linux-kernel - Re: [RFC 7/7] [media] rc: add support for IR LEDs driven through SPI

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Date:	Thu, 21 Jul 2016 11:22:09 +0100
From:	Sean Young <sean@...s.org>
To:	Andi Shyti <andi.shyti@...sung.com>
Cc:	Mauro Carvalho Chehab <mchehab@....samsung.com>,
	linux-media@...r.kernel.org, linux-kernel@...r.kernel.org,
	Andi Shyti <andi@...zian.org>
Subject: Re: [RFC 7/7] [media] rc: add support for IR LEDs driven through SPI

Hi Andi,

On Thu, Jul 21, 2016 at 10:09:26AM +0900, Andi Shyti wrote:
> > > +	ret = regulator_enable(idata->regulator);
> > > +	if (ret)
> > > +		return ret;
> > > +
> > > +	mutex_lock(&idata->mutex);
> > > +	idata->xfer.len = n;
> > > +	idata->xfer.tx_buf = buffer;
> > > +	mutex_unlock(&idata->mutex);
> > 
> > I'm not convinced the locking works here. You want to guard against 
> > someone modifying xfer while you are sending (so in spi_sync_transfer), 
> > which this locking is not doing. You could declare a 
> > local "struct spi_transfer xfer" and avoid the mutex altogether.
> 
> I cannot declare xfer locally because the spi framework needs
> a statically allocated xfer, so that either I dynamically
> allocate it in the function or I declare it global in idata.

It can be stack allocated for sync transfers. You might want to lock
the spi bus.

> With the mutex I would like to prevent different tasks to change
> the value at the same time, it's an easy case, it shouldn't make
> much difference.

That's cargo-cult locking. It does not achieve anything.


Sean