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 23:57:19 +0900
From:	Andi Shyti <andi@...zian.org>
To:	Sean Young <sean@...s.org>
Cc:	Andi Shyti <andi.shyti@...sung.com>,
	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 Sean,

> > > > +	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.

no, actually it's just dirty data and laziness, a memset to 0
fixes it :)

> > 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.

yes, as I said, it's not a big thing, I can remove the mutex.

Thanks,
Andi