a compile warning that nothing references sunxi_hwspinlock_ids
> - so then that will have to be marked __maybe_unused, or wrapped in an
> #if...
> 
> So better just leave it as:
> 	.of_match_table = sunxi_hwspinlock_ids,

Thank you for the explanation. I really like to know the details and reasons
behind this.

greetings,
Wilken

> Regards,
> Bjorn

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