Subject: Re: [PATCH] net: mdiobus: Prevent spike on MDIO bus reset signal

From: Russell King - ARM Linux admin <[email-masked]>
Date: Tue, 26 Jan 2021 13:49:38 +0000

On Tue, Jan 26, 2021 at 02:14:40PM +0100, Andrew Lunn wrote:
> On Tue, Jan 26, 2021 at 08:33:37AM +0100, Mike Looijmans wrote:
> > The mdio_bus reset code first de-asserted the reset by allocating with
> > GPIOD_OUT_LOW, then asserted and de-asserted again. In other words, if
> > the reset signal defaulted to asserted, there'd be a short "spike"
> > before the reset.
> > 
> > Instead, directly assert the reset signal using GPIOD_OUT_HIGH, this
> > removes the spike and also removes a line of code since the signal
> > is already high.
> 
> Hi Mike
> 
> This however appears to remove the reset pulse, if the reset line was
> already low to start with. Notice you left
> 
> fsleep(bus->reset_delay_us);
> 
> without any action before it? What are we now waiting for?  Most data
> sheets talk of a reset pulse. Take the reset line high, wait for some
> time, take the reset low, wait for some time, and then start talking
> to the PHY. I think with this patch, we have lost the guarantee of a
> low to high transition.
> 
> Is this spike, followed by a pulse actually causing you problems? If
> so, i would actually suggest adding another delay, to stretch the
> spike. We have no control over the initial state of the reset line, it
> is how the bootloader left it, we have to handle both states.

Andrew, I don't get what you're saying.

Here is what happens depending on the pre-existing state of the
reset signal:

Reset (previously asserted):   ~~~|_|~~~~|_______
Reset (previously deasserted): _____|~~~~|_______
                                  ^ ^    ^
                                  A B    C

At point A, the low going transition is because the reset line is
requested using GPIOD_OUT_LOW. If the line is successfully requested,
the first thing we do is set it high _without_ any delay. This is
point B. So, a glitch occurs between A and B.

We then fsleep() and finally set the GPIO low at point C.

Requesting the line using GPIOD_OUT_HIGH eliminates the A and B
transitions. Instead we get:

Reset (previously asserted)  : ~~~~~~~~~~|______
Reset (previously deasserted): ____|~~~~~|______
                                   ^     ^
                                   A     C

Where A and C are the points described above in the code. Point B
has been eliminated.

Therefore, to me the patch looks entirely reasonable and correct.

-- 
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