Em 20-03-2012 04:57, Guennadi Liakhovetski escreveu: > On Mon, 19 Mar 2012, Mauro Carvalho Chehab wrote: > >> Em 20-02-2012 16:23, Fabio Estevam escreveu: >>> On Mon, Feb 20, 2012 at 4:17 PM, Guennadi Liakhovetski >>> <g.liakhovetski@xxxxxx> wrote: >>>> On Mon, 20 Feb 2012, Fabio Estevam wrote: >>>> >>>>> Align mx3_camera driver with the other soc camera driver implementations >>>>> by allocating the camera object via kzalloc. >>>> >>>> Sorry, any specific reason, why you think this "aligning" is so important? >>> >>> Not really. >>> >>> Just compared it with all other soc camera drivers I found and >>> mx3_camera was the only one that uses "vzalloc" >>> >>> Any specific reason that requires mx3_camera to use "vzalloc" instead >>> of "kzalloc"? >> >> kzalloc() is more restrictive than vzalloc(). With v*alloc, it will allocate >> a virtual memory area, with can be discontinuous, while kzalloc will get >> a continuous area. >> >> The DMA logic need to be prepared for virtual memory, if v*alloc() is used >> (e. g. using videobuf2-vmalloc). >> >> As it is currently including media/videobuf2-dma-contig.h, I this patch >> makes sense on my eyes. > > Don't think so. vzalloc() is used in mx3_camera to allocate driver private > data objects and are never used for DMA, so, it doesn't have any > restrictions on contiguity, coherency, alignment etc. OK. In this case, using v*alloc()/vfree() won't be that different than k*alloc()/k*free(). > One could argue, that since the struct is anyway smaller than 1 page, it > anyway will be allocated in a physically contiguous memory area (will it?) > and so, maybe, kmalloc() is less heavy weight than vmalloc() and might > save a couple of CPU cycles, but I don't think it's anything important, > that we should be optimising for. Yes. There's another aspect to consider there: the vmalloc space is limited (there's a boot time parameter to regulate its size)[1]. I dunno why, nor what are the consequences of using a bigger value, but I think a big vmalloc size creates a big table to map between virtual/physical memory space. Yet, a single page is far below the vmaloc default max size, except if the system has very severe memory constraints. [1] On x86, I think that the default is 256MB. Several video adapters eat a lot of space there. I use a bigger value here, otherwise my 3-head system won't initialize all screens. Regards, Mauro -- 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
