linux-kernel - RE: [PATCH 2/6] Revert "arm64: dts: renesas: r8a7796: Enable DMA for SCIF2"

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Message-ID: <OSBPR01MB3174C93C0A49701EC72F9D82D8060@OSBPR01MB3174.jpnprd01.prod.outlook.com>
Date:   Mon, 20 May 2019 02:18:47 +0000
From:   Yoshihiro Shimoda <yoshihiro.shimoda.uh@...esas.com>
To:     "REE erosca@...ADIT-JV.COM" <erosca@...ADIT-JV.COM>,
        Geert Uytterhoeven <geert@...ux-m68k.org>
CC:     Eugeniu Rosca <roscaeugeniu@...il.com>,
        Geert Uytterhoeven <geert+renesas@...der.be>,
        Simon Horman <horms+renesas@...ge.net.au>,
        Chris Brandt <Chris.Brandt@...esas.com>,
        Wolfram Sang <wsa+renesas@...g-engineering.com>,
        Ulrich Hecht <ulrich.hecht+renesas@...il.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "George G . Davis" <george_davis@...tor.com>,
        Andy Lowe <andy_lowe@...tor.com>,
        Linux-Renesas <linux-renesas-soc@...r.kernel.org>,
        "open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" 
        <devicetree@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Magnus Damm <magnus.damm@...il.com>,
        Rob Herring <robh+dt@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Michael Rodin <mrodin@...adit-jv.com>,
        "REE erosca@...ADIT-JV.COM" <erosca@...ADIT-JV.COM>
Subject: RE: [PATCH 2/6] Revert "arm64: dts: renesas: r8a7796: Enable DMA for
 SCIF2"

Hi Eugeniu-san, Geert-san,

> From: Eugeniu Rosca, Sent: Tuesday, May 7, 2019 4:43 AM
<snip>
> > > [0] v5.0-rc6 commit 97f26702bc95b5 ("arm64: dts: renesas: r8a7796: Enable DMA for SCIF2")
> > > [1] v4.14.106 commit 703db5d1b1759f ("arm64: dts: renesas: r8a7796: Enable DMA for SCIF2")
> > > [2] scif (DEBUG) and rcar-dmac logs:
> > >     https://gist.github.com/erosca/132cce76a619724a9e4fa61d1db88c66
<snip>
> Enabling DEBUG in drivers/dma/sh/rcar-dmac.c, I can notice that one of
> the symptoms is a NULL dst_addr revealed by:
> 
> rcar-dmac e7300000.dma-controller: chan0: queue chunk (____ptrval____): 0@...fff800639eb8090 -> 0x0000000000000000
> 
> In working scenarios, dst_addr is never zero. Does it give any hints?

Thank you for the report! It's very helpful to me.
I think we should fix the sh-sci driver at least.

According to the [2] log above,

[    4.379716] sh-sci e6e88000.serial: sci_dma_tx_work_fn: ffff800639b55000: 0...0, cookie 126

This "0...0" means the s->tx_dma_len on the sci_dma_tx_work_fn will be zero. And,

> rcar-dmac e7300000.dma-controller: chan0: queue chunk (____ptrval____): 0@...fff800639eb8090 -> 0x0000000000000000

This means the chunk->dst_addr is not set to the "dst_addr" for SCIF because the len on rcar_dmac_chan_prep_sg is zero.
So, I'm thinking:
 - we have to fix the sh_sci driver to avoid "tx_dma_len = 0" transferring.

and 

 - also we have to fix the rcar-dmac driver to avoid this issue because the DMA Engine API
   guide doesn't prevent the len = 0.

Eugeniu-san, Geert-san, what do you think?

Best regards,
Yoshihiro Shimoda

>>
> > Thanks!
> 
> Likewise!
> 
> >
> > 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
> 
> --
> Best Regards,
> Eugeniu.