linux-kernel - Re: [alsa-devel] [PATCH] ASoC: mediatek: Use current HW pointer for pointer callback

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Date:	Thu, 03 Dec 2015 16:33:56 +0100
From:	Takashi Iwai <tiwai@...e.de>
To:	Mark Brown <broonie@...nel.org>
Cc:	Koro Chen <koro.chen@...iatek.com>, lgirdwood@...il.com,
	alsa-devel@...a-project.org, srv_heupstream@...iatek.com,
	s.hauer@...gutronix.de, linux-kernel@...r.kernel.org,
	linux-mediatek@...ts.infradead.org,
	linux-arm-kernel@...ts.infradead.org
Subject: Re: [alsa-devel] [PATCH] ASoC: mediatek: Use current HW pointer for pointer callback

On Thu, 03 Dec 2015 15:56:12 +0100,
Mark Brown wrote:
> 
> On Thu, Dec 03, 2015 at 12:07:26PM +0100, Takashi Iwai wrote:
> > Mark Brown wrote:
> > > On Thu, Dec 03, 2015 at 10:41:38AM +0100, Takashi Iwai wrote:
> 
> > > > While reading this patch, I wondered how regmap can be used safely in
> > > > an irq-disabled context.  Mark, do we have any API for that?
> 
> > > We can use user supplied locks or spin_lock_irqsave().
> 
> > I meant how to guarantee to make regmap_read() working in an already
> > spin-locked context, typically in an irq handler?  regmap_read()
> > involves with the regcache and it may invoke kmalloc().
> 
> I know that's what you meant - that should be done by preallocating the
> cache (which can be done with defaults) and providing your own lock
> if there's a spinlock already held (since we use _irqsave() which IIRC
> isn't nestable).

The extra lock should be fine unless any mutex is involved silently in
regmap code.  But preallocation sounds not so intuitive.

> We can also use GFP_ATOMIC for some of the allocations
> in reasonable use cases but it's not in general supported.

Yeah, but we have already map->alloc_flags, so we can use it
appropriately as a fallback?


Takashi
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