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

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Date:	Wed, 27 Aug 2014 14:56:41 -0700
From:	Andrew Bresticker <abrestic@...omium.org>
To:	Stephen Warren <swarren@...dotorg.org>
Cc:	Jassi Brar <jassisinghbrar@...il.com>,
	Thierry Reding <thierry.reding@...il.com>,
	"linux-tegra@...r.kernel.org" <linux-tegra@...r.kernel.org>,
	Rob Herring <robh+dt@...nel.org>,
	Pawel Moll <pawel.moll@....com>,
	Mark Rutland <mark.rutland@....com>,
	Ian Campbell <ijc+devicetree@...lion.org.uk>,
	Kumar Gala <galak@...eaurora.org>,
	Russell King <linux@....linux.org.uk>,
	Linus Walleij <linus.walleij@...aro.org>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Mathias Nyman <mathias.nyman@...el.com>,
	Grant Likely <grant.likely@...aro.org>,
	Alan Stern <stern@...land.harvard.edu>,
	Arnd Bergmann <arnd@...db.de>,
	Kishon Vijay Abraham I <kishon@...com>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"linux-usb@...r.kernel.org" <linux-usb@...r.kernel.org>
Subject: Re: [PATCH v2 2/9] mailbox: Add NVIDIA Tegra XUSB mailbox driver

On Wed, Aug 27, 2014 at 11:19 AM, Stephen Warren <swarren@...dotorg.org> wrote:
> On 08/27/2014 12:13 PM, Andrew Bresticker wrote:
>>
>> On Wed, Aug 27, 2014 at 10:50 AM, Stephen Warren <swarren@...dotorg.org>
>> wrote:
>>>
>>> On 08/27/2014 11:38 AM, Andrew Bresticker wrote:
>>>>
>>>>
>>>> On Mon, Aug 25, 2014 at 12:01 PM, Stephen Warren <swarren@...dotorg.org>
>>>> wrote:
>>>>>
>>>>>
>>>>> On 08/18/2014 11:08 AM, Andrew Bresticker wrote:
>>>>>>
>>>>>>
>>>>>> +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.
>>
>>
>> Perhaps I'm misunderstanding, but I don't see how MFD would prevent us
>> from having to map this register space in two different locations -
>> the XUSB FPCI address space cannot be divided cleanly between host and
>> mailbox registers.  Or are you saying that there should be a separate
>> device driver that exposes an API for accessing this register space,
>> like the Tegra fuse or PMC drivers?
>
>
> With MFD, there's typically a top-level driver for the HW module (or
> register space) that gets instantiated by the DT node. This driver then
> instantiates all the different sub-drivers that use that register space, and
> provides APIs for the sub-drivers to access the registers (either custom
> APIs or more recently by passing a regmap object down to the sub-drivers).
>
> This top-level driver is the only driver that maps the space, and can manage
> sharing the space between the various sub-drivers.

So if I'm understanding correctly, we end up with something like this:

usb@...90000 {
        compatible = "nvidia,tegra124-xusb";
        reg = <0x0 0x70090000 0x0 0x8000>, // xHCI host registers
              <0x0 0x70098000 0x0 0x1000>, // FPCI registers
              <0x0 0x70099000 0x0 0x1000>; // IPFS registers
        interrupts = <GIC_SPI 39 IRQ_TYPE_LEVEL_HIGH>, // host interrupt
                     <GIC_SPI 40 IRQ_TYPE_LEVEL_HIGH"; // mailbox interrupt

        host-controller {
                compatible = "nvidia,tegra124-xhci";
                ...
        };

        mailbox {
                compatible = "nvidia,tegra124-xusb-mailbox";
               ...
        };
};

To be honest though, this seems like overkill to share a couple of
registers when no other resources are shared between the two.
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