linux-kernel - Re: [PATCH 1/2] i2c: virtio: disable timeout handling

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Date:   Tue, 9 Nov 2021 10:22:21 +0530
From:   Viresh Kumar <viresh.kumar@...aro.org>
To:     Vincent Whitchurch <vincent.whitchurch@...s.com>
Cc:     "Chen, Conghui" <conghui.chen@...el.com>,
        "Deng, Jie" <jie.deng@...el.com>,
        Greg KH <gregkh@...uxfoundation.org>,
        Wolfram Sang <wsa@...nel.org>,
        "virtualization@...ts.linux-foundation.org" 
        <virtualization@...ts.linux-foundation.org>,
        "linux-i2c@...r.kernel.org" <linux-i2c@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        kernel <kernel@...s.com>
Subject: Re: [PATCH 1/2] i2c: virtio: disable timeout handling

On 03-11-21, 15:42, Vincent Whitchurch wrote:
> The suggested timeout is not meant to take into account the overhead of
> virtualization, but to be used by the virtio device as a timeout for the
> transaction on the I2C bus (presumably by programming this value to the
> physical I2C controller, if one exists).
> 
> Assume that userspace (or an I2C client driver) asks for a timeout of 20
> ms for a particular transfer because it, say, knows that the particular
> connected I2C peripheral either responds within 10 ms to a particular
> register read or never responds, so it doesn't want to waste time
> waiting unnecessarily long for the transfer to complete.
> 
> If the virtio device end does not have any information on what timeout
> is required (as in the current spec), it must assume some high value
> which will never cause I2C transactions to spuriously timeout, say 10
> seconds.  
>
> Even if the virtio driver is fixed to copy and hold all buffers to avoid
> memory corruption and to time out and return to the caller after the
> requested 20 ms, the next I2C transfer can not be issued until 10
> seconds have passed, since the virtio device end will still be waiting
> for the hardcoded 10 second timeout and may not respond to new requests
> until that transfer has timed out.

Okay, so this is more about making sure the device times-out before
the driver or lets say in an expected time-frame. That should be okay
I guess.

-- 
viresh