Re: [PATCH] zorro_esp: increase maximum dma length to 65536 bytes

[Finn Thain <fthain@telegraphics.com.au>]

On Sat, 9 Nov 2019, Kars de Jong wrote:

> When using this driver on a Blizzard 1260, there were failures whenever
> DMA transfers from the SCSI bus to memory of 65535 bytes were followed by a
> DMA transfer of 1 byte. This caused the byte at offset 65535 to be
> overwritten with 0xff. The Blizzard hardware can't handle single byte DMA
> transfers.
> 
> Besides this issue, limiting the DMA length to something that is not a
> multiple of the page size is very inefficient on most file systems.
> 

Makes sense. You may want to add,
Fixes: b7ded0e8b0d1 ("scsi: zorro_esp: Limit DMA transfers to 65535 bytes")

> It seems this limit was chosen because the DMA transfer counter of the ESP
> by default is 16 bits wide, thus limiting the length to 65535 bytes.
> However, the value 0 means 65536 bytes, which is handled by the ESP and the
> Blizzard just fine. It is also the default maximum used by esp_scsi when
> drivers don't provide their own dma_length_limit() function.
> 
> Signed-off-by: Kars de Jong <jongk@linux-m68k.org>
> ---
>  drivers/scsi/zorro_esp.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/scsi/zorro_esp.c b/drivers/scsi/zorro_esp.c
> index ca8e3abeb2c7..4448567c495d 100644
> --- a/drivers/scsi/zorro_esp.c
> +++ b/drivers/scsi/zorro_esp.c
> @@ -218,7 +218,7 @@ static int fastlane_esp_irq_pending(struct esp *esp)
>  static u32 zorro_esp_dma_length_limit(struct esp *esp, u32 dma_addr,
>  					u32 dma_len)
>  {
> -	return dma_len > 0xFFFF ? 0xFFFF : dma_len;
> +	return dma_len > (1U << 16) ? (1U << 16) : dma_len;
>  }
>  

Would it be safer to simply remove this code and leave 
esp_driver_ops.dma_length_limit == NULL for all board types?

-- 

>  static void zorro_esp_reset_dma(struct esp *esp)
>