Hello, On Tuesday, June 14, 2011 3:49 PM Arnd Bergmann wrote: > On Monday 13 June 2011, Marek Szyprowski wrote: > > cm_alloc/free are definitely not meant to be called from device drivers. > > They should be only considered as a backend for dma-mapping. > > > > 'Raw' contiguous memory block doesn't really make sense for the device > > drivers. What the drivers require is a contiguous memory block that is > > somehow mapped into respective bus address space, so dma-mapping > > framework is the right interface. > > > > alloc_pages(..., GFP_CMA) looks nice but in fact it is really impractical. > > The driver will need to map such buffer to dma context anyway, so imho > > dma_alloc_attributed() will give the drivers much more flexibility. In > > terms of dma-mapping the context argument isn't anything odd. > > Ok. > > > If possible I would like to make cma something similar to > > declare_dma_coherent()&friends, so the board/platform/bus startup code > > will just call declare_dma_contiguous() to enable support for cma for > > particular devices. > > Sounds good, I like that. Thanks. I thought a bit more on this and decided that I want to make this declare_dma_contiguous() optional for the drivers. It should be used only for some sophisticated cases like for example our video codec with two memory interfaces for 2 separate banks. By default the dma-mapping will use system-wide cma pool. (snipped) > > > * It requires you to pass the exact location of the area. I can see why > > > you want that on certain machines that require DMA areas to be spread > > > across multiple memory buses, but IMHO it's not appropriate for a > > > generic API. > > > > IMHO we can also use some NULL context to indicate some global, system > > wide CMA area and again -> in terms of dma-mapping api having a context > > isn't anything uncommon. > > Please explain the exact requirements that lead you to defining multiple > contexts. My feeling is that in most cases we don't need them and can > simply live with a single area. Depending on how obscure the cases are > where we do need something beyond that, we can then come up with > special-case solutions for them. Like it was already stated we need such feature for our multimedia codec to allocate buffers from different memory banks. I really don't see any problems with the possibility to have additional cma areas for special purposes. > > > * It requires you to hardcode the size in a machine specific source > file. > > > This probably seems to be a natural thing to do when you have worked > a > > > lot on the ARM architecture, but it's really not. We really want to > > > get rid of board files and replace them with generic probing based on > > > the device tree, and the size of the area is more policy than a > property > > > of the hardware that can be accurately described in the device tree > or > > > a board file. > > > > The problem is the fact that right now, we still have board files and we > > have to live with them for a while (with all advantages and > disadvantages). > > I hope that you won't require me to rewrite the whole support for all ARM > > platforms to get rid of board files to get CMA merged ;) > > Of course not. But we need to know what we want a platform with device > tree support to look like when it's using CMA, so we don't make it > harder to change the platforms over than it already is. > > > I see no problem defining CMA areas in device tree, as this is something > > really specific to particular board configuration. > > The problem with defining CMA areas in the device tree is that it's not > a property of the hardware, but really policy. The device tree file > should not contain anything related to a specific operating system > because you might want to boot something on the board that doesn't > know about CMA, and even when you only care about using Linux, the > implementation might change to the point where hardcoded CMA areas > no longer make sense. I really doubt that the device tree will carry only system-independent information. Anyway, the preferred or required memory areas/banks for buffer allocation is something that is a property of the hardware not the OS policy. > IMHO we should first aim for a solution that works almost everywhere > without the kernel knowing what board it's running on, and then we > can add quirks for devices that have special requirements. I think > the situation is similar to the vmalloc virtual address space, which > normally has a hardcoded size that works almost everywhere, but there > are certain drivers etc that require much more, or there are situations > where you want to make it smaller in order to avoid highmem. I'm trying to create something that will fulfill the requirements of my hardware, that's why I cannot focus on a generic case only. Best regards -- Marek Szyprowski Samsung Poland R&D Center -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxxx For more info on Linux MM, see: http://www.linux-mm.org/ . 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