Date:   Fri, 4 Dec 2020 17:21:17 +0100
From:   Maxime Ripard <maxime@...no.tech>
To:     Wilken Gottwalt <wilken.gottwalt@...teo.net>
Cc:     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 v2 2/2] hwspinlock: add sunxi hardware spinlock support

On Fri, Dec 04, 2020 at 04:35:12PM +0100, Wilken Gottwalt wrote:
> Adds the sunxi_hwspinlock driver for the hardware spinlock unit found in
> most of the sun8i and sun50i based SoCs.
> 
> This unit provides at least 32 spinlocks in hardware. The implementation
> supports 32, 64, 128 or 256 32bit registers, where one lock can be taken
> by reading a register and released by writing a 0 to it. This driver
> supports all 4 spinlock setups, but for now only the first setup (32
> locks) seem to exist in available devices. This spinlock unit is shared
> between all ARM cores and the embedded OpenRisc AR100 core, all of them
> can take/release a lock with a single cycle operation. It can be used to
> sync access to devices shared by the ARM cores and the OpenRisc core.
> 
> There are two ways to check if a lock is taken. The first way is to read
> a lock. If a 0 is returned, the lock was free and is taken now. If an 1
> is returned, the caller has to try again, which means the lock is
> currently taken. The second way is to read a 32bit wide status register
> where every bit represents one of the 32 first locks. According to the
> datasheets this status register supports the 32 first locks only. For
> this reason the lock read/write approach is used in this driver which
> can cover all 256 locks in the biggest setup.
> 
> Being able to use the status register to get knowledge about the locks
> makes it suitable to extended testing. It also can be used to bypass
> the Linux hwspinlock ABI completely and is not able to set locks. This
> is used in one of the tests to show that the driver works correctly.
> 
> To run all tests it is necessary to take locks on the OpenRisc core and
> show on the Linux side that the locks were taken by an external event.
> This can be achived by using the crust firmware. For this the crust
> firmware needs to be changed to take and release spinlocks (a simple
> MMIO operation on the hwlock registers), which is currently not
> supported by the current crust firmware. The necessary crust fork can
> be found here https://github.com/wgottwalt/crust (hwspinlock branch).
> It is also necessary to build u-boot with support for this crust/SCP
> firmware. This u-boot fork can be found here
> https://github.com/crust-firmware/u-boot (crust branch). For testing
> this driver it is also necessary to pick a device that is fully
> supported by crust. In this case a H5 based device works very well. In
> this test a Friendlyarm NanoPi NEO2 was used, which is fully supported
> by u-boot (also the fork) and current Linux kernels. In the crust fork
> it is necessary to go into debug menu of "make nconfig" and select the
> hwspinlock test loop, which uses the timeout functions of the crust
> firmware. It loops through the first 32 spinlocks and takes/releases a
> lock one after another using a timeout which can be set in the debug
> menu.
> 
> Test 1:
> This test was done with a mainline u-boot and a crust enabled u-boot.
> For this a simple second kernel module was used, found here
> https://github.com/wgottwalt/sunxi_hwspinlock/tree/main/test_module.
> 
> If run with mainline u-boot it shows that the Linux side correctly
> takes a lock, tries to recursively take a lock again (which does not
> happen) and releases a lock. Done for all 32 locks several times.
> 
> [  122.518435] [init]--- SUNXI HWSPINLOCK DRIVER TEST ---
> [  122.523810] [run ]--- testing locks 0 to 31 ---
> [  122.528370] [test] testing lock 0
> [  122.531686] [test]+++ attempt #0 succeded
> [  122.535709] [test]+++ attempt #1 succeded
> [  122.539729] [test]+++ attempt #2 succeded
> [  122.543752] [test] testing lock 1
> [  122.547072] [test]+++ attempt #0 succeded
> [  122.551093] [test]+++ attempt #1 succeded
> [  122.555113] [test]+++ attempt #2 succeded
> [  122.559129] [test] testing lock 2
> ...
> [  123.004812] [test] testing lock 31
> [  123.008208] [test]+++ attempt #0 succeded
> [  123.012228] [test]+++ attempt #1 succeded
> [  123.016250] [test]+++ attempt #2 succeded
> 
> Same test run with the hwspinlock test loop enabled in crust. Here the
> test module hits two locks exactly in the moment crust switches from
> lock 7 to 8. The test module code path is slower because of more code
> that needs to be executed which makes this rare scenario possible.
> 
> [  122.620656] [test] testing lock 6
> [  122.623966] [test]+++ attempt #0 succeded
> [  122.627988] [test]+++ attempt #1 succeded
> [  122.632008] [test]+++ attempt #2 succeded
> [  122.636044] [test] testing lock 7
> [  122.639364] [test] taking lock attempt #0 failed (-16)
> [  122.644507] [run ]--- testing specific lock 7 failed (-14) ---
> [  122.650345] [test] testing lock 8
> [  122.653671] [test] taking lock attempt #0 failed (-16)
> [  122.658818] [run ]--- testing specific lock 8 failed (-14) ---
> [  122.664658] [test] testing lock 9
