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

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Date:   Thu, 3 Sep 2020 10:10:30 +0200
From:   Ulf Hansson <ulf.hansson@...aro.org>
To:     Rich Felker <dalias@...c.org>, Christoph Hellwig <hch@....de>
Cc:     Geert Uytterhoeven <geert@...ux-m68k.org>,
        "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

On Thu, 3 Sep 2020 at 02:41, Rich Felker <dalias@...c.org> wrote:
>
> On Wed, Sep 02, 2020 at 05:51:16PM +0200, Geert Uytterhoeven wrote:
> > 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? ;-)
>
> Confirmed that this version of the patch works too. Thus,
>
> Tested-by: Rich Felker <dalias@...c.org>

I have applied the patch for fixes, thanks for testing!

Christoph, when it comes to the check of
"spi->master->dev.parent->dma_mask", I am keeping it for now. I am
simply not sure that all spi masters assign the pointer (even if most
are platform drivers). I think it's better that we remove that check
in a separate patch - to get it tested.

Kind regards
Uffe