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

From: Stephen Warren
Date: Wed Aug 27 2014 - 14:19:33 EST

On 08/27/2014 12:13 PM, Andrew Bresticker wrote:
On Wed, Aug 27, 2014 at 10:50 AM, Stephen Warren <swarren@xxxxxxxxxxxxx> wrote:
On 08/27/2014 11:38 AM, Andrew Bresticker wrote:

On Mon, Aug 25, 2014 at 12:01 PM, Stephen Warren <swarren@xxxxxxxxxxxxx>

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.

That said, I haven't noticed many MFD drivers for MMIO devices. I certainly have seen multiple different drivers just re-mapping shared registers for themselves. It is simpler and does work. However, people usually mutter about it when it happens, since it's not clear which drivers are using what from the IO mapping registry. Using MFD or similar would allow the sharing to work in a clean fashion.
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