linux-kernel - Re: [PATCH 2/4] msm_serial: Add devicetree support

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Date:	Tue, 16 Aug 2011 10:57:02 -0700
From:	David Brown <davidb@...eaurora.org>
To:	Arnd Bergmann <arnd@...db.de>
Cc:	David Brown <davidb@...eaurora.org>,
	linux-arm-kernel@...ts.infradead.org,
	Randy Dunlap <rdunlap@...otime.net>,
	Daniel Walker <dwalker@...o99.com>,
	Bryan Huntsman <bryanh@...eaurora.org>,
	Alan Cox <alan@...ux.intel.com>,
	Grant Likely <grant.likely@...retlab.ca>,
	linux-doc@...r.kernel.org, linux-arm-msm@...r.kernel.org,
	devicetree-discuss@...ts.ozlabs.org, linux-kernel@...r.kernel.org,
	linux-serial@...r.kernel.org
Subject: Re: [PATCH 2/4] msm_serial: Add devicetree support

On Sat, Aug 13, 2011 at 11:34:15PM +0200, Arnd Bergmann wrote:
> On Saturday 13 August 2011 12:46:45 David Brown wrote:
> > 
> > I'm not sure actually what is best to use here.  I'm thinking that the
> > 'lite' identifier should perhaps go away.  MSM's have two UARTS on
> > them, an older "simple" PIO type of UART, and a newer one that can do
> > DMA (called the hsuart for high-speed).  The hsuart can also be used
> > in a non-DMA driver in a mostly compatible way with the old UART.
> > 
> > For non-high-speed applications, the user will probably just want to
> > use the non-DMA driver.  My question is then: if the device tree
> > describes it as
> > 
> >         compatible = "qcom,msm-hsuart", "qcom,msm-uart";
> > 
> > and one driver matches qcom,msm-hsuart and another matches
> > qcom,msm-uart, which driver will get used.  Ideally, it would use the
> > earliest one in the list.
> > 
> > If that's the case, I'll get rid of the -lite suffix and just make the
> > non-DMA driver compatible with the plain "qcom,msm-uart".
> 
> I believe that unfortunately the answer is that the first driver that
> matches anything will get used. There are two possible ways that I can
> see to make it do what you want anyway:
> 
> 1. In the probe function for the slow driver, you return an error
> when the device you get passed matches "qcom,msm-hsuart", possibly
> dependent on whether the other driver also got built.
> 
> 2. You register one platform driver that handles both names and
> gives the device to just one of the two drivers. This would probably
> require linking the two drivers into the same module, or having
> the non-DMA speed driver just act as a library.

How about if I just keep it simple for now.  Since there isn't
actually a driver for the DMA version, this driver will handle both
UART blocks, so I'll just do the plain thing in the DT.

In the future, when a DMA-capable driver exists, we can figure out how
to determine which driver should be used.  At this point, I'm not even
sure what the correct answer will be, since a given configuration may
want to use non-DMA for one msm-hsuart device, and the DMA driver for
another.  It's kind of board/use specific, but beyond just describing
what the hardware is.

I've just sent new patches with this fixed up.

David

-- 
Sent by an employee of the Qualcomm Innovation Center, Inc.
The Qualcomm Innovation Center, Inc. is a member of the Code Aurora Forum.
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