linux-kernel - Re: [PATCH 1/1] regmap: regcache-rbtree: Use GFP

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Message-ID: <3879923.hy2mOjvtRz@ws-stein>
Date:	Mon, 20 Jul 2015 09:04:05 +0200
From:	Alexander Stein <alexander.stein@...tec-electronic.com>
To:	Mark Brown <broonie@...nel.org>
Cc:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/1] regmap: regcache-rbtree: Use GFP_ATOMIC when using spinlocks

On Thursday 16 July 2015 20:06:57, Mark Brown wrote:
> On Thu, Jul 16, 2015 at 05:48:52PM +0200, Alexander Stein wrote:
> 
> > When regmap locking is done using spinlocks (e.g. using
> > devm_regmap_init_mmio_clk) access is protected using spin_lock_irqsave.
> > So when calling regmap_write the first time and a node is about to be
> > inserted kzalloc must not be called with GFP_KERNEL. At this point
> 
> The expectation here is that we should either be using no or a flat
> cache here or (if we're using rbtree) providing register defaults to
> ensure that we never do allocations in the spinlock.  The rbtree code is
> written on the assumption that we only have to be faster than reading
> from a serial bus so I'd be worried about it not behaving at all nicely
> in a spinlock even ignoring this issue.

AFAICS even a flat cache seems also only be usefull when providing defaults, no? (Or having volatile registers).
So how to handle this properly? Bail out, if fast_io is available and cache_type != (REGCACHE_NONE || REGCACHE_FLAT)?

> Why are you using a dynamically allocated rbtree for a device like this?

On my way home, I came to the same question. In fact this is not a driver written by myself, but from here http://git.freescale.com/git/cgit.cgi/ppc/sdk/linux.git/tree/drivers/video/fsl-dcu-fb.c#n1024.
I guess as this is a mmio device and things like regcache_cache_only() are used, REGCACHE_FLAT seems appropriate.

> > interrupts are disabled. This fixes the following warning:
> > [    8.605433] WARNING: CPU: 0 PID: 130 at kernel/lockdep.c:2740 lockdep_trace_alloc+0x124/0x128()
> > [    8.614096] DEBUG_LOCKS_WARN_ON(irqs_disabled_flags(flags))
> 
> Please don't paste entire backtraces into commit messages, they're
> enormous and don't add any value.  If you feel a backtrace is useful
> edit down the relevant context.

Thanks, noted.

Best regards,
Alexander
-- 
Dipl.-Inf. Alexander Stein
SYS TEC electronic GmbH
alexander.stein@...tec-electronic.com

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