> [  122.667971] [test]+++ attempt #0 succeded
> [  122.671989] [test]+++ attempt #1 succeded
> 
> Test 2:
> This is a more complex test that uses the status register to bypass the
> Linux hwspinlock ABI. For this to work a slightly modified driver is
> used and can be found here
> https://github.com/wgottwalt/sunxi_hwspinlock/tree/main/
> modified_sunxi_hwspinlock
> This modified driver splits the 4K memory range into two and leaves the
> status register untouched, so that it can be used by another test kernel
> module which can be found here
> https://github.com/wgottwalt/sunxi_hwspinlock/tree/main/test2_module
> It is also necessary to change the device tree entries to get both
> kernel modules working in parallel.
> 
> hwspinlock-mod@...8000 {
>         compatible = "allwinner,sun50i-hwspinlock-mod";
>         reg = <0x01c18000 0x4 0x01c18100 0x400>;
>         clocks = <&ccu CLK_BUS_SPINLOCK>;
>         clock-names = "ahb";
>         resets = <&ccu RST_BUS_SPINLOCK>;
>         reset-names = "ahb";
>         status = "okay";
> };
> 
> hwspinlock-stat@...8010 {
>         compatible = "allwinner,sun50i-hwspinlock-stat";
>         reg = <0x01c18010 0x4>;
>         status = "okay";
> };
> 
> The extended test kernel module supports 4 different modes of the test,
> 2 of them are sufficient to show the spinlock mechanism working.
> 
> Mode 1:
> This one reads and prints the status register continuously. The crust
> firmware and the test are set to a hwlock timeout of 1 second. The test
> kernel module code runs a bit slower because of more code executed and
> you can see how one lock is missed between entry 2 and 3.
> 
> > modprobe sunxi_hwspinlock_test2 mode=1 loops=10
> [  187.167074] [init]--- SUNXI HWSPINLOCK DRIVER TEST ---
> [  187.172636] [sreg] 00000000_00010000_00000000_00000000
> [  188.196287] [sreg] 00000000_00001000_00000000_00000000
> [  189.220285] [sreg] 00000000_00000010_00000000_00000000
> [  190.244286] [sreg] 00000000_00000001_00000000_00000000
> [  191.268285] [sreg] 00000000_00000000_10000000_00000000
> [  192.292294] [sreg] 00000000_00000000_01000000_00000000
> [  193.316302] [sreg] 00000000_00000000_00100000_00000000
> [  194.340285] [sreg] 00000000_00000000_00010000_00000000
> [  195.364285] [sreg] 00000000_00000000_00001000_00000000
> [  196.388284] [sreg] 00000000_00000000_00000100_00000000
> 
> Mode 3:
> This mode combines the Linux hwspinlock ABI approach from test 1 and the
> status register access. The "after" reads show the locks taken by the
> Linux driver and the crust firmware.
> 
> [  439.138476] [test] testing lock 13
> [  439.141894] [sreg] before take 00000000_00000100_00000000_00000000
> [  439.148083] [sreg] after take 00000000_00000110_00000000_00000000
> [  439.154189] [sreg] after recursive take 00000000_00000110_00000000_00000000
> [  439.161162] [sreg] after untake 00000000_00000010_00000000_00000000
> [  439.167435] [test]+++ attempt #0 succeded
> [  439.171458] [sreg] before take 00000000_00000010_00000000_00000000
> [  439.177649] [sreg] after take 00000000_00000110_00000000_00000000
> [  439.183751] [sreg] after recursive take 00000000_00000110_00000000_00000000
> [  439.190725] [sreg] after untake 00000000_00000010_00000000_00000000
> [  439.196992] [test]+++ attempt #1 succeded
> [  439.201018] [sreg] before take 00000000_00000010_00000000_00000000
> [  439.207212] [sreg] after take 00000000_00000110_00000000_00000000
> [  439.213322] [sreg] after recursive take 00000000_00000110_00000000_00000000
> [  439.220290] [sreg] after untake 00000000_00000010_00000000_00000000
> [  439.226559] [test]+++ attempt #2 succeded
> [  439.230576] [test] testing lock 14
> [  439.233996] [sreg] before take 00000000_00000010_00000000_00000000
> [  439.240177] [test] taking lock attempt #0 failed (-16)
> [  439.245322] [run ]--- testing specific lock 14 failed (-14) ---
> [  439.251252] [test] testing lock 15
> [  439.254677] [sreg] before take 00000000_00000010_00000000_00000000
> [  439.260865] [sreg] after take 00000000_00000011_00000000_00000000
> [  439.266975] [sreg] after recursive take 00000000_00000011_00000000_00000000
> [  439.273943] [sreg] after untake 00000000_00000010_00000000_00000000
> [  439.280212] [test]+++ attempt #0 succeded
> [  439.284235] [sreg] before take 00000000_00000010_00000000_00000000
> [  439.290428] [sreg] after take 00000000_00000011_00000000_00000000
> [  439.296534] [sreg] after recursive take 00000000_00000011_00000000_00000000
> [  439.303502] [sreg] after untake 00000000_00000010_00000000_00000000
> [  439.309774] [test]+++ attempt #1 succeded

Most of this should be in the cover letter (the details on how to make the tests mostly).

However, I mentioned in your first version that some comments on the
previous drivers posted still applied to you, and you missed those
comments apparently. See here for more details:
https://patchwork.kernel.org/project/linux-arm-kernel/patch/20200210170143.20007-2-nborisov@suse.com/#23148161

Most importantly, the driver name and compatible need to be changed.

Maxime

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