On Fri, 13 Aug 2010, Janusz Krzysztofik wrote: > Thursday 12 August 2010 23:38:17 Guennadi Liakhovetski napisał(a): > > On Sun, 1 Aug 2010, Janusz Krzysztofik wrote: > > > Friday 30 July 2010 20:49:05 Janusz Krzysztofik napisaÅ‚(a): > > > > > > I think the right way would be if implemented at the videobuf-core level. > > > Then, drivers should be able to initialize both paths, providing queue > > > callbacks for both sets of methods, contig and sg, for videobuf sole use. > > > > Ok, here're my thoughts about this: > > > > 1. We've discussed this dynamic switching a bit on IRC today. The first > > reaction was - you probably should concentrate on getting the contiguous > > version to work reliably. I.e., to reserve the memory in the board init > > code similar, how other contig users currently do it. > > I already tried before to find out how I could allocate memory at init without > reinventing a new videobuf-dma-contig implementation. Since in the > Documentation/video4linux/videobuf I've read that videobuf does not currently > play well with drivers that play tricks by allocating DMA space at system > boot time, I've implemented the alternate sg path. > > If it's not quite true what the documentation says and you can give me a hint > how this could be done, I might try again. For an example look at arch/arm/mach-mx3/mach-pcm037.c::pcm037_camera_alloc_dma(). > > But given problems > > with this aproach in the current ARM tree [1], this might be a bit > > difficult. Still, those problems have to be and will be fixed somehow > > eventually, so, you might prefer to still just go that route. > > My board uses two drivers that allocate dma memory at boot time: > drivers/video/omap/lcdc.c and sounc/soc/omap/omap-pcm.c. Both use > alloc_dma_writecombine() for this and work without problems. dma_alloc_writecombine() also allocates contiguous RAM, right? And it doesn't use device "local" memory. So, it's chances to fail are the same as those of dma_alloc_coherent() in the absence of device own memory. I guess, the sound driver doesn't need much RAM, but if you build your LCDC driver as a module and load it later after startup, it might get problems allocating RAM for the framebuffer. > > 2. If you do want to do the switching - we also discussed, how forthcoming > > changes to the videobuf subsystem will affest this work. I do not think it > > would be possible to implement this switching in the videobuf core. > > OK, I should have probably said that it looked not possible for me to do it > without any additional support implemented at videobuf-core (or soc_camera) > level. > > > Remember, with the videobuf API you first call the respective > > implementation init method, which doesn't fail. Then, in REQBUFS ioctl you > > call videobuf_reqbufs(), which might already fail but normally doesn't. > > The biggest problem is the mmap call with the contig videobuf > > implementation. This one is likely to fail. So, you would have to catch > > the failing mmap, call videobuf_mmap_free(), then init the SG videobuf, > > request buffers and mmap them... > > That's what I've already discovered, but failed to identify a place in my code > where I could intercept this failing mmap without replacing parts of the > videobuf code. Right, ATM soc-camera just calls videobuf_mmap_mapper() directly in its mmap method. I could add a callback to struct soc_camera_host_ops like int (*mmap)(struct soc_camera_device *, struct vm_area_struct *) and modify soc_camera_mmap() to check, whether the host driver has implemented it. If so - call it, otherwise call videobuf_mmap_mapper() directly just like now. So, other drivers would not have to be modified. And you could implement that .mmap() method, call videobuf_mmap_mapper() yourself, and if it fails for contig, fall back to SG. > > With my 2 patches from today, there is > > only one process (file descriptor, to be precise), that manages the > > videobuf queue. So, this all can only be implemented in your driver. > > The only way I'm yet able to invent is replacing the > videobuf_queue->int_ops->mmap_mapper() callback with my own wrapper that > would intercept a failing videobuf-dma-contig version of mmap_mapper(). This > could be done in my soc_camera_host->ops->init_videobuf() after the > videobuf-dma-contig.c version of the videobuf_queue->int_ops->mmap_mapper() > is installed with the videobuf_queue_dma_contig_init(). > > Is this method close to what you have on mind? See, if the above idea would suit your needs. Thanks Guennadi --- Guennadi Liakhovetski, Ph.D. Freelance Open-Source Software Developer http://www.open-technology.de/ -- 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
