netdev - Re: [PATCH 3/3] net: smsc911x: add u16 workaround for pxa platforms

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Date:   Sat, 8 Oct 2016 20:28:59 -0500
From:   Rob Herring <robh@...nel.org>
To:     Robert Jarzmik <robert.jarzmik@...e.fr>
Cc:     Mark Rutland <mark.rutland@....com>,
        Nicolas Pitre <nico@...xnic.net>,
        Russell King - ARM Linux <linux@...linux.org.uk>,
        Arnd Bergmann <arnd@...db.de>, netdev@...r.kernel.org,
        devicetree@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/3] net: smsc911x: add u16 workaround for pxa platforms

On Thu, Oct 06, 2016 at 08:47:13AM +0200, Robert Jarzmik wrote:
> Robert Jarzmik <robert.jarzmik@...e.fr> writes:
> 
> > Mark Rutland <mark.rutland@....com> writes:
> >
> >> On Mon, Oct 03, 2016 at 06:11:23PM +0200, Robert Jarzmik wrote:
> >>> Mark Rutland <mark.rutland@....com> writes:
> >>> 
> >>> reg-u16-align4 tells that a specific hardware doesn't support 16 bit writes not
> >>> being 32 bits aligned, or said differently that a "store" 16 bits wide on an
> >>> address of the format 4*n + 2 deserves a special handling in the driver, while a
> >>> store 16 bits wide on an address of the format 4*n can follow the simple casual
> >>> case.
> >>
> >> If I've understood correctly, effectively the low 2 address lines to the
> >> device are hard-wired to zero, e.g. a 16-bit access to 4*n + 2 would go
> >> to 4*n + 0 on the device? Or is the failure case distinct from that?
> > It is distinct.
> >
> > The "awful truth" is that an FPGA lies between the system bus and the
> > smc91c111. And this FPGA cannot handle correctly the 4*n + 2 u16 writes.
> >
> >> Do we have other platforms where similar is true? e.g. u8 accesses
> >> requiring 16-bit alignment?
> >
> > Not really, ie. not with a alignement requirement.
> >
> > But there are of course these ones are handled by reg-io-width and the
> > SMC_USE_xxx_BITS flags as far as I understand it. These cases are when a
> > platform declares SMC91X_USE_16BIT or SMC91X_USE_32BIT, but not SMC91X_USE_8BIT,
> > which would make me think of :
> >  - CONFIG_SH_SH4202_MICRODEV,
> >  - CONFIG_M32R
> >  - several omap1 boards
> >  - 1 sa1100 board
> >  - several MMP and realview boards
> >
> > With all these platforms, each u8 access is replaced with a u16 access and a
> > mask / shift + mask.
> 
> Or so what should I call this entry if reg-u16-align4 is not a good candidate ?

This seems broken in a very platform specific way, so perhaps something 
named based on the platform.

Rob