linux-kernel - Re: [PATCH] mmc: sdhci-tegra: Set DMA mask

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Date:	Fri, 26 Feb 2016 12:31:36 +0100
From:	Arnd Bergmann <arnd@...db.de>
To:	Alexandre Courbot <gnurou@...il.com>
Cc:	Alexandre Courbot <acourbot@...dia.com>,
	Ulf Hansson <ulf.hansson@...aro.org>,
	Stephen Warren <swarren@...dotorg.org>,
	Thierry Reding <thierry.reding@...il.com>,
	linux-mmc <linux-mmc@...r.kernel.org>,
	"linux-tegra@...r.kernel.org" <linux-tegra@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] mmc: sdhci-tegra: Set DMA mask

On Friday 26 February 2016 16:24:34 Alexandre Courbot wrote:
> On Thu, Feb 25, 2016 at 11:52 PM, Arnd Bergmann <arnd@...db.de> wrote:
> >> Actually even if we specify a dma-ranges on the parent DT node, the
> >> DMA range will still be limited to 32 bits because of the following
> >> code in of_dma_configure():
> >>
> >>     /*
> >>      * Set default coherent_dma_mask to 32 bit.  Drivers are expected to
> >>      * setup the correct supported mask.
> >>      */
> >>     if (!dev->coherent_dma_mask)
> >>         dev->coherent_dma_mask = DMA_BIT_MASK(32);
> >>
> >>     /*
> >>      * Set it to coherent_dma_mask by default if the architecture
> >>      * code has not set it.
> >>      */
> >>     if (!dev->dma_mask)
> >>         dev->dma_mask = &dev->coherent_dma_mask;
> >>
> >>     ....
> >>     /* gets dma-ranges into dma_addr and size */
> >>     ....
> >>
> >>
> >>     *dev->dma_mask = min((*dev->dma_mask),
> >>                  DMA_BIT_MASK(ilog2(dma_addr + size)));
> >>
> >> So unless the DMA mask is set on the device before of_dma_configure()
> >> is called, the min() statement will choose the 32 bits mask that has
> >> been previously set. So IIUC in any case, the driver will need to call
> >> dma_set_mask()
> >
> > Yes, the driver definitely has to call dma_set_mask(), the property of
> > the parent bus is used to make that fail when the bus doesn't support
> > it.
> 
> And that's where things seem to stop working: the driver's probe
> function is invoked by the platform bus, *after* of_dma_configure() is
> called. So unless I am missing something there is no way for the
> driver to set the DMA mask in such a way that of_dma_configure() can
> see it and do the right thing.
> 
> In other words, most of the DMA mask logic in of_dma_configure()
> doesn't seem to have any effect (and a 32 bits mask will be set), at
> least on the platform bus.

That is correct: of_dma_configure has to set a 32-bit DMA mask
because that is the default that we expect to see in Linux drivers.

A lot of drivers don't call dma_set_mask() at all, so this is
the most reasonable value that typically works, unless the
device is more limited, or you want the extra performance you
get on devices that actually support a bigger mask.

> >> Can I have your thoughts on this? Am I missing something?
> >
> > One point: I think the dma_set_mask() probably should be in the
> > generic part of the sdhci driver, not the tegra specific portion.
> 
> Ok, but then how does the generic part of the driver knows which DMA
> mask applies to the device?

If dma_set_mask() succeeds when passed a 64-bit mask, the driver
can pass high addresses into dma_map_*() and put the result into
the 64-bit DMA registers of the device. That is all the driver
needs to know here.

When the bus is more limited than the device, we either have
an swiotlb/iommu that will use bounce buffers to map dma_map_* work
anyway (using low DMA addresses for high memory), or we don't have
an swiotlb and the dma_set_mask() operation has to fail.

	Arnd