Re: [PATCH 0/2] serial: sh-sci: Fix .flush_buffer() issues

[Geert Uytterhoeven <geert@linux-m68k.org>]

Hi Eugeniu,

On Wed, Jun 26, 2019 at 7:34 PM Eugeniu Rosca <roscaeugeniu@gmail.com> wrote:
> On Mon, Jun 24, 2019 at 02:35:38PM +0200, Geert Uytterhoeven wrote:
> > This patch series attempts to fix the issues Eugeniu Rosca reported
> > seeing, where .flush_buffer() interfered with transmit DMA operation[*].
> >
> > There's a third patch "dmaengine: rcar-dmac: Reject zero-length slave
> > DMA requests", which is related to the issue, but further independent,
> > hence submitted separately.
> >
> > Eugeniu: does this fix the issues you were seeing?
>
> Many thanks for both sh-sci and the rcar-dmac patches.
> The fixes are very much appreciated.
>
> > Geert Uytterhoeven (2):
> >   serial: sh-sci: Fix TX DMA buffer flushing and workqueue races
> >   serial: sh-sci: Terminate TX DMA during buffer flushing
> >
> >  drivers/tty/serial/sh-sci.c | 33 ++++++++++++++++++++++++---------
> >  1 file changed, 24 insertions(+), 9 deletions(-)
>
> I reserved some time to get a feeling about how the patches behave on
> a real system (H3-ES2.0-ULCB-KF-M06), so here come my observations.

Thanks for your extensive testing!

> First of all, the issue I have originally reported in [0] is only
> reproducible in absence of [4]. So, one of my questions would be how
> do you yourself see the relationship between [1-3] and [4]?

I consider them independent.
Just applying [4] would fix the issue for the console only, while the
race condition can still be triggered on other serial ports.

> That said, all my testing assumes:
>  - Vanilla tip v5.2-rc6-15-g249155c20f9b with [4] reverted.
>  - DEBUG is undefined in {sh-sci.c,rcar-dmac.c}, since I've noticed
>    new issues arising in the debug build, which are unrelated to [0].
>
> Below is the summary of my findings:
>
>  Version         IS [0]       Is console       Error message when
> (vanilla+X)    reproduced?  usable after [0]   [0] is reproduced
>                              is reproduced?
>  ------------------------------------------------------------
>  -[4]             Yes           No                [5]
>  -[4]+[1]         Yes           No                -
>  -[4]+[2]         Yes           Yes               [5]
>  -[4]+[3]         Yes           Yes               [6]
>  -[4]+[1]+[2]     No            -                 -
>  -[4]+[1]+[2]+[3] No            -                 -
>  pure vanilla     No            -                 -
>
> This looks a little too verbose, but I thought it might be interesting.

Thanks, it's very helpful to provide these results.

> The story which I see is that [1] does not fix [0] alone, but it seems
> to depend on [2]. Furthermore, if cherry picked alone, [1] makes the
> matters somewhat worse in the sense that it hides the error [5].

OK.

> My only question is whether [1-3] are supposed to replace [4] or they
> are supposed to happily coexist. Since I don't see [0] being reproduced

They are meant to coexist.

> with [1-3], I personally prefer to re-enable DMA on SCIF (when the
> latter is used as console) so that more features and code paths are
> exercised to increase test coverage.

If a serial port is used as a console, the port is used for both DMA
(normal use) and PIO (serial console output).  The latter can have a
negative impact on the former, aggravating existing bugs, or triggering
more races, even in the hardware.  So I think it's better to be more
cautious and keep DMA disabled for the console.

> [0] https://lore.kernel.org/lkml/20190504004258.23574-3-erosca@de.adit-jv.com/
> [1] https://patchwork.kernel.org/patch/11012983/
>     ("serial: sh-sci: Fix TX DMA buffer flushing and workqueue races")
> [2] https://patchwork.kernel.org/patch/11012987/
>     ("serial: sh-sci: Terminate TX DMA during buffer flushing")
> [3] https://patchwork.kernel.org/patch/11012991/
>     ("dmaengine: rcar-dmac: Reject zero-length slave DMA requests")
> [4] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=099506cbbc79c0
>     ("serial: sh-sci: disable DMA for uart_console")
>
> [5] rcar-dmac e7300000.dma-controller: Channel Address Error
> [6] rcar-dmac e7300000.dma-controller: rcar_dmac_prep_slave_sg: bad parameter: len=1, id=19
>     sh-sci e6e88000.serial: Failed preparing Tx DMA descriptor

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-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