linux-kernel - Re: [PATCH v16 07/14] counter: Add character device interface

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Date:   Mon, 27 Sep 2021 12:20:00 +0100
From:   Jonathan Cameron <Jonathan.Cameron@...wei.com>
To:     William Breathitt Gray <vilhelm.gray@...il.com>
CC:     Jonathan Cameron <jic23@...nel.org>,
        <linux-stm32@...md-mailman.stormreply.com>,
        <kernel@...gutronix.de>, <a.fatoum@...gutronix.de>,
        <kamel.bouhara@...tlin.com>, <gwendal@...omium.org>,
        <alexandre.belloni@...tlin.com>, <david@...hnology.com>,
        <linux-iio@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        <linux-arm-kernel@...ts.infradead.org>, <syednwaris@...il.com>,
        <patrick.havelange@...ensium.com>, <fabrice.gasnier@...com>,
        <mcoquelin.stm32@...il.com>, <alexandre.torgue@...com>,
        <o.rempel@...gutronix.de>, <jarkko.nikula@...ux.intel.com>,
        Dan Carpenter <dan.carpenter@...cle.com>
Subject: Re: [PATCH v16 07/14] counter: Add character device interface

On Mon, 27 Sep 2021 19:21:17 +0900
William Breathitt Gray <vilhelm.gray@...il.com> wrote:

> On Sun, Sep 26, 2021 at 04:15:42PM +0100, Jonathan Cameron wrote:
> > On Mon, 20 Sep 2021 19:09:13 +0900
> > William Breathitt Gray <vilhelm.gray@...il.com> wrote:
> >   
> > > On Sun, Sep 12, 2021 at 05:18:42PM +0100, Jonathan Cameron wrote:  
> > > > On Fri, 27 Aug 2021 12:47:51 +0900
> > > > William Breathitt Gray <vilhelm.gray@...il.com> wrote:
> > > >     
> > > > > This patch introduces a character device interface for the Counter
> > > > > subsystem. Device data is exposed through standard character device read
> > > > > operations. Device data is gathered when a Counter event is pushed by
> > > > > the respective Counter device driver. Configuration is handled via ioctl
> > > > > operations on the respective Counter character device node.
> > > > > 
> > > > > Cc: David Lechner <david@...hnology.com>
> > > > > Cc: Gwendal Grignou <gwendal@...omium.org>
> > > > > Cc: Dan Carpenter <dan.carpenter@...cle.com>
> > > > > Cc: Oleksij Rempel <o.rempel@...gutronix.de>
> > > > > Signed-off-by: William Breathitt Gray <vilhelm.gray@...il.com>    
> > > > 
> > > > Hi William,
> > > > 
> > > > Why the bit based lock?  It feels like a mutex_trylock() type approach or
> > > > spinlock_trylock() would be a more common solution to this problem.
> > > > There is precedence for doing what you have here though so I'm not that
> > > > worried about it.    
> > > 
> > > Hi Jonathan,
> > > 
> > > We originally used a mutex for this, but Jarkko discovered that this
> > > produced a warning because chrdev_lock would be held when returning to
> > > user space:
> > > https://lore.kernel.org/linux-arm-kernel/YOq19zTsOzKA8v7c@shinobu/T/#m6072133d418d598a5f368bb942c945e46cfab9a5
> > > 
> > > Following David Lechner's suggestion, I decided to reimplement
> > > chrdev_lock as a bitmap using an atomic flag.  
> > 
> > Ok.  I'm not sure bit lock was quite what was intended (as there is only one of them)
> > but I suppose it doesn't greatly matter.  
> 
> It didn't cross my mind before, but would declaring chrdev_lock as an
> atomic_t be a more appropriate solution here because we have only one
> flag?
> 
> William Breathitt Gray
> 

It would be less esoteric.  This was the first time I've ever come across the bitlock stuff
whereas atomics are an every day thing.

Thanks,

Jonathan