linux-kernel - Re: [PATCH] drivers/block/xsysace - replace in(out)_8/in(out)_be16/in(out)

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Date:	Thu, 7 Feb 2013 14:51:49 +0000
From:	Grant Likely <grant.likely@...retlab.ca>
To:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
Cc:	Alexey Brodkin <Alexey.Brodkin@...opsys.com>,
	Michal Simek <monstr@...str.eu>, Arnd Bergmann <arnd@...db.de>,
	Vineet Gupta <Vineet.Gupta1@...opsys.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Alan Cox <alan@...rguk.ukuu.org.uk>,
	Geert Uytterhoeven <geert@...ux-m68k.org>,
	dahinds@...rs.sourceforge.net
Subject: Re: [PATCH] drivers/block/xsysace - replace in(out)_8/in(out)_be16/in(out)_le16
 with generic iowrite(read)8/16(be)

On Thu, Feb 7, 2013 at 2:39 PM, Grant Likely <grant.likely@...retlab.ca> wrote:
> On Wed, Feb 6, 2013 at 9:35 PM, Benjamin Herrenschmidt
> <benh@...nel.crashing.org> wrote:
>>>  In fact, the driver already knows about this and figures
>>> out at runtime how the device is wired up to the bus. This is not the
>>> problem.
>>
>> Except that this is very gross, especially when you observe that in the
>> busted "big endian" case, it has to byteswap the bloody data port.
>>
>> So you end up having to do that gross hack with separate accessors for
>> registers vs. data and not able to use the _rep variants, which also
>> means that on platforms like ppc, you end up with a memory barrier on
>> every access (or more), which is going to slow things down enormously.
>
> I don't see why the _rep variants aren't usable here. The only reason
> I didn't use them when I wrote the driver in the first place was I was
> a n00b kernel hacker and I didn't know they were there.

The 8-bit variant is different though because the hardware requires
pingponging between odd and even byte addresses to flush the fifo.
Reading a data port even address (0x40) gives the least significant
byte. Reading from an odd address (0x41) give the MSB and pops the
data off the FIFO. So, yes, the _rep variant can't be used in 8-bit
mode. It should still be fine in 16-bit.

page 45: http://www.xilinx.com/support/documentation/data_sheets/ds080.pdf
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