From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Rick L. Vinyard, Jr." Subject: Re: [PATCH 1/3] picolcd: driver for PicoLCD HID device Date: Thu, 25 Feb 2010 08:34:42 -0700 Message-ID: <7316226fbd1cb53b7f90965c54acd343.squirrel@intranet.cs.nmsu.edu> References: <20100221002001.0a7e05a7@neptune.home> <20100224170049.0d04af3c@neptune.home> <201002241927.53532.oliver@neukum.org> Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7BIT Return-path: Received: from mail.cs.nmsu.edu ([128.123.64.3]:41008 "EHLO mail.cs.nmsu.edu" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1759190Ab0BYPfF (ORCPT ); Thu, 25 Feb 2010 10:35:05 -0500 In-Reply-To: Sender: linux-input-owner@vger.kernel.org List-Id: linux-input@vger.kernel.org To: Jiri Kosina Cc: Oliver Neukum , Bruno =?iso-8859-1?Q?Pr=C3=A9mont?= , linux-input@vger.kernel.org, linux-usb@vger.kernel.org, linux-fbdev@vger.kernel.org, linux-kernel@vger.kernel.org, Nicu Pavel 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