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

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Message-ID: <871szx9rhc.fsf@belgarion.home>
Date:   Mon, 03 Oct 2016 21:12:31 +0200
From:   Robert Jarzmik <robert.jarzmik@...e.fr>
To:     Mark Rutland <mark.rutland@....com>
Cc:     Rob Herring <robh+dt@...nel.org>, 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

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.

Cheers.

-- 
Robert