Re: [PATCH v2 2/9] mailbox: Add NVIDIA Tegra XUSB mailbox driver

[Stephen Warren <swarren@wwwdotorg.org>]

On 08/27/2014 11:38 AM, Andrew Bresticker wrote:
> On Mon, Aug 25, 2014 at 12:01 PM, Stephen Warren <swarren@wwwdotorg.org> wrote:
>> On 08/18/2014 11:08 AM, Andrew Bresticker wrote:
>>>
>>> The Tegra xHCI controller's firmware communicates requests to the host
>>> processor through a mailbox interface.  While there is only a single
>>> communication channel, messages sent by the controller can be divided
>>> into two groups: those intended for the PHY driver and those intended
>>> for the host-controller driver.  This mailbox driver exposes the two
>>> channels and routes incoming messages to the appropriate channel based
>>> on the command encoded in the message.
>>
>>
>>> diff --git a/drivers/mailbox/tegra-xusb-mailbox.c
>>> b/drivers/mailbox/tegra-xusb-mailbox.c

>>> +static struct mbox_chan *mbox_cmd_to_chan(struct tegra_xusb_mbox *mbox,
>>> u32 cmd)
>>> +{
>>> +       switch (cmd) {
>>> +       case MBOX_CMD_INC_FALC_CLOCK:
>>> +       case MBOX_CMD_DEC_FALC_CLOCK:
>>> +       case MBOX_CMD_INC_SSPI_CLOCK:
>>> +       case MBOX_CMD_DEC_SSPI_CLOCK:
>>> +       case MBOX_CMD_SET_BW:
>>> +               return &mbox->mbox.chans[TEGRA_XUSB_MBOX_CHAN_HOST];
>>> +       case MBOX_CMD_SAVE_DFE_CTLE_CTX:
>>> +       case MBOX_CMD_START_HSIC_IDLE:
>>> +       case MBOX_CMD_STOP_HSIC_IDLE:
>>> +               return &mbox->mbox.chans[TEGRA_XUSB_MBOX_CHAN_PHY];
>>> +       default:
>>> +               return NULL;
>>> +       }
>>> +}
>>
>>
>> This makes me think that the CHAN_HOST/CHAN_PHY values are purely a facet of
>> the Linux driver's message de-multiplexing, rather than anything to do with
>> the HW.
>
> Yup, they are...
>
>> I'm not even sure if it's appropriate for the low-level mailbox driver to
>> know about the semantics of the message, rather than simply sending them on
>> to the client driver? Perhaps when drivers register(?) for callbacks(?) for
>> messages, they should state which types of messages they want to listen to?
>
> So there's not really a way for the client driver to tell the mailbox
> driver which types of messages it wants to listen to on a particular
> channel with the mailbox framework - it simply provides a way for
> clients to bind with channels.  I think there are a couple of options
> here, either: a) have a channel per message (as you mentioned in the
> previous patch), which allows the client to only register for messages
> (channels) it wants to handle, or b) extend the mailbox framework to
> allow shared channels so that both clients can receive messages on the
> single channel and handle messages appropriately.   The disadvantage
> of (a) is that the commands are firmware defined and could
> theoretically change between releases of the firmware, though I'm not
> sure how common that is in practice.  So that leaves (b) - Jassi, what
> do you think about having shared (non-exclusive) channels?

Another alternative might be for each client driver to hard-code a 
unique dummy channel ID so that each client still gets a separate 
exclusive channel, but then have the mbox driver broadcast each message 
to each of those channels. I'm not sure that would be any better though; 
adding (b) as an explicit option to the mbox subsystem would almost 
certainly be cleaner.

>>> +static int tegra_xusb_mbox_probe(struct platform_device *pdev)
>>
>>
>>> +       res = platform_get_resource(pdev, IORESOURCE_MEM, 0);
>>>
>>> +       if (!res)
>>> +               return -ENODEV;
>>
>>
>> Should devm_request_mem_region() be called here to claim the region?
>
> No, the xHCI host driver also needs to map these registers, so they
> cannot be mapped exclusively here.

That's unfortunate. Having multiple drivers with overlapping register 
regions is not a good idea. Can we instead have a top-level driver map 
all the IO regions, then instantiate the various different 
sub-components internally, and divide up the address space. Probably via 
MFD or similar. That would prevent multiple drivers from touching the 
same register region.