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

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Date:   Mon, 3 Jun 2019 08:23:13 +0200
From:   Geert Uytterhoeven <geert@...ux-m68k.org>
To:     Finn Thain <fthain@...egraphics.com.au>
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

Hi Finn,

On Mon, Jun 3, 2019 at 1:32 AM Finn Thain <fthain@...egraphics.com.au> wrote:
> On Sun, 2 Jun 2019, Geert Uytterhoeven wrote:
> > 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.

So you do have future sharing in mind...

> > 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

I meant that apart from the code containing m68k assembler source, it
is not related to arch/m68k/, and thus belongs to the driver.
There are several other drivers that contain pieces of assembler code.

> 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.?

If the code is used by multiple SCSI drivers, you can move it to a header
file under drivers/scsi/.
If the code is shared by drivers belonging to multiple subsystems, you can
move it to a header file under include/linux/.

Anyone who has a better solution?
Thanks!

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds