linux-kernel - Re: Fwd: [Proposal] PM sleep children of inactive I2C bus segments off Masters in multi-master systems

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Message-ID: <CAAVjN7fGRzyu7gxavJUay+K_Mm-0t1T_0Bysk6-sdaqu5=5k5A@mail.gmail.com>
Date:	Wed, 1 Oct 2014 13:03:59 -0700
From:	Danielle Costantino <danielle.costantino@...il.com>
To:	Guenter Roeck <groeck@...iper.net>
Cc:	linux-i2c <linux-i2c@...r.kernel.org>,
	Bjorn Helgaas <bhelgaas@...gle.com>,
	Jiri Kosina <jkosina@...e.cz>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"David S. Miller" <davem@...emloft.net>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	"linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
	Rajat Jain <rajatjain@...iper.net>
Subject: Re: Fwd: [Proposal] PM sleep children of inactive I2C bus segments
 off Masters in multi-master systems

On Wed, Oct 1, 2014 at 12:41 PM, Guenter Roeck <groeck@...iper.net> wrote:
> On Wed, Oct 01, 2014 at 11:44:59AM -0700, Danielle Costantino wrote:
>> Re-sending Proposal:
>>
>> Currently I2C mux devices that support multiple master arbitration are
>> the i2c-mux-pca9541 and i2c-arb-gpio-challenge drivers. I propose to
>> add the ability to configure an interrupt pin from the Master Selector
>> device to indicate that bus ownership has been lost. Once the device
>> loses ownership, all of its children should enter a pm sleep mode (as
>> you can't talk to them at this point) until master-ship has been
>> reacquired.
>>
> Not sure I understand what you are proposing here.

Lets say you have a active - standby based multi-master system. If the
other master forced arbitration (took mastership) the next transation
on any slaves of that bus would return EAGAIN or EBUSY.

Another point is that once mastership has been lost, the configuration
of the devices on that bus are no longer known to be valid...therefor
a re-init of those devices may be needed.

> A typical use case would be a power supply such as the one supported by
> drivers/hwmon/lineage-pem.c from both an active and standby system
> controller. The power supply needs to be accessible from both controllers.
> If one controller looses access, it can only mean that it did not follow
> the access protocol. Similar, one controller enforcing access means that
> it either does not follow the access protocol either, or that the other
> end did not follow the protocol (or maybe the other end died).
>
> In all cases, loss of access does not mean that the end device can or should
> be put in sleep mode, not even logically. All it means is that there was
> an access protocol error. Not sure if there is anything that can be done
> about that, but putting the device into sleep mode does not seem to be
> an appropriate response to me.
>
>> This has come up as an issue when the master loses control over a bus
>> the return code of all transactions to its lave devices is EIO (not
>> very helpful).
>>
> But, again, doesn't that mean that there was some access protocol error ?
> Shouldn't it try to re-acquire mastership instead, and block all accesses
> to slaves until it acquired it ?

EIO is such a generic error it makes finding weather there was a
problem communicating or is no longer master of the bus segment.

>
> Thanks,
> Guenter

Thanks

-- 
- Danielle Costantino
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