Re: [PATCH 5/7] scsi: mac_scsi: Fix pseudo DMA implementation, take 2
From: Finn Thain
Date: Sun Jun 02 2019 - 19:36:37 EST
On Sun, 2 Jun 2019, Geert Uytterhoeven wrote:
> Hi Finn,
> On Sun, Jun 2, 2019 at 3:29 AM Finn Thain <fthain@xxxxxxxxxxxxxxxxxxx>
> > A system bus error during a PDMA transfer can mess up the calculation
> > of the transfer residual (the PDMA handshaking hardware lacks a byte
> > counter). This results in data corruption.
> > The algorithm in this patch anticipates a bus error by starting each
> > transfer with a MOVE.B instruction. If a bus error is caught the
> > transfer will be retried. If a bus error is caught later in the
> > transfer (for a MOVE.W instruction) the transfer gets failed and
> > subsequent requests for that target will use PIO instead of PDMA.
> > This avoids the "!REQ and !ACK" error so the severity level of that
> > message is reduced to KERN_DEBUG.
> > Cc: Michael Schmitz <schmitzmic@xxxxxxxxx>
> > Cc: Geert Uytterhoeven <geert@xxxxxxxxxxxxxx>
> > Cc: stable@xxxxxxxxxxxxxxx # v4.14+
> > Fixes: 3a0f64bfa907 ("mac_scsi: Fix pseudo DMA implementation")
> > Reported-by: Chris Jones <chris@xxxxxxxxxxxxxxxx>
> > Tested-by: Stan Johnson <userm57@xxxxxxxxx>
> > Signed-off-by: Finn Thain <fthain@xxxxxxxxxxxxxxxxxxx>
> Thanks for your patch!
> > ---
> > arch/m68k/include/asm/mac_pdma.h | 179 +++++++++++++++++++++++++++
> > drivers/scsi/mac_scsi.c | 201 ++++++++-----------------------
> Why have you moved the PDMA implementation to a header file under
> arch/m68k/? Do you intend to reuse it by other drivers?
There are a couple of reasons: the mac_esp driver also uses PDMA and the
NuBus PowerMac port also uses mac_scsi.c. OTOH, the NuBus PowerMac port is
still out-of-tree, and it is unclear whether the mac_esp driver will ever
benefit from this code.
> If not, please keep it in the driver, so (a) you don't need an ack from
> me ;-), and (b) your change may be easier to review.
I take your wink to mean that you don't want to ask the SCSI maintainers
to review m68k asm. Putting aside the code review process for a moment, do
you have an opinion on the most logical way to organise this sort of code,
from the point-of-view of maintainability, re-usability, readability etc.?