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 19:28:24 +0400
From:	Alexey Brodkin <Alexey.Brodkin@...opsys.com>
To:	Grant Likely <grant.likely@...retlab.ca>
CC:	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	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 02/07/2013 07:23 PM, Grant Likely wrote:
> On Thu, Feb 7, 2013 at 3:12 PM, Alexey Brodkin
> <Alexey.Brodkin@...opsys.com> wrote:
>> On 02/07/2013 06:51 PM, Grant Likely wrote:
>>>
>>> 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
>>>
>>
>> Ok, so may I do a re-spin with these changes:
>
> There are two things here. 1) changing the accessors used. 2)
> switching the endianess as a bug fix. Any changes to the endian access
> should be a separate patch which a description of what is needed.
>
>> 1. In "ace_in_be16" use "ioread16be"
>> 2. In "ace_out_be16" use "iowrite16be"
>> 3. In "ace_in_le16" use "ioread16"
>> 4. In "ace_out_le16" use "iowrite16"
>
> Yes
>
>> 5. In "ace_datain_le16" use "ioread16_rep"
>> 6. In "ace_dataout_le16" use "iowrite16_rep"
>
> Maybe. In a separate patch. Hmmm... I guess there isn't an
> ioread16be_rep variant. Oh well. Check first with Michal on LE
> microblaze before making the change. If it doesn't work for him the
> more understanding is needed. I was pretty sure the LE variant already
> worked.
>
>> not sure about items for "ace_datain/out_be16" - what about _rep options
>> here?
>
> ioread16_rep should be fine. The ace_data{in,out}_be16 routines need
> to use the LE accessor. The existing code is definitely correct in
> this respect.

Well, if "ioread16_rep" is used in both "ace_data{in,out}_be16" and 
"ace_data{in,out}_le16" then what is a difference between them?
Whether there's a subtle difference still exists and I cannot see it or 
unified accessor could be used from now on at least for data access.

What do you think?

-Alexey
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