Re: [PATCH RFC v3 2/2] m68k/q40: add data_swab option for pata_falcon to byte-swap disk data

Michael Schmitz
2023-08-17T19:15:59Z

Hi Finn.

On 17/08/23 22:15, Finn Thain wrote:
> On Thu, 17 Aug 2023, Michael Schmitz wrote:
>
>> Some users of pata_falcon on Q40 have IDE disks in classic
>> little endian byte order, whereas legacy disks use 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 3 and 4.
> Many would say that's off by one, as it's popular to number the LSB as bit
> zero.

Old FORTRAN programmers never die. They just cause havoc in C.

I'll fix that.

>
>> 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 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 | 29 ++++++++++++++++++++++++++++-
>> 1 file changed, 28 insertions(+), 1 deletion(-)
>>
>> diff --git a/drivers/ata/pata_falcon.c b/drivers/ata/pata_falcon.c
>> index 346259e3bbc8..cec3c3a6eed9 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;
>>
>> @@ -165,10 +178,20 @@ 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;
>>
>> if (base_res) { /* only Q40 has IO resources */
>> + if (pdev->id)
>> + pata_falcon_swap_mask >>= 2;
>> + priv->swap_mask = pata_falcon_swap_mask;
>> io_offset = 0x10000;
>> reg_scale = 1;
>> base = (void __iomem *)base_res->start;
>> @@ -178,6 +201,7 @@ static int __init pata_falcon_init_one(struct platform_device *pdev)
>> &base_res->start,
>> &ctl_res->start);
>> } else {
>> + priv->swap_mask = pata_falcon_swap_mask;
>> base = (void __iomem *)base_mem_res->start;
>> ctl_base = (void __iomem *)ctl_mem_res->start;
>>
>> @@ -199,6 +223,9 @@ 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;
>>
>> + if (priv->swap_mask)
>> + priv->swap_data = 1;
>> +
>> irq_res = platform_get_resource(pdev, IORESOURCE_IRQ, 0);
>> if (irq_res && irq_res->start > 0) {
>> irq = irq_res->start;
>>
> I think the patch could be simplified by dropping the dynamic memory
> allocation...

If using a private data pointer field for an int bitmask is acceptable
use ...

I'll see if I can find any performance impact from the additional
swap_data flag. If there's none, your solution may be cleaner.

Cheers,

Michael

>
> diff --git a/drivers/ata/pata_falcon.c b/drivers/ata/pata_falcon.c
> index 346259e3bbc8..d057f11ec02d 100644
> --- a/drivers/ata/pata_falcon.c
> +++ b/drivers/ata/pata_falcon.c
> @@ -33,6 +33,11 @@
> #define DRV_NAME "pata_falcon"
> #define DRV_VERSION "0.1.0"
>
> +static int pata_falcon_data_swab;
> +
> +module_param_named(data_swab, pata_falcon_data_swab, int, 0444);
> +MODULE_PARM_DESC(data_swab, "Data byte swap enable/disable bitmap (0x1==drive1, 0x2==drive2, 0x4==drive3, 0x8==drive4, default: 0)");
> +
> static const struct scsi_host_template pata_falcon_sht = {
> ATA_PIO_SHT(DRV_NAME),
> };
> @@ -50,7 +55,7 @@ static unsigned int pata_falcon_data_xfer(struct ata_queued_cmd *qc,
>
> if (dev->class == ATA_DEV_ATA && cmd &&
> !blk_rq_is_passthrough(scsi_cmd_to_rq(cmd)))
> - swap = 0;
> + swap = (int)ap->private_data & BIT(dev->devno);
>
> /* Transfer multiple of 2 bytes */
> if (rw == READ) {
> @@ -199,6 +204,8 @@ 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;
>
> + ap->private_data = (void *)((pata_falcon_data_swab >> (2 * pdev->id)) & 3);
> +
> irq_res = platform_get_resource(pdev, IORESOURCE_IRQ, 0);
> if (irq_res && irq_res->start > 0) {
> irq = irq_res->start;