linux-kernel - Re: [PATCH] usb: ftdi-elan: Remove deprecated create_singlethread

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Date:	Tue, 26 Jul 2016 11:34:21 -0400
From:	Tejun Heo <tj@...nel.org>
To:	Bhaktipriya Shridhar <bhaktipriya96@...il.com>
Cc:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Saurabh Karajgaonkar <skarajga@...teon.com>,
	linux-usb@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] usb: ftdi-elan: Remove deprecated
 create_singlethread_workqueue

On Tue, Jul 26, 2016 at 10:47:20AM +0530, Bhaktipriya Shridhar wrote:
> The status workqueue is involved in initializing the Uxxx and polling
> the Uxxx until a supported PCMCIA CardBus device is detected.
> It then starts the command and respond workqueues and then loads the
> module that handles the device, after which it just polls the Uxxx
> looking for card ejects.
> 
> The command and respond workqueues are involved in implementing a command
> sequencer for communicating with the firmware on the other side of
> the FTDI chip in the Uxxx.
> 
> These workqueues have only a single work item each and hence they do not
> require ordering. Also, none of the above workqueues are being used on a
> memory recliam path. Hence, the singlethreaded workqueues have been
> replaced with the use of system_wq.
> 
> System workqueues have been able to handle high level of concurrency
> for a long time now and hence it's not required to have a singlethreaded
> workqueue just to gain concurrency. Unlike a dedicated per-cpu workqueue
> created with create_singlethread_workqueue(), system_wq allows multiple
> work items to overlap executions even on the same CPU; however, a
> per-cpu workqueue doesn't have any CPU locality or global ordering
> guarantee unless the target CPU is explicitly specified and thus the
> increase of local concurrency shouldn't make any difference.
> 
> The work items have been sync cancelled because they are self-requeueing
> and need to wait for the in-flight work item to finish before proceeding
> with destruction. Hence, they have been sync cancelled in
> ftdi_status_cancel_work(), ftdi_command_cancel_work() and
> ftdi_response_cancel_work(). These functions are called in
> ftdi_elan_exit() to ensure that there are no pending work items while
> disconnecting the driver.
> 
> Signed-off-by: Bhaktipriya Shridhar <bhaktipriya96@...il.com>

Acked-by: Tejun Heo <tj@...nel.org>

Thanks.

-- 
tejun