linux-kernel - Re: [PATCH V6 05/12] mailbox: Add NVIDIA Tegra XUSB mailbox driver

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Date:	Tue, 2 Dec 2014 11:06:18 -0800
From:	Andrew Bresticker <abrestic@...omium.org>
To:	Thierry Reding <thierry.reding@...il.com>
Cc:	Stephen Warren <swarren@...dotorg.org>,
	Alexandre Courbot <gnurou@...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>,
	Jassi Brar <jassisinghbrar@...il.com>,
	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>, Olof Johansson <olof@...om.net>,
	Kishon Vijay Abraham I <kishon@...com>,
	Felipe Balbi <balbi@...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 V6 05/12] mailbox: Add NVIDIA Tegra XUSB mailbox driver

On Tue, Dec 2, 2014 at 1:47 AM, Thierry Reding <thierry.reding@...il.com> wrote:
> On Mon, Nov 24, 2014 at 04:17:17PM -0800, 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
>> physical 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.  The requesting driver is assigned
>> one of two virtual channels when the single physical channel is
>> requested.  All incoming messages are sent to both virtual channels.
>>
>> Signed-off-by: Andrew Bresticker <abrestic@...omium.org>
>> Reviewed-by: Stephen Warren <swarren@...dia.com>
>> ---
>> Thierry,
>>
>> I've left this as a separate driver because I don't think it should be
>> combined with the xHCI host.  Stephen and I discussed this earlier in
>> review of v1 of this series and he agreed that the mailbox belongs in
>> its own DT node and driver.
>
> My objection wasn't strictly against merging the drivers. What I don't
> want to see is two devices sharing the same memory-mapped regions in DT.
>
> That's based on earlier discussion with Stephen as well, because we have
> had the same problem with USB and USB-PHY before. Unless I completely
> misinterpreted we came to the conclusion that we should avoid this in
> the future.

Yes, I understand your objection to having multiple drivers map the
same IO memory, however Stephen and I discussed this before [1] and
came to the conclusion that, although mapping the MMIO region in both
drivers was non-ideal, the alternatives had their own drawbacks as
well and that the approach I had taken was fine.

> Like I said, this doesn't mean that both need to be in the same driver.
> We could use MFD for example, or provide an entry point in the mailbox
> driver that can be called from the XUSB driver to instantiate the
> mailbox (which is really the same as MFD).

This does not look like the typical MFD, but I would agree that it's
the only reasonable alternative.  If Lee is fine with it, then I
suppose I could go the MFD route...  Lee: for context, the mailbox
registers live in one of the host-controller's MMIO regions - there
are no other shared resources.  This has been discussed, along with a
possible binding, in [1] as well.

[1] https://lkml.org/lkml/2014/8/27/727
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