linux-kernel - Re: [PATCH] mailbox: txdone_method shouldn't always be reset

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Date:   Thu, 16 Nov 2017 17:24:44 +0000
From:   Sudeep Holla <sudeep.holla@....com>
To:     Jassi Brar <jassisinghbrar@...il.com>,
        Bjorn Andersson <bjorn.andersson@...aro.org>
Cc:     Sudeep Holla <sudeep.holla@....com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Alexey Klimov <klimov.linux@...il.com>
Subject: Re: [PATCH] mailbox: txdone_method shouldn't always be reset

(- Remove  Alexey Klimov's ARM id as he has left the company, adding his
personal id instead)

On 16/11/17 16:51, Jassi Brar wrote:
> On Thu, Nov 16, 2017 at 11:01 AM, Bjorn Andersson
> <bjorn.andersson@...aro.org> wrote:
>> A client that knows how to drive txdone would temporarily "upgrade" the
>> method to TXDONE_BY_ACK. But with the introduction of commit 33cd7123ac0ba
>> ("mailbox: reset txdone_method TXDONE_BY_POLL if client knows_txdone")
>> there is no longer a distinction between a channel in "upgraded" state
>> or a channel for a controller that only supports TXDONE_BY_ACK. So upon
>> freeing the channel it will be "downgraded" to TXDONE_BY_POLL.
>>
>> But a channel that operates with the txdone method of TXDONE_BY_POLL
>> requires that the controller implements the last_tx_done callback and
>> that the associated hrtimer was initialized when the controller was
>> registered.
>>
>> So the core now relies on the fact that subsequent calls to
>> mbox_request_channel() "upgrades" the channel to TXDONE_BY_ACK or it
>> will dereference the non-initialized hrtimer.
>>
> I think we just need avoid the channel not running under POLL
> 
> Does this fix your issue?
> 

Unfortunately only Alexey had fast reproducer of the issue reported with
original commit. I will try to run on my setup, but it was hard to
reproduce(mainly lack of firmware support for multiple virtual channels).
I will try my best to test this.

Sorry for not considering temporary "upgrade" use case.

-- 
Regards,
Sudeep