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

[James Bottomley <jejb@linux.ibm.com>]

On Sat, 2019-11-09 at 20:14 +0100, 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.

Have you tested this on any other hardware?  the reason most legacy
hardware would have a setting like this is because they have a two byte
transfer length register and zero doesn't mean 65536.  If this is the
case for any of the cards the zorro_esp drives, it might be better to
lower the max length to 61440 (64k-4k) so the residual is a page.

James