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. Arnd -- To unsubscribe from this list: send the line "unsubscribe linux-mmc" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
