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Message-ID: <954848a9ee2967908bc40069c94cea4407e7de7f.camel@ew.tq-group.com>
Subject: Re: [PATCH 8/8] gpio: tqmx86: fix broken IRQ_TYPE_EDGE_BOTH
 interrupt type
From: Matthias Schiffer <matthias.schiffer@ew.tq-group.com>
To: Andrew Lunn <andrew@lunn.ch>
Cc: Linus Walleij <linus.walleij@linaro.org>, Bartosz Golaszewski
 <brgl@bgdev.pl>, linux-gpio@vger.kernel.org, linux-kernel@vger.kernel.org, 
 Gregor Herburger <gregor.herburger@tq-group.com>, linux@ew.tq-group.com
Date: Wed, 29 May 2024 14:44:30 +0200
In-Reply-To: <9f8107e1-4a80-4f20-8862-f85aed578cc6@lunn.ch>
References: <cover.1716967982.git.matthias.schiffer@ew.tq-group.com>
	 <2c265b6bcfcde7d2327b94c4f6e3ad6d4f1e2de7.1716967982.git.matthias.schiffer@ew.tq-group.com>
	 <9f8107e1-4a80-4f20-8862-f85aed578cc6@lunn.ch>
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On Wed, 2024-05-29 at 14:37 +0200, Andrew Lunn wrote:
> On Wed, May 29, 2024 at 09:45:20AM +0200, Matthias Schiffer wrote:
> > The TQMx86 GPIO controller only supports falling and rising edge
> > triggers, but not both. Fix this by implementing a software both-edge
> > mode that toggles the edge type after every interrupt.
>=20
> Do you have a real use case for this, one that will handle lost
> interrupts because it cannot swap edge quick enough?
>=20
> I personally would not do this, because it is dangerous, it gives the
> impression the device can do both, when in fact it cannot reliably.
>=20
> For me, the correct fix is to return EOPNOTSUPP or EINVAL for BOTH.
>=20

This was the first thing I tried as well, but it seems that supporting IRQ_=
TYPE_EDGE_BOTH is
mandatory for all GPIO drivers (not doing so results in various error messa=
ges when attemting to use
*any* type of interrupt for the GPIO; I don't remember the exact errors). F=
or this reason, several
drivers implement a similar software solution when the hardware doesn't sup=
port it.

Many drivers implement this in a fragile way that will easily break when an=
 edge is missed. On the
TQMx86 we are lucky, and this software implementation is actually robust an=
d will not stop reporting
edges when one is missed. The reason is explained in detail in the long com=
ment added by this patch.

Matthias