linux-kernel - RE: [PATCH 2/3] usb: ehci: fsl: Update register accessing for arm/arm64 platforms

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Message-ID: <Pine.LNX.4.44L0.1901101018160.1497-100000@iolanthe.rowland.org>
Date:   Thu, 10 Jan 2019 10:29:06 -0500 (EST)
From:   Alan Stern <stern@...land.harvard.edu>
To:     Ran Wang <ran.wang_1@....com>
cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "linux-usb@...r.kernel.org" <linux-usb@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH 2/3] usb: ehci: fsl: Update register accessing for
 arm/arm64 platforms

On Thu, 10 Jan 2019, Ran Wang wrote:

> Hi Alan,
> 
> > -----Original Message-----
> > From: Alan Stern <stern@...land.harvard.edu>
> > Sent: Wednesday, January 09, 2019 23:14
> > To: Ran Wang <ran.wang_1@....com>
> > Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>; linux-
> > usb@...r.kernel.org; linux-kernel@...r.kernel.org
> > Subject: RE: [PATCH 2/3] usb: ehci: fsl: Update register accessing for
> > arm/arm64 platforms
> > 
> > On Wed, 9 Jan 2019, Ran Wang wrote:
> > 
> > > > Why do you change this writel() into iowrite32be() but leave other
> > > > instances of writel() unchanged?  Was this a mistake?
> > >
> > > Yes, I didn't notice there are other writel() used in this file.
> > > However, as I know, on both powerpc and arm SoC, EHCI FSL IP's memory
> > > mapped register block is always Big-endian, so I'd like to replace all
> > > writel() with iowrite32be() in this file. Is it necessary?
> > >
> > > Or I just replace them with ehci_writel() and select
> > CONFIG_USB_EHCI_BIG_ENDIAN_MMIO?
> > 
> > That should work okay.  ehci_fsl_setup() sets ehci->big_endian_desc and
> > ehci->big_endian_mmio according to the platform data, so you just have
> > to make sure the platform data is initialized correctly.
> >
> OK, so I should not change writel() into iowrite32be() at that
> place you mentioned, and still using iowrite32be()/ioread32be() to replace
> clrsetbits_be32(), am I right?

Actually, I think it would be good to use ehci_writel() and
ehci_readl() everywhere except in fsl_ehci_drv_probe().

But if you prefer to use iowrite32be() and ioread32be() instead, that's
probably okay.  (However, it won't work if anyone ever produces a
little-endian version of the IP.)

Alan Stern