Re: [PATCH 2/3] m68k/q40: add data_swab option for pata_falcon to byte-swap disk data

[Damien Le Moal <dlemoal@kernel.org>]

On 2023/08/18 7:12, 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>
> Signed-off-by: Michael Schmitz <schmitzmic@gmail.com>
> 
> ---
> 
> Changes since RFC v4:
> 
> Geert Uytterhoeven:
> - don't shift static module parameter for drive 3/4 bitmask
> - simplify bit mask calculation to always use pdev->id
> 
> Finn Thain:
> - correct bit numbers for drive 3/4
> 
> Changes since RFC v3:
> 
> - split off this byte swap handling into separate patch
> 
> - add hint regarding third and fourth drive on Q40
> 
> Finn Thain:
> - rename module parameter to 'data_swab' to better reflect its use
> 
> William Sowerbutts:
> - correct IDE drive number used in data swap conditional
> ---
>  drivers/ata/pata_falcon.c | 26 +++++++++++++++++++++++++-
>  1 file changed, 25 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/ata/pata_falcon.c b/drivers/ata/pata_falcon.c
> index 346259e3bbc8..90488f565d6f 100644
> --- a/drivers/ata/pata_falcon.c
> +++ b/drivers/ata/pata_falcon.c
> @@ -33,6 +33,16 @@
>  #define DRV_NAME "pata_falcon"
>  #define DRV_VERSION "0.1.0"
>  
> +static int pata_falcon_swap_mask;
> +
> +module_param_named(data_swab, pata_falcon_swap_mask, int, 0444);
> +MODULE_PARM_DESC(data_swab, "Data byte swap enable/disable bitmap (0x1==drive1, 0x2==drive2, 0x4==drive3, 0x8==drive4, default==0)");
> +
> +struct pata_falcon_priv {
> +	unsigned int swap_mask;
> +	bool swap_data;
> +};
> +
>  static const struct scsi_host_template pata_falcon_sht = {
>  	ATA_PIO_SHT(DRV_NAME),
>  };
> @@ -44,13 +54,15 @@ static unsigned int pata_falcon_data_xfer(struct ata_queued_cmd *qc,
>  	struct ata_device *dev = qc->dev;
>  	struct ata_port *ap = dev->link->ap;
>  	void __iomem *data_addr = ap->ioaddr.data_addr;
> +	struct pata_falcon_priv *priv = ap->private_data;
>  	unsigned int words = buflen >> 1;
>  	struct scsi_cmnd *cmd = qc->scsicmd;
> +	int dev_id = dev->devno;
>  	bool swap = 1;
>  
>  	if (dev->class == ATA_DEV_ATA && cmd &&
>  	    !blk_rq_is_passthrough(scsi_cmd_to_rq(cmd)))
> -		swap = 0;
> +		swap = priv->swap_data && (priv->swap_mask & BIT(dev_id));
>  
>  	/* Transfer multiple of 2 bytes */
>  	if (rw == READ) {
> @@ -123,6 +135,7 @@ static int __init pata_falcon_init_one(struct platform_device *pdev)
>  	struct resource *base_res, *ctl_res, *irq_res;
>  	struct ata_host *host;
>  	struct ata_port *ap;
> +	struct pata_falcon_priv *priv;
>  	void __iomem *base, *ctl_base;
>  	int irq = 0, io_offset = 1, reg_scale = 4;
>  
> @@ -165,6 +178,13 @@ static int __init pata_falcon_init_one(struct platform_device *pdev)
>  	ap->pio_mask = ATA_PIO4;
>  	ap->flags |= ATA_FLAG_SLAVE_POSS | ATA_FLAG_NO_IORDY;
>  
> +	priv = devm_kzalloc(&pdev->dev,
> +		sizeof(struct pata_falcon_priv), GFP_KERNEL);
> +	if (!priv)
> +		return -ENOMEM;
> +
> +	ap->private_data = priv;
> +
>  	/* N.B. this assumes data_addr will be used for word-sized I/O only */
>  	ap->ioaddr.data_addr = (void __iomem *)base_mem_res->start;
>  
> @@ -199,6 +219,10 @@ static int __init pata_falcon_init_one(struct platform_device *pdev)
>  	ap->ioaddr.altstatus_addr	= ctl_base + io_offset;
>  	ap->ioaddr.ctl_addr		= ctl_base + io_offset;
>  
> +	priv->swap_mask = pata_falcon_swap_mask >> (2 * pdev->id);

I do not understand the shift here. It seems that it will lead to
priv->swap_mask always being 0...

> +	if (priv->swap_mask)
> +		priv->swap_data = 1;

I do not understand why priv->swap_data is needed, given that it is set to 1 if
priv->swap_mask != 0, the above:

	swap = priv->swap_data && (priv->swap_mask & BIT(dev_id));

should be equivalent to the simpler:

	swap = priv->swap_mask & BIT(dev_id);

No ? Am I missing something ?

> +
>  	irq_res = platform_get_resource(pdev, IORESOURCE_IRQ, 0);
>  	if (irq_res && irq_res->start > 0) {
>  		irq = irq_res->start;

-- 
Damien Le Moal
Western Digital Research