linux-kernel - Re: [PATCH v2] mmc: mmc_spi: Allow the driver to be built when CONFIG_HAS

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Date:   Wed, 2 Sep 2020 17:51:16 +0200
From:   Geert Uytterhoeven <geert@...ux-m68k.org>
To:     Rich Felker <dalias@...c.org>
Cc:     Ulf Hansson <ulf.hansson@...aro.org>,
        Christoph Hellwig <hch@....de>,
        "linux-mmc@...r.kernel.org" <linux-mmc@...r.kernel.org>,
        Mark Brown <broonie@...nel.org>,
        Linux-sh list <linux-sh@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] mmc: mmc_spi: Allow the driver to be built when
 CONFIG_HAS_DMA is unset

Hi Rich,

On Wed, Sep 2, 2020 at 5:43 PM Rich Felker <dalias@...c.org> wrote:
> On Wed, Sep 02, 2020 at 10:31:47AM +0200, Ulf Hansson wrote:
> > On Tue, 1 Sep 2020 at 17:40, Christoph Hellwig <hch@....de> wrote:
> > > On Tue, Sep 01, 2020 at 05:36:17PM +0200, Ulf Hansson wrote:
> > > > > I still don't think this makes sense, as the dma_mask should always
> > > > > be non-NULL here.
> > > >
> > > > If that is the case, I wonder how the driver could even have worked without DMA.
> > > >
> > > > Because in the existing code, host->dma_dev gets assigned to
> > > > spi->master->dev.parent->dma_mask - which seems to turn on the DMA
> > > > usage in the driver.
> > > >
> > > > What am I missing?
> > >
> > > Do you know of other non-DMA users?  For SH nommu it probably worked
> >
> > I don't know of other non-DMA users. As I said, I wish someone could
> > step in and take better care of mmc_spi - as I know it's being used a
> > lot.
> >
> > > because SH nommu used to provide a DMA implementation that worked
> > > fine for streaming maps, but was completely broken for coherent
> > > allocation.  And this driver appears to only use the former.
> >
> > Alright, so you are saying the DMA support may potentially never have
> > been optional to this driver. In any case, I can remove the check in
> > $subject patch, as it shouldn't matter.
>
> DMA support was always optional, because even on systems where DMA is
> present, it doesn't necessarily mean the SPI controller uses DMA. In
> particular, pure bit-banged SPI via GPIOs doesn't have DMA, but has
> always worked. See my previous reply to Christoph about host->dma_dev
> for my current-best understanding of what's going on here.
>
> > Anyway, let's see what Rich thinks of this. I am curious to see if the
> > patch works on his SH boards - as I haven't been able to test it.
>
> I'll rebuild and retest just to confirm, but I already tested a
> functionally equivalent patch that just did the #ifdef inline (rather
> than moving the logic out to separate functions) and it worked fine.

Hence, Tested-by? ;-)

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