linux-kernel - Re: [PATCH v4 22/22] phy: Add support for Qualcomm's USB HS phy

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Date:   Thu, 15 Sep 2016 13:29:06 +0800
From:   Peter Chen <hzpeterchen@...il.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>,
        Kishon Vijay Abraham I <kishon@...com>,
        devicetree@...r.kernel.org,
        Fabien Lahoudere <fabien.lahoudere@...labora.co.uk>
Subject: Re: [PATCH v4 22/22] phy: Add support for Qualcomm's USB HS phy

On Wed, Sep 14, 2016 at 10:42:50AM -0700, Stephen Boyd wrote:
> > > 
> > > Hmm.. maybe the confusion is in which registers we should be able to
> > > access? Are we talking about the ULPI viewport MMIO register space or
> > > the ULPI registers that we access through the viewport? I have a
> > > hw_phymode_configure() inside of of ci_ulpi_init() so that the
> > > identification registers through the ULPI viewport read properly
> > > (assuming there aren't other power requirements like regulators). If we
> > > don't set the portsc.pts before using the viewport, the viewport doesn't
> > > work and reads timeout. So we really don't touch the ULPI registers
> > > except for the scratch space and the id registers until after the phy is
> > > properly powered on with clks and regulators, because the only place we
> > > touch them after doing the id checking is in this phy driver in
> > > qcom_usb_hs_phy_power_on(). We've "solved" the chicken-egg problem where
> > > we don't know which device driver to probe because the phy needs to be
> > > powered on to read the id registers to know which device driver to use
> > > by using DT to match up device drivers instead.
> > > 
> > > [1] https://www.sparkfun.com/datasheets/Components/SMD/ULPI_v1_1.pdf
> > 
> > Ok, ulpi phy works like USB device on USB bus which create device at
> > runtime. So, like some hard-wired USB devices, it may needs power
> > sequence too, otherwise, how it knows which driver can loads.
> > 
> 
> Yes. We use the DT compatible string to ignore any issues with reading
> the device ids when the device is powered off. Unlike USB though, we
> have device drivers for the ULPI PHYs that do the power sequencing along
> with other initializations, so using a common pwrseq layer seems like
> overkill just so we can read the id registers.

If the attempt to read id registers will not hang the system, it can
work. Like the case [1], without PHY ref_clk, it will hang when
configure PHY mode (visit portsc.pts), it has no chance to read id
registers. So, I still think the ULPI bus driver needs to do power
sequence for its children, unless you have some place to open the
ref_clk.

> 
> Are there any concerns with this patch? Or can they be reapplied?

For this patch, it is ok for me.


[1] http://www.spinics.net/lists/linux-usb/msg146336.html
-- 

Best Regards,
Peter Chen