netdev - Re: [net-next-2.6 PATCH 2/3] fs_enet: Add support for MPC512x to fs

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Date:	Wed, 10 Feb 2010 11:20:25 +0100
From:	Wolfgang Grandegger <wg@...ndegger.com>
To:	David Miller <davem@...emloft.net>
CC:	agust@...x.de, netdev@...r.kernel.org, dzu@...x.de, wd@...x.de,
	jcrigby@...il.com, kosmo@...ihalf.com, linuxppc-dev@...abs.org,
	grant.likely@...retlab.ca
Subject: Re: [net-next-2.6 PATCH 2/3] fs_enet: Add support for MPC512x to
 fs_enet driver

Wolfgang Grandegger wrote:
> Hi David,
> 
> David Miller wrote:
>> From: Anatolij Gustschin <agust@...x.de>
>> Date: Tue, 9 Feb 2010 15:23:17 +0100
>>
>>> In my understanding, in the ESP scsi driver the set of defines for
>>> the register offsets is common for all chip drivers. The chip driver
>>> methods for register access translate the offsets because the
>>> registers on some chips are at different intervals (4-byte, 1-byte,
>>> 16-byte for mac_esp.c). But the register order is the same for
>>> different chips.
>>>
>>> In our case non only the register order is not the same for 8xx
>>> FEC and 5121 FEC, but there are also other differences, different
>>> reserved areas between several registers, some registers are
>>> available only on 8xx and some only on 5121.
>> That only means you would need to use a table based register address
>> translation scheme, rather than a simple calculation.  Something
>> like:
>>
>> static unsigned int chip_xxx_table[] =
>> {
>> 	[GENERIC_REG_FOO]	 = CHIP_XXX_FOO,
>> 	...
>> };
>>
>> static u32 chip_xxx_read_reg(struct chip *p, unsigned int reg)
>> {
>> 	unsigned int reg_off = chip_xxx_table[reg];
>>
>> 	return readl(p->regs + reg_off);
>> }
>>
>> And this table can have special tokens in entries for
>> registers which do not exist on a chip, so you can trap
>> attempted access to them in these read/write handlers.
> 
> Yes, that could be done, but to honest, I do not see any improvement in
> respect to the previous patch where the register offset were defined via
> pointers within a structure.
> 
>> Please stop looking for excuses to fork this driver, a
>> unified driver I think can be done cleanly.
> 
> Other people suggested to fork the driver because it's getting too ugly.

That said, I think there is consensus that it does not make sense, and
it's even not possible, to provide a kernel image which runs on both,
the 8xx and the mpc512x. Therefore, there is also no need for sharing
this driver at run time. Compile time selection would allow a more
elegant and transparent implementation. Would that be an acceptable
solution?

Wolfgang.
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