linux-kernel - Re: [PATCH] NFC: Driver for Inside Secure MicroRead NFC chip

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Date:	Thu, 10 Mar 2011 17:20:53 +0100
From:	Arnd Bergmann <arnd@...db.de>
To:	Waldemar.Rymarkiewicz@...to.com, matti.j.aaltonen@...ia.com
Cc:	linux-i2c@...r.kernel.org, linux-kernel@...r.kernel.org,
	hthebaud@...idefr.com
Subject: Re: [PATCH] NFC: Driver for Inside Secure MicroRead NFC chip

On Thursday 10 March 2011, Waldemar.Rymarkiewicz@...to.com wrote:

> >What I suggest you do is to work with the maintainers of the existing
> >pn544 driver (Matti and Jari) to create an NFC core library 
> >that takes care of the character device interface and that can 
> >be shared between the two drivers. Instead of each driver 
> >registering a misc device, make it call a 
> >nfc_device_register() function that is implemented in a common module.
> 
> I've been already thinking about that and it's seems like next obvious step.

Ok, cool.

> >mdev, rx_waitq and mutex would go into the common module.
> >I would expect that you also need a tx_waitq. What happens 
> >when the buffer is full?
> 
> Do you mean info->buff ?

Oh, I see you simply do

       ret = i2c_master_send(client, info->buf, len);
       usleep_range(1000, 10000);

and assume that the buffer can always be written within a milisecond,
so you just slow down output enough to never have to worry about it,
right?

A nicer solution would be to have an interrupt driven output
so you know when the i2c buffers have been flushed.

> >Note that the microread_is_busy() logic does not protect you 
> >from having multiple concurrent readers, because multiple 
> >threads may share a single file descriptor.
> 
> It's just used to ensure that only one reader can open the device.
> It's called only in open callback.
> The mutex actually secures concurrent read operations. 

So if having multiple readers is safe (though possibly not
meaningful), I guess you don't really need the microread_is_busy()
logic.

I suppose it doesn't hurt either, it just seems a bit pointless
when it does the right thing most of the time, but not always.

	Arnd
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