Date:   Tue, 24 Nov 2020 15:28:21 +0100
From:   Maxime Ripard <maxime@...no.tech>
To:     Samuel Holland <samuel@...lland.org>
Cc:     Wilken Gottwalt <wilken.gottwalt@...teo.net>,
        linux-kernel@...r.kernel.org, Ohad Ben-Cohen <ohad@...ery.com>,
        Bjorn Andersson <bjorn.andersson@...aro.org>,
        Baolin Wang <baolin.wang7@...il.com>,
        Rob Herring <robh+dt@...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 Mon, Nov 23, 2020 at 09:35:52PM -0600, Samuel Holland wrote:
> On 11/23/20 12:32 PM, Wilken Gottwalt wrote:
> > On Sat, 21 Nov 2020 17:44:18 +0100
> > Maxime Ripard <maxime@...no.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@...no.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
> 
> Crust does not yet support the H2+/H3 (it is active WIP). H5 should work
> well.
> 
> > 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.
> 
> Correct. There is currently no hwspinlock driver in crust. For testing,
> you can poke MMIO from the main loop, near the call to scpi_poll() in
> common/system.c. You can use the timeout.h functions for timing.

Yeah, that would be enough for me. We only really need to make sure that
the concurrency is properly handled to merge the driver

> If you want to write a full driver, I would like to know how you expect
> to use the hwspinlocks. Allocating the locks has to be coordinated among
> all of the users: Linux, U-Boot, crust, any other ARISC firmware, etc.

while that can come as a second step and I wouldn't make it a
requirement for Linux to merged the hwspinlock driver.

Maxime

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