linux-kernel - Re: [PATCH 1/3] picolcd: driver for PicoLCD HID device

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Message-ID: <7316226fbd1cb53b7f90965c54acd343.squirrel@intranet.cs.nmsu.edu>
Date:	Thu, 25 Feb 2010 08:34:42 -0700
From:	"Rick L. Vinyard, Jr." <rvinyard@...nmsu.edu>
To:	"Jiri Kosina" <jkosina@...e.cz>
Cc:	"Oliver Neukum" <oliver@...kum.org>,
	Bruno PrÃ©mont <bonbons@...ux-vserver.org>,
	linux-input@...r.kernel.org, linux-usb@...r.kernel.org,
	linux-fbdev@...r.kernel.org, linux-kernel@...r.kernel.org,
	"Nicu Pavel" <npavel@...ner.com>
Subject: Re: [PATCH 1/3] picolcd: driver for PicoLCD HID device

Jiri Kosina wrote:
> On Wed, 24 Feb 2010, Rick L. Vinyard, Jr. wrote:
>
>> The key difference is the replacement of spin_lock() with spin_trylock()
>> such that if the non-interrupt code has already obtained the lock, the
>> interrupt will not deadlock but instead take the else path and schedule
>> a
>> framebuffer update at the next interval.
>
> Why is _irqsave() and/or deferred work not enough? The aproach with
> _trylock() seems to be overly complicated for no good reason (I personally
> become very suspicious every time I see code that is using _trylock()).
>

I was concerned about _irqsave() because the lock is split across two
functions to protect the urb after it is handed off to the usb subsystem
with usb_submit_urb(). It's locked in g13_fb_send() and unlocked in the
urb completion callback.

As for deferred work, the g13_fb_send() is the I/O portion of the deferred
framebuffer callback. I was concerned that without a lock one deferred
update could hand the urb off to the usb subsystem and a second could try
to access it before it was handed back to the driver.

In this case the _trylock() would fail and in the else patch we would
defer yet again until the next update cycle.

I took this approach because usb_interrupt_msg() couldn't be used from an
interrupt context, such as a resume hook because eventually down the chain
it does a wait_for_completion_timeout().

It has the added benefit of reusing the urb instead of creating a new one
for each framebuffer sent out, but that wasn't a reason... just a side
effect.

The downside is that I had to manage the urb.

One thing I could do is forget about directly calling g13_fb_send() from
any context and instead use the deferred framebuffer workqueue.

That's probably a simpler approach anyway.

> [ by the way, Rick, are you planning to resubmit the G13 driver with
> incorporated feedback from the last review round? ]
>

Yes. I just wanted to get the details of suspend/resume worked out before
resubmitting.

-- 

Rick

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