linux-kernel - Re: [PATCH] mailbox/bcm2835: Fix mailbox full detection.

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Message-ID: <CABb+yY1t4P4LmnV3ymnNtfV6Q2D_9nkpfxJHQC8ot-ky+nzTfQ@mail.gmail.com>
Date:	Fri, 29 May 2015 12:55:52 +0530
From:	Jassi Brar <jassisinghbrar@...il.com>
To:	Stephen Warren <swarren@...dotorg.org>
Cc:	Eric Anholt <eric@...olt.net>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	linux-rpi-kernel@...ts.infradead.org
Subject: Re: [PATCH] mailbox/bcm2835: Fix mailbox full detection.

On Fri, May 29, 2015 at 2:15 AM, Stephen Warren <swarren@...dotorg.org> wrote:
> On 05/13/2015 02:10 PM, Eric Anholt wrote:
>>
>> With the VC reader blocked and the ARM writing, MAIL0_STA reads empty
>> permanently while MAIL1_STA goes from empty (0x40000000) to non-empty
>> (0x00000001-0x00000007) to full (0x80000008).
>>
>> This bug ended up having no effect on us, because all of our
>> transactions in the client driver were synchronous and under a mutex.
>
>
> If you could get someone at the RPi Foundation or Broadcom to update the
> register descriptions and example code at the following URLs, that would be
> rather useful. Otherwise, this code will appear incorrect when compared
> against the documentation:
>
> https://github.com/raspberrypi/firmware/wiki/Mailboxes
> ("Mailbox registers" at the bottom)
>
> https://github.com/raspberrypi/firmware/wiki/Accessing-mailboxes
> ("Sample code")
>
>> diff --git a/drivers/mailbox/bcm2835-mailbox.c
>> b/drivers/mailbox/bcm2835-mailbox.c
>
>
>> @@ -117,7 +118,7 @@ static bool bcm2835_last_tx_done(struct mbox_chan
>> *link)
>>         bool ret;
>>
>>         spin_lock(&mbox->lock);
>> -       ret = !(readl(mbox->regs + MAIL0_STA) & ARM_MS_FULL);
>> +       ret = !(readl(mbox->regs + MAIL1_STA) & ARM_MS_FULL);
>
>
> What does "tx done" mean semantically?
>
> If "tx done" means "remote side received all our messages", then surely this
> should check MAIL1_STA for emptiness, which is different to the "not full"
> check implemented here?
>
If only one message is active at any time we may have either FULL or
EMPTY status(?), so that might explain why this patch works as well.
But yes, it should check for emptiness ideally.
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