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

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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@xxxxxxxxxxxxxx>
Cc: Finn Thain <fthain@xxxxxxxxxxxxxx>
Cc: Geert Uytterhoeven <geert@xxxxxxxxxxxxxx>
Tested-by: William R Sowerbutts <will@xxxxxxxxxxxxxx>
Signed-off-by: Michael Schmitz <schmitzmic@xxxxxxxxx>
Reviewed-by: Sergey Shtylyov <s.shtylyov@xxxxxx>

[...]

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



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