Re: [PATCH v2] zorro_esp: Limit DMA transfers to 65536 bytes (except on Fastlane)

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

On Tue, 12 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.
> 
> 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.
> 
> The limit of 65536 bytes can be used by all boards except the Fastlane. The
> old driver used a limit of 65532 bytes (0xfffc), which is reintroduced in
> this patch.
> 
> Fixes: b7ded0e8b0d1 ("scsi: zorro_esp: Limit DMA transfers to 65535 bytes")
> Signed-off-by: Kars de Jong <jongk@linux-m68k.org>

Reviewed-by: Finn Thain <fthain@telegraphics.com.au>

> ---
>  drivers/scsi/zorro_esp.c | 11 +++++++++--
>  1 file changed, 9 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/scsi/zorro_esp.c b/drivers/scsi/zorro_esp.c
> index ca8e3abeb2c7..a23a8e5794f5 100644
> --- a/drivers/scsi/zorro_esp.c
> +++ b/drivers/scsi/zorro_esp.c
> @@ -218,7 +218,14 @@ 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;
> +}
> +
> +static u32 fastlane_esp_dma_length_limit(struct esp *esp, u32 dma_addr,
> +					u32 dma_len)
> +{
> +	/* The old driver used 0xfffc as limit, so do that here too */
> +	return dma_len > 0xfffc ? 0xfffc : dma_len;
>  }
>  
>  static void zorro_esp_reset_dma(struct esp *esp)
> @@ -604,7 +611,7 @@ static const struct esp_driver_ops fastlane_esp_ops = {
>  	.esp_write8		= zorro_esp_write8,
>  	.esp_read8		= zorro_esp_read8,
>  	.irq_pending		= fastlane_esp_irq_pending,
> -	.dma_length_limit	= zorro_esp_dma_length_limit,
> +	.dma_length_limit	= fastlane_esp_dma_length_limit,
>  	.reset_dma		= zorro_esp_reset_dma,
>  	.dma_drain		= zorro_esp_dma_drain,
>  	.dma_invalidate		= fastlane_esp_dma_invalidate,
>