Date:   Sun, 15 Jan 2017 10:47:22 +1100 (AEDT)
From:   Finn Thain <fthain@...egraphics.com.au>
To:     Michael Schmitz <schmitzmic@...il.com>
cc:     Bartlomiej Zolnierkiewicz <b.zolnierkie@...sung.com>,
        Tejun Heo <tj@...nel.org>,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        linux-ide@...r.kernel.org,
        Linux/m68k <linux-m68k@...ts.linux-m68k.org>,
        Linux Kernel Development <linux-kernel@...r.kernel.org>,
        Andreas Schwab <schwab@...ux-m68k.org>
Subject: Re: [PATCH 0/3] ata: add m68k/Atari Falcon PATA support


On Sat, 14 Jan 2017, Michael Schmitz wrote:

> Am 13.01.2017 um 15:33 schrieb Finn Thain:
> 
> >> The case I'm worried about is both IDE and SCSI raising an interrupt. 
> >> We don't currently mask the IDE/ST-DMA interrupt so a stacked 
> >> interrupt must be processed in the same pass as the initial interrupt 
> >> or it will get dropped. We'd have to peek at the DMA registers to 
> >> check the SCSI or floppy interrupt status, and we just can't safely 
> >> do that. So races of this kind are currently prevented by including 
> >> IDE in the IRQ locking process.
> >>
> >> Whether it's possible to mask the interrupt, do one pass, unmask and 
> >> process the second interrupt I don't know.
> > 
> > Would that require handling the SCSI DMA interrupt in the first pass? 
> > Or handling IDE first, and ensuring that the IDE handler does not 
> > access ST-DMA registers? What about FDC?
> 
> Handling the IDE interrupt first I think, then looking at the DMA (for 
> SCSI or FDC).
> 

For the sake of discussion, I'll assume that the FDC driver will not be 
using DMA. (Perhaps FDC and SCSI can share the ST-DMA chip, using the 
present locking mechanism, but it would not simplify things much: when IDE 
no longer participates in that locking mechanism then both FDC and SCSI 
drivers have to solve the same issues.)

> > The atari_scsi handler accesses the ST-DMA registers; it can do so 
> > because it knows that any DMA must have completed -- it can infer this 
> > because a simultaneous pending interrupt from FDC or IDE is impossible 
> > due to stdma_lock().
> 
> libata dropped the locking (and does not use IDE interrupts at present 
> so it seems to be safe. Still testing - I've seen IO errors, and that's 
> a bit of a worry).
> 

What compiler are you using, BTW? Are you still using the gcc-4.6.3 m68k 
cross-compiler from kernel.org? I had to abandon it in order to get my 
SCSI driver patches to work.

> > Your suggestion would seem to allow other pending interrupts, hence 
> > the atari_scsi interrupt handler logic has to be tossed out. What 
> > logic would replace it?
> 
> I need to think about that some more - if no DMA is in progress we can 
> safely peek at the SCSI registers. So the logic could be changed to test 
> for DMA operation first, and just try and service the interrupt if DMA 
> wasn't active.
> 

OK, so based on the above, we handle the possible IDE interrupt (without 
checking DMA registers), handle the possible FDC interrupt (again without 
checking DMA registers) and finally handle the possible SCSI interrupt.

The core 5380 driver knows whether or not it has started a DMA. The 
atari_scsi driver also knows that no other Falcon driver uses DMA. So the 
atari_scsi handler only has to figure out whether the interrupt was 
asserted by the ST-DMA chip or the 5380 chip, or neither. (The "neither" 
possility arises when IDE ditches the the stdma.c lock mechanism.)

Without the stdma.c lock, any or all of these interrupts could be asserted 
simultaneously, so the IDE and FDC drivers need to be able to do the right 
thing in the presence of the other interrupts and do so without accessing 
the ST-DMA chip. And the SCSI interrupt handler needs to do the right 
thing when there is no DMA interrupt, and yet a DMA is running.

Perhaps we could reverse the algorithm in scsi_falcon_intr(): if 
NCR5380_intr() completes with IRQ_HANDLED and the core 5380 driver is no 
longer in DMA, then check the ST-DMA registers for errors etc.

Alternatively, if NCR5380_intr() returns IRQ_NONE, then do nothing at all, 
on the basis that the interrupt was handled by FDC or IDE. In this 
situation, I gather that atari_scsi could miss out on a DMA completion 
interrupt from the ST-DMA chip, which could lead to command timeout?

> If DMA has been in progress, I'm not sure that we can figure out if it's 
> still active from looking at the status register (that is, whether bits 
> 0 or 1 are set while DMA is ongoing). We'd have to peek at the DMA 
> status register (or DMA address registers) without first stopping DMA, 
> which the current driver does. The docs seem to advise against that. If 
> DMA was in progress, stopping it would likely leave us with residual 
> bytes to be transferred -

I can't comment on the Profibuch doc or the ST-DMA chip details (Andreas?)

I suspect it has to be tried.

> we'd have to handle that transfer as we would any other DMA error (from 
> memory, probably best to retry the entire command, or transfer the 
> remaining bytes using PIO if we're sure no bytes have been lost).
> 

Allowing the command to fail should be fine so long as the 5380 driver 
sends the correct result code to the mid-layer. To attempt to complete the 
command after a failed DMA is needless complexity, and it's a trick that 
probably can't be pulled off reliably anyway.

-- 

> > If all else fails, perhaps we could inhibit DMA entirely when the new 
> > ATA driver is loaded. Then we can just dispatch the ST-DMA irq like a 
> > shared irq. I'm sure that atari_scsi can work without DMA. No idea 
> > about the FDC driver though (ataflop.c).
> 
> Yes, SCSI can work using PIO but's it a real dog. Been there, done that 
> (about 20 years ago). I know nothing about the FDC chip though.
> 
> > Another solution would be to dedicate the DMA function to atari_scsi, 
> > and then mask the FDC and IDE interrupts during each DMA transfer. But 
> > once again, this would mean changing the FDC driver to eliminate DMA, 
> > if that is possible. From the schematic it looks the the FDC chip, 
> > "AJAX", is another custom ... 
> > http://dev-docs.atariforge.org/files/Falcon030_Schematic.pdf
> > 
> > Unfortunately my grasp of the ST hardware reflects my inability to 
> > read German; those who can may want to take a look at "ATARI Profibuch 
> > ST-STE-TT.pdf".
> 
> I'll reread the ST-DMA description (and the FDC one).
> 
> Let me know if you think this could work ...
> 
> Cheers,
> 
> 	Michael
> 

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