Hello, On Thursday, April 21, 2011 2:00 PM Arnd Bergmann wrote: > On Thursday 21 April 2011, Marek Szyprowski wrote: > > On Wednesday, April 20, 2011 6:07 PM Arnd Bergmann wrote: > > > On Wednesday 20 April 2011, Marek Szyprowski wrote: > > > > The only question is how a device can allocate a buffer that will be > most > > > > convenient for IOMMU mapping (i.e. will require least entries to > map)? > > > > > > > > IOMMU can create a contiguous mapping for ANY set of pages, but it > performs > > > > much better if the pages are grouped into 64KiB or 1MiB areas. > > > > > > > > Can device allocate a buffer without mapping it into kernel space? > > > > > > Not today as far as I know. You can register coherent memory per device > > > using dma_declare_coherent_memory(), which will be used to back > > > dma_alloc_coherent(), but I believe it is always mapped right now. > > > > This is not exactly what I meant. > > > > As we have IOMMU, the device driver can access any system memory. However > > the performance will be better if the buffer is composed of larger > contiguous > > parts (like 64KiB or 1MiB). I would like to avoid putting logic that > manages > > buffer allocation into the device drivers. It would be best if such > buffers > > could be allocated by a single call to dma-mapping API. > > > > Right now there is dma_alloc_coherent() function, which is used by the > > drivers to allocate a contiguous block of memory and map it to DMA > addresses. > > With IOMMU implementation it is quite easy to provide a replacement for > it > > that will allocate some set of pages and map into device virtual address > > space as a contiguous buffer. > > > > This will have the advantage that the same multimedia device driver > > will work on both systems - Samsung S5PC110 (without IOMMU) and Exynos4 > > (with IOMMU). > > Right. > > > However dma_alloc_coherent() besides allocating memory also implies some > > particular type of memory mapping for it. IMHO it might be a good idea to > > separate these 2 things (allocation and mapping) somewhere in the future. > > > > On systems with IOMMU the dma_map_sg() can be also used to create a > mapping > > in device virtual address space, but the driver will still need to > allocate > > the memory by itself. > > Note that dma_map_sg() is the "streaming mapping", which provides a > cacheable > buffer all the time, while dma_alloc_coherent() and is the "coherent > mapping". Ok. > There is also dma_alloc_noncoherent(), which you can use to allocate a > buffer > for the streaming mapping. This is currently not implemented on ARM, but if > I understand you correctly, adding this would do what you want. Ok, I got it. Implementing dma_alloc_noncoherent() as well as dma_map_sg() for non-IOMMU cases also makes some sense and will simplify the drivers imho. > > > Ok, I see. Having one device per channel as you suggested could > probably > > > work around this, and it's at least consistent with how you'd represent > > > IOMMUs in the device tree. It is not ideal because it makes the video > > > driver more complex when it now has to deal with multiple struct device > > > that it binds to, but I can't think of any nicer way either. > > > > Well, this will definitely complicate the codec driver. I wonder if > allowing > > the driver to kmalloc(sizeof(struct device))) and copy the relevant data > > from the 'proper' struct device will be better idea. It is still hack but > > definitely less intrusive for the driver. > > No, I think that would be much worse, it definitely destroys all kinds of > assumptions that the core code makes about devices. However, I don't think > it's much of a problem to just create two child devices and use them > from the main driver, you don't really need to create a device_driver > to bind to each of them. I must have missed something. Video codec is a platform device and struct device pointer is gathered from it (&pdev->dev). How can I define child devices and attach them to the platform device? Best regards -- Marek Szyprowski Samsung Poland R&D Center -- To unsubscribe from this list: send the line "unsubscribe linux-media" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
