linux-kernel - Re: [PATCH V1 3/4] usb: serial: f81534: add output pin control

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Message-ID: <20180102092745.GG16993@localhost>
Date:   Tue, 2 Jan 2018 10:27:45 +0100
From:   Johan Hovold <johan@...nel.org>
To:     "Ji-Ze Hong (Peter Hong)" <hpeter@...il.com>
Cc:     Johan Hovold <johan@...nel.org>, gregkh@...uxfoundation.org,
        linux-usb@...r.kernel.org, linux-kernel@...r.kernel.org,
        "Ji-Ze Hong (Peter Hong)" <hpeter+linux_kernel@...il.com>
Subject: Re: [PATCH V1 3/4] usb: serial: f81534: add output pin control

On Tue, Jan 02, 2018 at 11:24:26AM +0800, Ji-Ze Hong (Peter Hong) wrote:
> Hi Johan,
> 
> >> In this code, I'm only read/write 3 registers of 0x2ae8, 0x2a90, 0x2a80,
> >> but some register will read/write more than once. Should I change the
> >> code from port_probe() to attach() and re-write it as:
> >> 	1: read the 3 register
> >> 	2: change them will 12 pin desire value
> >> 	3: write it back
> >> Is it ok?
> > 
> > Do you expect these pins to ever be changed after probe? If not, then
> > perhaps it can be moved to attach(), but otherwise I guess they should
> > be set at port_probe(). By using shadow registers, you should be able to
> > reduce the number of device accesses, but perhaps it's not worth the
> > complexity.
> > 
> > Do you have a rough idea about how long these register updates take? I
> > was just worried that these changes will add up to really long probe
> > times.
> > 
> 
> I had measured the time of the loop in f81534_set_port_output_pin() via
> getnstimeofday() with 685.410 ~ 3681.682us per port, but normally with
> 600~800us per port. So I prefer remain the current method of
> f81534_set_port_output_pin(). Is it ok?

That should be fine. Thanks for verifying.

Johan