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

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Date:   Thu, 06 Oct 2016 08:47:13 +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

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 ?

Cheers.

-- 
Robert