Re: Sun3 SCSI DMA, was Re: converting the NCR5380 drivers away from scsi_register

[Michael Schmitz <schmitzmic@gmail.com>]

Hi Finn,
> On Mon, 11 Aug 2014, Sam Creasey wrote:
>
>   
>> On Fri, Aug 08, 2014 at 08:46:08PM +1200, Michael Schmitz wrote:
>>     
>>>> IIRC, the DMA controller for the sun3's NCR5380 implementation is 
>>>> extremely fussy about what happens in which phase, and it's quick to 
>>>> anger if you don't handle everything exactly how it expects.
>>>>  
>>>>         
>>> Does the DMA controller also sit in between the bus and the NCR chip, 
>>> as on Falcon? Otherwise, I can't see how it would matter if we just 
>>> bypass it and use PIO instead.
>>>
>>> The PIO code from the Atari driver worked just fine for the Mac 5380 
>>> (in fact, the first Mac driver used PIO for everything, including data 
>>> in/out).
>>>       
>> PIO definitely works on the Sun3 implementation under the right 
>> circumstances, I ran it in PIO mode only for a while (much like the Mac 
>> driver).
>>
>> I can't remember the bus configuration offhand.
>>     
>
> It might not be a big problem: sun3_scsi does a lot of PIO a lot as it is.
>
> For NCR5380_reselect(), atari_NCR5380.c apparently uses only PIO whereas 
> sun3_NCR5380.c will use PIO up to a 128 byte size limit (beyond which, 
> only DMA is used).
>   

Transfer sizes for message in during reselection are only a few bytes. 
According to atari_dma_xfer_len(), DMA reads must be multiple of 512 
bytes (writes can be rounded up, reads cannot). PIO is the only option 
for Atari, I'm afraid.

> For NCR5380_information_transfer() and PHASE_DATAIN, atari will use PIO 
> for transfersize <= 31 bytes whereas sun3 will use PIO for <= 128 bytes. 
>   

IIRC the ST-DMA can't be programmed for shorter transfers than 512 bytes. 
The condition

(transfersize = NCR5380_dma_xfer_len(instance,cmd,phase)) > 31)

enforces that - transfersize will be 0 on reads that are not multiples 
of 512 bytes (and even some where the advertised transfer size does meet 
that condition, but the command is not guranteed to be a block mode 
command (tape reads).

> However, atari never uses DMA here if cmd->device->borken.
>   

Different semantics of borken, I presume. On Atari, this means a DMA 
transfer has failed earlier.

> When cmd->device->borken, I assume sun3_NCR5380 inhibits PIO because PIO 
> was itself expected to be problematic?
>
> OTOH, if PIO works up to 128 bytes in all cases, why not larger transfers? 
>   

Performance, perhaps. PIO would work, but hog the CPU. In fact, PIO for 
everything _does_ work, on Falcon as well as Mac (I tried the PIO only 
mode on the Falcon before trying on Mac). Did I eve mention how reallly 
incredibly painfully slow that is?

> Does the sun3_scsi driver still work after removing #define REAL_DMA?
>
> For NCR5380_information_transfer() and PHASE_CMDOUT, the sun3 version 
> applies the same size limit but only does DMA setup if cmd type is 
> REQ_TYPE_FS (i.e. filesystem request). This command is then sent by PIO, 
> so the DMA setup here seems to assume that the next phase will always be 
> PHASE_DATAIN...
>   

Might be a similar limitation to certain transfer sizes here. Block mode 
vs. char mode.

> This would seem to imply that PIO always works when not REQ_TYPE_FS, such 
> that no size limit is applicable...
>
> BTW, testing sun3_dma_setup_done against cmd pointers looks very 
> unreliable to me: unique cmds only have unique pointers until freed by the 
> scsi mid-layer. After that, the same pointer is likely be re-used.
>
>   
>>>> I definitely remember that the DMA setup logic from the atari version 
>>>> did not work at all on sun3.
>>>>         
>>> No surprise there - DMA is implementation specific and needs to be 
>>> handled on a per-case basis.
>>>       
>
> Perhaps atari_NCR5380 could use some of the sun3_NCR5380 code. E.g. atari 
> avoids DMA entirely for reselect, and (only in some phases) when 
> cmd->device->borken.
>   

I don't think we can use DMA for the reselect message in transfers. Data 
phase transfer is handled by the coroutine after reselection anyway - 
that one will use DMA.

Cheers,

    Michael