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

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Message-ID: <CABb+yY0WFxk75zPV5SV=9eCTi3ZTbaL9JYr4zH+5T11CLgkXUA@mail.gmail.com>
Date:   Thu, 16 Nov 2017 22:36:09 +0530
From:   Jassi Brar <jassisinghbrar@...il.com>
To:     Bjorn Andersson <bjorn.andersson@...aro.org>
Cc:     Sudeep Holla <sudeep.holla@....com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Alexey Klimov <alexey.klimov@....com>
Subject: Re: [PATCH] mailbox: txdone_method shouldn't always be reset

On Thu, Nov 16, 2017 at 10:21 PM, Jassi Brar <jassisinghbrar@...il.com> 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
>
And also detect if the channel was originally driven under POLL before
mbox_request_channel.
So this is finally what I think we need.

diff --git a/drivers/mailbox/mailbox.c b/drivers/mailbox/mailbox.c
index 674b35f..95e480e 100644
--- a/drivers/mailbox/mailbox.c
+++ b/drivers/mailbox/mailbox.c
@@ -124,7 +124,8 @@ static enum hrtimer_restart txdone_hrtimer(struct
hrtimer *hrtimer)
        for (i = 0; i < mbox->num_chans; i++) {
                struct mbox_chan *chan = &mbox->chans[i];

-               if (chan->active_req && chan->cl) {
+               if (chan->active_req && chan->cl &&
+                   chan->txdone_method == TXDONE_BY_POLL) {
                        txdone = chan->mbox->ops->last_tx_done(chan);
                        if (txdone)
                                tx_tick(chan, 0);
@@ -418,7 +419,7 @@ void mbox_free_channel(struct mbox_chan *chan)
        spin_lock_irqsave(&chan->lock, flags);
        chan->cl = NULL;
        chan->active_req = NULL;
-       if (chan->txdone_method == TXDONE_BY_ACK)
+       if (chan->txdone_method == TXDONE_BY_ACK && chan->mbox->txdone_poll)
                chan->txdone_method = TXDONE_BY_POLL;

        module_put(chan->mbox->dev->driver->owner);