linux-kernel - Re: [PATCH v3 1/2] gpio: Add gpio latch driver

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Message-ID: <20220922133105.GN986@pengutronix.de>
Date:   Thu, 22 Sep 2022 15:31:05 +0200
From:   Sascha Hauer <s.hauer@...gutronix.de>
To:     Serge Semin <fancer.lancer@...il.com>
Cc:     linux-gpio@...r.kernel.org, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Linus Walleij <linus.walleij@...aro.org>,
        Bartosz Golaszewski <brgl@...ev.pl>, kernel@...gutronix.de
Subject: Re: [PATCH v3 1/2] gpio: Add gpio latch driver

On Wed, Sep 14, 2022 at 05:03:10PM +0300, Serge Semin wrote:
> > +				    unsigned int offset, bool val)
> > +{
> > +	int latch = offset / priv->n_latched_gpios;
> > +	int i;
> > +
> > +	assign_bit(offset, priv->shadow, val);
> > +
> 
> > +	for (i = 0; i < priv->n_latched_gpios; i++)
> > +		set(priv->latched_gpios->desc[i],
> > +		    test_bit(latch * priv->n_latched_gpios + i, priv->shadow));
> 
> -> duration?
> 
> > +
> > +	set(priv->clk_gpios->desc[latch], 1);
> 
> -> duration?
> 
> > +	set(priv->clk_gpios->desc[latch], 0);
> 
> I am pretty much sure there must be some duration between the actions
> above *. See for instance the tw and (tsu + th) timing requirements in
> the next edge-triggered flip-flops:
> https://www.ti.com/lit/ds/symlink/sn74lv74a.pdf?ts=1663163389954&ref_url=https%253A%252F%252Fwww.google.com%252F
> 
> The durations are normally small (ns or a bit smaller) but still need
> to be added anyway.
> 
> Note since the durations are device-specific an additional DT-property array
> with durations should be added too.

Do you think a fixed udelay(1) would be enough for now? Bigger delays
shouldn't be needed and smaller delays expand to udelay(1) anyway on
architectures not providing an architecture specific ndelay().

Sascha

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