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Date:   Mon, 23 Nov 2020 19:32:06 +0100
From:   Wilken Gottwalt <wilken.gottwalt@posteo.net>
To:     Maxime Ripard <maxime@cerno.tech>
Cc:     linux-kernel@vger.kernel.org, Ohad Ben-Cohen <ohad@wizery.com>,
        Bjorn Andersson <bjorn.andersson@linaro.org>,
        Baolin Wang <baolin.wang7@gmail.com>,
        Rob Herring <robh+dt@kernel.org>, Chen-Yu Tsai <wens@csie.org>,
        Jernej Skrabec <jernej.skrabec@siol.net>
Subject: Re: [PATCH 2/2] hwspinlock: add sunxi hardware spinlock support
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On Sat, 21 Nov 2020 17:44:18 +0100
Maxime Ripard <maxime@cerno.tech> wrote:

> On Sat, Nov 21, 2020 at 08:22:55PM +0800, fuyao wrote:
> > On Fri, Nov 20, 2020 at 05:42:31PM +0100, Maxime Ripard wrote:
> > > Hi,
> > > 
> > > On Thu, Nov 19, 2020 at 11:13:43AM +0100, Wilken Gottwalt wrote:
> > > > On Thu, 19 Nov 2020 08:15:23 +0100
> > > > Maxime Ripard <maxime@cerno.tech> wrote:
> > > > > > can you help me here a bit? I still try to figure out how to do patch sets
> > > > > > properly. Some kernel submitting documentation says everything goes into the
> > > > > > coverletter and other documentation only tells how to split the patches. So
> > > > > > what would be the right way? A quick example based on my patch set would be
> > > > > > really helpful.
> > > > > 
> > > > > I mean, the split between your patches and so on is good, you got that right
> > > > > 
> > > > > The thing I wanted better details on is the commit log itself, so the
> > > > > message attached to that patch.
> > > > 
> > > > Ah yes, I think I got it now. So basically add a nice summary of the coverletter
> > > > there.
> > > 
> > > Yes, a bit more context as well. Eventually, this should be the
> > > motivation on why this patch is useful. So what it can be used for, what
> > > are the challenges, how it was tested, etc.
> > > 
> > > The cover letter is usually here more to provide some meta-context: what
> > > you expect from the maintainers / reviewers if it's an RFC, if there's
> > > any feature missing or that could be added later on, etc.
> > > 
> > > > > > > Most importantly, this hwspinlock is used to synchronize the ARM cores
> > > > > > > and the ARISC. How did you test this driver?
> > > > > > 
> > > > > > Yes, you are right, I should have mentioned this. I have a simple test kernel
> > > > > > module for this. But I must admit, testing the ARISC is very hard and I have
> > > > > > no real idea how to do it. Testing the hwspinlocks in general seems to work
> > > > > > with my test kernel module, but I'm not sure if this is really sufficient. I
> > > > > > can provide the code for it if you like. What would be the best way? Github?
> > > > > > Just mailing a patch?
> > > > > > 
> > > > > > The test module produces these results:
> > > > > > 
> > > > > > # insmod /lib/modules/5.9.8/kernel/drivers/hwspinlock/sunxi_hwspinlock_test.ko 
> > > > > > [   45.395672] [init] sunxi hwspinlock test driver start
> > > > > > [   45.400775] [init] start test locks
> > > > > > [   45.404263] [run ] testing 32 locks
> > > > > > [   45.407804] [test] testing lock 0 -----
> > > > > > [   45.411652] [test] taking lock attempt #0 succeded
> > > > > > [   45.416438] [test] try taken lock attempt #0
> > > > > > [   45.420735] [test] unlock/take attempt #0
> > > > > > [   45.424752] [test] taking lock attempt #1 succeded
> > > > > > [   45.429556] [test] try taken lock attempt #1
> > > > > > [   45.433823] [test] unlock/take attempt #1
> > > > > > [   45.437862] [test] testing lock 1 -----
> > > > > 
> > > > > That doesn't really test for contention though, and dealing with
> > > > > contention is mostly what this hardware is about. Could you make a small
> > > > > test with crust to see if when the arisc has taken the lock, the ARM
> > > > > cores can't take it?
> > > > 
> > > > So the best solution would be to write a bare metal program that runs on the
> > > > arisc and can be triggered from the linux side (the test kernel module) to take
> > > > a spinlock ... or at least take spinlocks periodically for a while and watch it
> > > > on the linux side. Okay, I think I can do this. Though, I have to dig through
> > > > all this new stuff first.
> > > 
> > > It doesn't have to be super complicated, just a loop that takes a lock,
> > > sleeps for some time, and releases the lock should be enough to at least
> > > validate that the lock is actually working
> > >
> >
> > I think the difficulty is the bare metal program in arsic has little
> > documentation.
> 
> crust has mostly figured it out:
> https://github.com/crust-firmware/crust

I actually have serious trouble to get crust running. It compiles for H2+/H3, but
I can't figure out if it runs at all. I will switch to a H5 based device which is
confirmed to work. If I see this correctly crust is doing nothing with spinlocks
yet, so I may end up also working on crust, adding the spinlocks there too. Don't
know yet how long I will take to understand every detail, but I will report
progress.

Greetings,
Wilken