linux-kernel - Re: [PATCH 1/2] usb: chipidea: Reduce ULPI PHY reset pulse to datasheet spec of 1us

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Date:	Mon, 06 Jul 2015 14:54:07 +0200
From:	Mike Looijmans <mike.looijmans@...ic.nl>
To:	David Laight <David.Laight@...LAB.COM>,
	"'Peter Chen'" <peter.chen@...escale.com>
CC:	"lars@...afoo.de" <lars@...afoo.de>,
	"linux-usb@...r.kernel.org" <linux-usb@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/2] usb: chipidea: Reduce ULPI PHY reset pulse to datasheet
 spec of 1us

On 02-07-15 11:39, David Laight wrote:
> From: Peter Chen
>> Sent: 30 June 2015 03:06
>> On Fri, Jun 26, 2015 at 03:47:03PM +0200, Mike Looijmans wrote:
>>> The datasheet for the 334x PHY mentions that a reset can be performed:
>>> "... by bringing the pin low for a minimum of 1 microsecond and
>>> then high."
>>> A delay of 5ms to implement that seems overly long, so reduce it to
>>> just 1us.
>>> As for the delay after reset, the datasheet only mentioned that the
>>> chip will assert the DIR output. 1ms seems like a safe time to wait
>>> for that to happen, so no change there.
>>>
>>> Signed-off-by: Mike Looijmans <mike.looijmans@...ic.nl>
>>> ---
>>>   drivers/usb/chipidea/core.c | 2 +-
>>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>>
>>> diff --git a/drivers/usb/chipidea/core.c b/drivers/usb/chipidea/core.c
>>> index e970863..c865abe 100644
>>> --- a/drivers/usb/chipidea/core.c
>>> +++ b/drivers/usb/chipidea/core.c
>>> @@ -664,7 +664,7 @@ static int ci_hdrc_create_ulpi_phy(struct device *dev, struct ci_hdrc *ci)
>>>   			dev_err(dev, "Failed to request ULPI reset gpio: %d\n", ret);
>>>   			return ret;
>>>   		}
>>> -		msleep(5);
>>> +		udelay(1);
>
> If the spec says 1us I'd delay for longer just to make sure.
> And add a comment saying that the reset needs to be 1us long.

Why? It's not as if the chip would reset any "better" if you pull the line 
longer. For some of these USB PHYs, the internal regulators and other analog 
parts will be disabled as well and the longer you keep it in reset, the longer 
it will take the device to start back up again.
The 1us value already takes into account a generous compensation for 
temperature and other factors, and probably the reset will work just fine even 
if you only use half the time.
Even if I'd agree with you here, I think you'll also have to admit that 5000x 
the value in the datasheet might be a bit over the top.

M.
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