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

[Yoshihiro Shimoda]

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@0xffff800639eb8090 -> 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@0xffff800639eb8090 -> 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@xxxxxxxxxxxxxx
> >
> > 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.