linux-kernel - Re: [PATCH 5/7] scsi: mac_scsi: Fix pseudo DMA implementation, take 2

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Date:   Mon, 3 Jun 2019 09:32:36 +1000 (AEST)
From:   Finn Thain <fthain@...egraphics.com.au>
To:     Geert Uytterhoeven <geert@...ux-m68k.org>
cc:     "James E.J. Bottomley" <jejb@...ux.ibm.com>,
        "Martin K. Petersen" <martin.petersen@...cle.com>,
        Michael Schmitz <schmitzmic@...il.com>,
        scsi <linux-scsi@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        stable <stable@...r.kernel.org>,
        linux-m68k <linux-m68k@...ts.linux-m68k.org>
Subject: Re: [PATCH 5/7] scsi: mac_scsi: Fix pseudo DMA implementation, take
 2

On Sun, 2 Jun 2019, Geert Uytterhoeven wrote:

> Hi Finn,
> 
> On Sun, Jun 2, 2019 at 3:29 AM Finn Thain <fthain@...egraphics.com.au> 
> wrote:
> > 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@...il.com>
> > Cc: Geert Uytterhoeven <geert@...ux-m68k.org>
> > Cc: stable@...r.kernel.org # v4.14+
> > Fixes: 3a0f64bfa907 ("mac_scsi: Fix pseudo DMA implementation")
> > Reported-by: Chris Jones <chris@...tin-jones.com>
> > Tested-by: Stan Johnson <userm57@...oo.com>
> > Signed-off-by: Finn Thain <fthain@...egraphics.com.au>
> 
> 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.?

Thanks.

-- 

> Thanks!
> 
> Gr{oetje,eeting}s,
> 
>                         Geert
> 
>