linux-kernel - Re: [PATCH 09/10] sh: don't allow non-coherent DMA for NOMMU

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Date:   Mon, 31 Aug 2020 13:28:50 +0200
From:   Ulf Hansson <ulf.hansson@...aro.org>
To:     Christoph Hellwig <hch@....de>, Rich Felker <dalias@...c.org>
Cc:     Yoshinori Sato <ysato@...rs.sourceforge.jp>,
        Linux-sh list <linux-sh@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        "linux-mmc@...r.kernel.org" <linux-mmc@...r.kernel.org>,
        linux-spi@...r.kernel.org
Subject: Re: [PATCH 09/10] sh: don't allow non-coherent DMA for NOMMU

On Sat, 29 Aug 2020 at 10:31, Christoph Hellwig <hch@....de> wrote:
>
> On Fri, Aug 28, 2020 at 11:09:43AM -0400, Rich Felker wrote:
> > > However, by looking at the code, one get the feeling that the DMA
> > > support is somewhat prepared to be made optional. I guess it has never
> > > been really tested, as the Kconfig option has "depends on HAS_DMA"  -
> > > and it's been like that as long as I can remember.
> >
> > It always worked on our "byte-banged" SPI controller, with no DMA
> > controller present, before Christoph's changes in this patch series,
>
> Before that nommu sh builds provided a DMA mapping implementation
> that even worked for the streaming side (dma_map_*), but would corrupt
> data if you used dma_alloc_coherent memory to communicate with the
> device.

I see.

>
> > and seems to be working now (although I have some other, hopefully
> > unrelated regressions to debug) with #ifdef CONFIG_HAS_DMA around the
> > if (spi->master->dev.parent->dma_mask) block in mmc_spi_probe. That's
> > probably not the right fix though -- why isn't it checking
> > host->dma_dev instead and only attempting DMA setup if dma_dev is
> > non-null?
>
> I don't think dma_dev can be NULL right now.  dma_dev is assigned here:
>
>         if (spi->master->dev.parent->dma_mask) {
>                 struct device   *dev = spi->master->dev.parent;
>
>                 host->dma_dev = dev;
>
> but for any OF or real bus device dma_mask never is zero (it actually is
> a pointer), and the value of it also is initialized to 32-bit by default,
> making this effectively an "if (1) {".  The driver needs some way to
> communicate if a given device actually is DMA capable or not. Or is that
> purely a factor of the platform which would be a little strange.

Okay, thanks for clarifying things.

>
> In which case we should do something like:
>
>
> diff --git a/drivers/mmc/host/mmc_spi.c b/drivers/mmc/host/mmc_spi.c
> index 39bb1e30c2d722..3b0cc9a70e6432 100644
> --- a/drivers/mmc/host/mmc_spi.c
> +++ b/drivers/mmc/host/mmc_spi.c
> @@ -1374,7 +1374,7 @@ static int mmc_spi_probe(struct spi_device *spi)
>         if (!host->data)
>                 goto fail_nobuf1;
>
> -       if (spi->master->dev.parent->dma_mask) {
> +       if (IS_ENABLED(CONFIG_HAS_DMA)) {

I guess "#ifdef CONFIG_HAS_DMA", would work as well as CONFIG_HAS_DMA
can't be "m".

I would also suggest moving the DMA setup things into a separate
function. Then when CONFIG_HAS_DMA is unset, we can use a stub.

>                 struct device   *dev = spi->master->dev.parent;
>
>                 host->dma_dev = dev;

Let me cook a patch that we can try.

Kind regards
Uffe