linux-kernel - Re: [PATCH v5 1/4] mfd: add support for Diolan DLN-2 devices

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Date:	Wed, 24 Sep 2014 17:54:15 +0300
From:	Octavian Purdila <octavian.purdila@...el.com>
To:	Johan Hovold <johan@...nel.org>
Cc:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Linus Walleij <linus.walleij@...aro.org>,
	Alexandre Courbot <gnurou@...il.com>, wsa@...-dreams.de,
	Samuel Ortiz <sameo@...ux.intel.com>,
	Lee Jones <lee.jones@...aro.org>,
	Arnd Bergmann <arnd@...db.de>,
	Daniel Baluta <daniel.baluta@...el.com>,
	Laurentiu Palcu <laurentiu.palcu@...el.com>,
	linux-usb@...r.kernel.org, lkml <linux-kernel@...r.kernel.org>,
	"linux-gpio@...r.kernel.org" <linux-gpio@...r.kernel.org>,
	linux-i2c@...r.kernel.org
Subject: Re: [PATCH v5 1/4] mfd: add support for Diolan DLN-2 devices

On Wed, Sep 24, 2014 at 4:54 PM, Johan Hovold <johan@...nel.org> wrote:
> On Wed, Sep 24, 2014 at 04:36:22PM +0300, Octavian Purdila wrote:
>> On Wed, Sep 24, 2014 at 1:48 PM, Johan Hovold <johan@...nel.org> wrote:
>> > On Fri, Sep 19, 2014 at 11:22:42PM +0300, Octavian Purdila wrote:
>>
>> <snip>
>>
>> >> + * dln2_dev.mod_rx_slots and then the echo header field to index the
>> >> + * slots field and find the receive context for a particular
>> >> + * request.
>> >> + */
>> >> +struct dln2_mod_rx_slots {
>> >> +     /* RX slots bitmap */
>> >> +     unsigned long bmap;
>> >> +
>> >> +     /* used to wait for a free RX slot */
>> >> +     wait_queue_head_t wq;
>> >> +
>> >> +     /* used to wait for an RX operation to complete */
>> >> +     struct dln2_rx_context slots[DLN2_MAX_RX_SLOTS];
>> >> +
>> >> +     /* device has been disconnected */
>> >> +     bool disconnected;
>> >
>> > This belongs in the dln2_dev struct.
>> >
>> > I think you're overcomplicating the disconnect handling by intertwining
>> > it with your slots.
>> >
>> > Add a lock, an active-transfer counter, a disconnected flag, and a wait
>> > queue to struct dln2_dev.
>> >
>>
>> I agree that disconnected is better suited in dln2_dev.
>>
>> However, I don't think that we need the active-transfer counter and a
>> new wait queue. We can simply use the existing waiting queues and the
>> implicit alloc_rx_slot()/free_rx_slot() calls to see if we are still
>> waiting for I/O.
>
> Just because you can reuse them doesn't mean it's a good idea. By
> separating a generic disconnect solution from your custom slot
> implementation we get something that is way easier to verify for
> correctness and that could also be reused in other drivers.
>

Maybe I miss-understood what you are proposing, let me try to
summarize it to see if I got it right.

You are suggesting to add a counter, increment it in alloc_rx_slot(),
decrement it in free_rx_slot(). Then add a new waitqueue in dln2_dev
and in free_rx_slot() wake it up while in disconnect do a wait_event()
on it and check for the counter. Also, alloc_rx_slot() should fail if
the disconnect flag is set.

In this case we are still coupled to the slots implementation, in the
sense that you would need to understand the slots implementation to
understand how the disconnect works. We are also doing two wake-up
operations which I find redundant and which does not add much value in
clarity (since we still need to wake-up all completions for each
handle).

I do agree that using a counter instead of checking the bitmaps is
cleaner though.
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