Re: [PATCH RFC] regmap: maple: Switch to use irq-safe locking

[Andy Yan <andy.yan@rock-chips.com>]

Hi Mark and Cristian,
also cc Sasha,

On 8/15/24 03:04, Mark Brown wrote:
> On Wed, Aug 14, 2024 at 01:20:21AM +0300, Cristian Ciocaltea wrote:
> 
>> Commit 3d59c22bbb8d ("drm/rockchip: vop2: Convert to use maple tree
>> register cache") enabled the use of maple tree register cache in
>> Rockchip VOP2 driver.  However, building the kernel with lockdep support
>> indicates locking rules violation when trying to unload the rockchipdrm
>> module:
> 
>> [ 48.360258] ========================================================
>> [ 48.360829] WARNING: possible irq lock inversion dependency detected
>> [ 48.361400] 6.11.0-rc1 #40 Not tainted
>> [ 48.361743] --------------------------------------------------------
>> [ 48.362311] modprobe/685 just changed the state of lock:
>> [ 48.362790] ffff0000087fa798 (&mt->ma_lock){+...}-{2:2}, at: regcache_maple_exit+0x6c/0xe0
> 
> Please think hard before including complete backtraces in upstream
> reports, they are very large and contain almost no useful information
> relative to their size so often obscure the relevant content in your
> message. If part of the backtrace is usefully illustrative (it often is
> for search engines if nothing else) then it's usually better to pull out
> the relevant sections.
> 
>> The problem is that the regmap lock could be taken by an IRQ context,
>> interrupting the irq-unsafe maple tree lock, which may result in a lock
>> inversion deadlock scenario.
> 
>> Switch to use irq-safe locking in the maple tree register cache.
> 
> I'd have a bigger question here which is why the driver is using a
> dynamically allocated register cache in a hardirq context, especially
> with no defaults provided?  Anything except the flat cache might do

Do you mean the current code we call devm_regmap_init_mmio in vop2_bind function ?

> allocations at runtime which might include in interrupt context unless
> the caller is very careful and since the lockdep warning triggered it's
> clear that this driver isn't.  The core will be doing atomic allocations
> for MMIO but that's not something we want to be doing as a matter of
> course...  I would generally expect drivers to try to ensure that any
> registers are cached outside of the interrupt handler, usually by
> specifying defaults or touching all registers during setup.
> 
> Without having done a full analysis it also looks like the marking of
> volatile registers isn't right, it's not immediately clear that the
> interrupt status and clear registers are volatile and they ought to be.
> None of the registers accessed in interrupt context look like they
> should be cached at all unless there's something triggered via the DRM
> vblank calls.
> 

I think the interrupt status and clear registers should also be marked as volatile.
But this is not releated to the current issue, right?
> It might be safer to fall back to the rbtree cache for this device since
> rbtree doesn't force an extra level of locking on us, though like I say

I also think fall back to rbtree would work. I had a similar thought the first
time I encountered this issue[0]. But i try to  keep maple tree as is based
on a much more modern data structure.

> I'm not convinced that what the driver is doing with caching is a super
> good idea.  Though probably what the driver is doing should work.
The registers of VOP are quite special: Each write operation to the register
does not take effect immediately, it only take effect after the next VBLANK if
we write the CFGONE register.
So we need a cache to record what we wrote to register before.

> 


[0]https://patchwork.kernel.org/project/linux-rockchip/patch/20231217084415.2373043-1-andyshrk@163.com/

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