Subject: [PATCH 02/10] mailbox: Enable BCM2835 mailbox support

From: [email-masked] (Stephen Warren)
Date: Fri, 06 Mar 2015 13:29:31 -0700

On 03/06/2015 01:00 PM, Eric Anholt wrote:
> Stephen Warren <[email-masked]> writes:
>
>> On 03/04/2015 11:20 AM, Eric Anholt wrote:
>>> Stephen Warren <[email-masked]> writes:
>>>
>>>> On 03/02/2015 01:54 PM, Eric Anholt wrote:
>>>>> From: Lubomir Rintel <[email-masked]>
>>>>>
>>>>> Implement BCM2835 mailbox support as a device registered with
>>>>> the general purpose mailbox framework. Implementation based on
>>>>> commits by Lubomir Rintel [1], Suman Anna and Jassi Brar [2] on
>>>>> which to base the implementation.
>>>>
>>>>> diff --git a/drivers/mailbox/bcm2835-mailbox.c
>>>>> b/drivers/mailbox/bcm2835-mailbox.c
>>
>>>>> +static irqreturn_t bcm2835_mbox_irq(int irq, void *dev_id) +{
>>>>> +	struct bcm2835_mbox *mbox = (struct bcm2835_mbox *) dev_id; +
>>>>> struct device *dev = mbox->dev; + +	while (!(readl(mbox->regs +
>>>>> MAIL0_STA) & ARM_MS_EMPTY)) { +		u32 msg = readl(mbox->regs +
>>>>> MAIL0_RD); +		unsigned int chan = MBOX_CHAN(msg); + +		if
>>>>> (!mbox->channel[chan].started) { +			dev_err(dev, "Reply on
>>>>> stopped channel %d\n", chan); +			continue; +		} +
>>>>> dev_dbg(dev, "Reply 0x%08X\n", msg); +
>>>>> mbox_chan_received_data(mbox->channel[chan].link, +			(void *)
>>>>> MBOX_DATA28(msg)); +	} +	rmb(); /* Finished last mailbox read.
>>>>> */
>>>>
>>>> I know the PDF mentioned in the comment earlier in the patch says
>>>> to put in barriers between accesses to different peripherals,
>>>> which this seems compliant with, but I don't see quite what this
>>>> barrier achieves. I think the PDF is talking generalities, not
>>>> imposing a rule that must be blindly followed. Besides, if
>>>> there's a context-switch you can't actually implement the rules
>>>> the PDF suggests. What read operation is this barrier attempting
>>>> to ensure happens after reading all mailbox messages and any
>>>> associated DRAM buffer?
>>>
>>> Looking at this bit of code in particular:
>>>
>>> "As interrupts can appear anywhere in the code so you should
>>> safeguard those. If an interrupt routine reads from a peripheral
>>> the routine should start with a memory read barrier. If an
>>> interrupt routine writes to a peripheral the routine should end
>>> with a memory write barrier."
>>
>> I can see that doing that would be safe, but I don't think following
>> those rules is actually necessary in the general case. Following those
>> rules would cause lots of unnecessary barriers in code.
>>
>> Barriers shouldn't be used arbitrarily at the start/end of ISRs, but
>> rather at specific chosen locations in the code that actually need to
>> enforce some kind of memory ordering.
>
> According to the architecture docs, if you don't RMB at the start of
> your ISR, then the following timeline could get the wrong values:

Which architecture doc and section/... specifically?

> some other device driver               our isr
> ------------------------               -------
> rmb()
> read from device
>                                         read from device
>                                         examine value read
>                                         exit isr
> examine value raed.
>
> The ISR could get the device driver's value.  This is made explicit in
> footnote 2 on page 7.

Are you saying that the "read from device" in the right -hand "our isr" 
column could end up returning the value actually read during "read from 
device" in the left-hand "some other device driver" column, or vice-versa?

That doesn't make any sense.

Barriers are about ensuring that accesses happen (are visible or 
complete) in the desired order, not about ensuring that the results of 
two unrelated reads don't get swapped.