linux-kernel - Re: [PATCH v6 07/12] usb: chipidea: add a usb2 driver for ci13xxx

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Date:	Fri, 26 Sep 2014 09:20:54 +0200
From:	Arnd Bergmann <arnd@...db.de>
To:	linux-arm-kernel@...ts.infradead.org, balbi@...com
Cc:	thomas.petazzoni@...e-electrons.com, zmxu@...vell.com,
	devicetree@...r.kernel.org,
	Antoine Tenart <antoine.tenart@...e-electrons.com>,
	linux-usb@...r.kernel.org, linux-kernel@...r.kernel.org,
	alexandre.belloni@...e-electrons.com,
	Peter Chen <peter.chen@...escale.com>, p.zabel@...gutronix.de,
	jszhang@...vell.com, sebastian.hesselbarth@...il.com
Subject: Re: [PATCH v6 07/12] usb: chipidea: add a usb2 driver for ci13xxx

On Thursday 25 September 2014 19:39:34 Felipe Balbi wrote:
> > 
> > why would a glue layer need to access registers from the core ? That
> > sounds very odd. I haven't seen that and will, definitely, NACK such a
> > patch 
> > 
> > can you further describe why you think a glue layer might need to access
> > core IP's registers ?
> 
> I just realised we're talking about chipidea here... in any case, it's
> still valid to ask why would glue need to fiddle with core IP's
> registers.

Generally, the glue driver wouldn't access the registers, but I don't
think it's important to prevent it from doing that. In some cases, 
a glue driver needs to override a function of the core driver, e.g.
to work around an errata. We have a lot of those quirks in ATA drivers,
one example from ahci_mvebu.c is

static void ahci_mvebu_regret_option(struct ahci_host_priv *hpriv)
{
        /*
         * Enable the regret bit to allow the SATA unit to regret a
         * request that didn't receive an acknowlegde and avoid a
         * deadlock
         */
        writel(0x4, hpriv->mmio + AHCI_VENDOR_SPECIFIC_0_ADDR);
        writel(0x80, hpriv->mmio + AHCI_VENDOR_SPECIFIC_0_DATA);
}

	Arnd
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