linux-kernel - Re: [PATCH 1/2] [IDE] Platform IDE driver (was: MMIO IDE driver)

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Date:	Wed, 25 Jul 2007 23:16:12 +0400
From:	Sergei Shtylyov <sshtylyov@...mvista.com>
To:	Scott Wood <scottwood@...escale.com>
Cc:	Vitaly Bordug <vitb@...nel.crashing.org>,
	linux-ide@...r.kernel.org, linux-kernel@...r.kernel.org,
	linuxppc-dev@...abs.org
Subject: Re: [PATCH 1/2] [IDE] Platform IDE driver (was: MMIO IDE driver)

Hello.

Scott Wood wrote:

>>> +    hwif->hw.io_ports[IDE_DATA_OFFSET] = port;
>>> +
>>> +    port += (1 << pdata->ioport_shift);
>>> +    for (i = IDE_ERROR_OFFSET; i <= IDE_STATUS_OFFSET;
>>> +         i++, port += (1 << pdata->ioport_shift))
>>
>>
>>
>>     Looks like shift doesn't buy as anything, why not just use stride?

> It doesn't buy us anything in here, but it's conceivable that someone 
> may want to write a driver that uses a shift in the I/O accessor rather 
> than an array of port offsets,

    It wouldn't be IDE driver then, and neither it would be libata which also 
does this another way this (despite pata_platform uses shifts too -- not in 
the accessors, so no speed loss).

> and it's easier to convert a shift to a stride than the other way around
 > (not all architectures have an
> equivalent of the cntlzw innstruction, and shift makes it clear that the 
> stride must be power-of-two).  Plus, using shift is consistent with what 
> we do on ns16550.

    Why the heck should we care about the UART code taling about IDE?!
    So, let me consider your argument purely speculative and invalid. ;-)

> -Scott

WBR, Sergei
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