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

[Alexandre Courbot]

On 02/26/2016 08:31 PM, Arnd Bergmann wrote:
> On Friday 26 February 2016 16:24:34 Alexandre Courbot wrote:
> > On Thu, Feb 25, 2016 at 11:52 PM, Arnd Bergmann <arnd@xxxxxxxx> 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.

Ok, I think I understand. I was expecting of_dma_configure() to do more 
than it actually needs to and be the final arbiter of a device's DMA 
mask. That's obviously not the case - thanks for the thorough explanation.

Let me send a v2 of this to see if I got it properly.