linux-kernel - Re: [PATCH] i2c-axxia: support slave mode

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Date:   Wed, 7 Aug 2019 17:19:44 +0200
From:   Wolfram Sang <wsa@...-dreams.de>
To:     "Adamski, Krzysztof (Nokia - PL/Wroclaw)" 
        <krzysztof.adamski@...ia.com>
Cc:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-i2c@...r.kernel.org" <linux-i2c@...r.kernel.org>,
        "Sverdlin, Alexander (Nokia - DE/Ulm)" <alexander.sverdlin@...ia.com>
Subject: Re: [PATCH] i2c-axxia: support slave mode

Hi Krzysztof,

> Good point. I'll remove those verbose messages and maybe leave one or
> two debug messages with just a summary of the status which will
> hopefully be a good compromise. Will that be ok?

Very likely :)

> BTW, I have added this synchronize_irq() in unreg_slave callback just to
> make sure it is save to set idev->slave to NULL already. Most of the
> controllers do not have such a guard and I'm wondering why that wouldn't
> be a problem for them. Like the i2c-rcar.c - isn't there a small race
> condition if some slave interrupt triggers just before ICSIER is cleared
> and somehow does not finish before priv->slave is set to NULL? This is
> the situation I was afraid of and tried to solve by using this
> synchronize_irq().

You have an important point there. unreg_slave is protected by the
adapter lock but this won't help if another master is requesting
something, thus, causing interrupts.

Phew, nearly all slave implementation need to fix this!

Thanks a lot,

   Wolfram


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