linux-kernel - Re: [PATCH v5 03/23] usb: ulpi: Support device discovery via DT

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Date:   Fri, 11 Nov 2016 13:02:48 +0200
From:   Heikki Krogerus <heikki.krogerus@...ux.intel.com>
To:     Stephen Boyd <stephen.boyd@...aro.org>
Cc:     linux-usb@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-kernel@...r.kernel.org, linux-arm-msm@...r.kernel.org,
        Andy Gross <andy.gross@...aro.org>,
        Bjorn Andersson <bjorn.andersson@...aro.org>,
        Neil Armstrong <narmstrong@...libre.com>,
        Arnd Bergmann <arnd@...db.de>, Felipe Balbi <balbi@...nel.org>,
        Peter Chen <peter.chen@....com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        devicetree@...r.kernel.org, Rob Herring <robh+dt@...nel.org>
Subject: Re: [PATCH v5 03/23] usb: ulpi: Support device discovery via DT

On Mon, Oct 17, 2016 at 06:56:16PM -0700, Stephen Boyd wrote:
> The qcom HSIC ULPI phy doesn't have any bits set in the vendor or
> product ID registers. This makes it impossible to make a ULPI
> driver match against the ID registers. Add support to discover
> the ULPI phys via DT help alleviate this problem. In the DT case,
> we'll look for a ULPI bus node underneath the device registering
> the ULPI viewport (or the parent of that device to support
> chipidea's device layout) and then match up the phy node
> underneath that with the ULPI device that's created.
> 
> The side benefit of this is that we can use standard properties
> in the phy node like clks, regulators, gpios, etc. because we
> don't have firmware like ACPI to turn these things on for us. And
> we can use the DT phy binding to point our phy consumer to the
> phy provider.
> 
> The ULPI bus code supports native enumeration by reading the
> vendor ID and product ID registers at device creation time, but
> we can't be certain that those register reads will succeed if the
> phy is not powered up. To avoid any problems with reading the ID
> registers before the phy is powered we fallback to DT matching
> when the ID reads fail.
> 
> If the ULPI spec had some generic power sequencing for these
> registers we could put that into the ULPI bus layer and power up
> the device before reading the ID registers. Unfortunately this
> doesn't exist and the power sequence is usually device specific.
> By having the device matched up with DT we can avoid this
> problem.
> 
> Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
> Cc: Heikki Krogerus <heikki.krogerus@...ux.intel.com>
> Cc: <devicetree@...r.kernel.org>
> Cc: Rob Herring <robh+dt@...nel.org>
> Signed-off-by: Stephen Boyd <stephen.boyd@...aro.org>

Acked-by: Heikki Krogerus <heikki.krogerus@...ux.intel.com>

-- 
heikki