Re: [PATCH v4 2/2] ata: pata_falcon: add data_swab option to byte-swap disk data

[Michael Schmitz <schmitzmic@gmail.com>]

Hi Damien,

On 24/08/23 11:03, Damien Le Moal wrote:
> On 8/24/23 01:35, Sergey Shtylyov wrote:
>> On 8/23/23 1:13 AM, Michael Schmitz wrote:
>>
>>> Some users of pata_falcon on Q40 have IDE disks in default
>>> IDE little endian byte order, whereas legacy disks use
>>> host-native big-endian byte order as on the Atari Falcon.
>>>
>>> Add module parameter 'data_swab' to allow connecting drives
>>> with non-native data byte order. Drives selected by the
>>> data_swap bit mask will have their user data byte-swapped to
>>> host byte order, i.e. 'pata_falcon.data_swab=2' will byte-swap
>>> all user data on drive B, leaving data on drive A in native
>>> byte order. On Q40, drives on a second IDE interface may be
>>> added to the bit mask as bits 2 and 3.
>>>
>>> Default setting is no byte swapping, i.e. compatibility with
>>> the native Falcon or Q40 operating system disk format.
>>>
>>> Cc: William R Sowerbutts <will@sowerbutts.com>
>>> Cc: Finn Thain <fthain@linux-m68k.org>
>>> Cc: Geert Uytterhoeven <geert@linux-m68k.org>
>>> Tested-by: William R Sowerbutts <will@sowerbutts.com>
>>> Signed-off-by: Michael Schmitz <schmitzmic@gmail.com>
>> Reviewed-by: Sergey Shtylyov <s.shtylyov@omp.ru>
>>
>> [...]
>>
>>> diff --git a/drivers/ata/pata_falcon.c b/drivers/ata/pata_falcon.c
>>> index 3841ea200bcb..7cf15bd9764a 100644
>>> --- a/drivers/ata/pata_falcon.c
>>> +++ b/drivers/ata/pata_falcon.c
>> [...]
>>> @@ -194,6 +199,9 @@ static int __init pata_falcon_init_one(struct platform_device *pdev)
>>>   	ata_port_desc(ap, "cmd %px ctl %px data %pa",
>>>   		      base, ctl_base, &ap->ioaddr.data_addr);
>>>   
>>> +	ap->private_data = (void *)(uintptr_t)(pdev->id > 0 ?
>>> +		pata_falcon_swap_mask >> 2 : pata_falcon_swap_mask);
>>     How about:
>>
>> 	ap->private_data = (void *)(uintptr_t)(pata_falcon_swap_mask >>
>> 			   (pdev->id > 0 ? 2 : 0));
> This is so hard to decode... Let's please spell this out.
> Something like:
>
> 	int shift;
>
> 	if (pdev->id)

Atari Falcon has pdev->id==-1, so this must be 'if (pdev->id > 0)' here.

(Testing for pdev->id nonzero did work in earlier versions of my patch 
because patch 3 changed the platform device ID on Atari. That patch had 
the potential to confuse user space so I dropped it from v3 on.)

> 		shift = 2;
> 	else
> 		shift = 0;

No need for the 'else' if we initialize shift above (as is done for irq, 
io_offset and reg_shift).

> 	ap->private_data = (uintptr_t)(pata_falcon_swap_mask >> shift);
>
> This is initialization, so no need to try to optimize and rather privilege clear
> code.

Hmm - or maybe I'll leave that spelled out as you suggest.

Cheers,

     Michael

>
>> [...]
>>
>> MBR, Sergey