Hi Felipe, On Fri, May 14, 2010 at 10:27 PM, Felipe Contreras <felipe.contre...@gmail.com> wrote: > Hi, > > I spent some time looking deeper into this patch series, and I have some > doubts. > > On Sun, May 2, 2010 at 8:47 PM, Ohad Ben-Cohen <o...@wizery.com> wrote: >> On Sun, May 2, 2010 at 4:17 PM, Felipe Contreras >> <felipe.contre...@gmail.com> wrote: >>> On Sat, May 1, 2010 at 11:44 PM, Ohad Ben-Cohen <o...@wizery.com> wrote: >>>> The patchset renames the flush ioctl to begin_dma_to_dsp and >>>> the invalidate ioctl to begin_dma_from_dsp. Both functions >>>> eventually call dma_map_sg, with the former requesting a >>>> DMA_BIDIRECTIONAL direction, and the latter requesting a >>>> DMA_FROM_DEVICE direction. >>>> In addition, the patchset adds two new APIs which calls dma_unmap_sg: >>>> end_dma_to_dsp and end_dma_from_dsp. >>>> >>>> Ideally, there would be only a single begin_dma command and a single >>>> end_dma one, which would accept an additional parameter that will >>>> determine the direction of the transfer. Such an approach would be more >>>> versatile and cleaner, but it would also break all user space apps that >>>> use dspbridge today. >>> >>> If I understand correctly all user-space apps would be broken anyway >>> because they are not issuing the end_dma calls. At the very least they >>> need to be updated to use them. >> >> Basically you're right, but the patches currently silently allow >> today's user space >> to somehow work (because if you don't use dma bounce buffers and you feel >> lucky >> about speculative prefetching then you might get away with not calling >> dma unmap). >> I did that on purpose, to ease testing & migration, but didn't >> explicitely said it because >> frankly it's just wrong. > > I looked into the dma code (I guess it's in arch/arm/mm/dma-mapping.c) > and I don't understand what dma_unmap_sg is supposed to do.
Portable code must use the dma_unmap_* APIs. As you mentioned, one of the things it's crucial for (but not used in our case) is bounce buffers: e.g. when doing a DMA from a device using a buffer that is out of the platform's DMA reach, the DMA API uses bounce buffers: data DMA'ed to that bounce buffer, and then the dma unmap copies it back to the original buffer). Another thing unmap is taking care of is speculative prefetch: modern ARM processors can access the buffer during DMA in order to speculatively prefetch data into the cache. Naturally this can seriously break a DMA from a device, so unmap is invalidating the caches to prevent this. The current dspbridge cache API is mostly relying on the application to do the right thing: application programmer should decide when to clean, flush or invalidate the caches in order to have a successful DMA operation. This is prone to mistakes, it's not portable, and of course not upstreamable. Using the standard DMA API we take the freedom from the application programmer - we just ask him/her to tell us before a DMA operation begins, and after it ends. The DMA API is then responsible to do the right thing. > first some "safe buffer" is searched, and if there isn't any... then > it depends on the direction whether something is actually done or not. > > I guess it depends whether our arch has dmabounce or not, which I have > no idea, but if we do, wouldn't skiping dma_unmap calls leak some > massive amount of "safe buffers"? > >>> Also, in Nokia we patched the bridgedriver to be able to have the 3 >>> operations available from user-space (clean, inv, and flush), so we >>> would be very interested in having the direction of the transfer >>> available. >> >> Thanks, that's important to know. > > It's not that critical, but I guess it can't hurt to do the right thing. > >> What do you say about the following plan then: >> - I'll add a single pair of begin_dma and end_dma commands that will >> take the additional >> direction parameter as described above. I'll then covert the existing >> flush & invalidate commands to use this begin_dma command supplying it >> the appropriate direction argument. > > Sounds perfect, but I wonder if the deprecated flush & invalidate > would really work. See previous comments. > >> - In a separate patch, I'll deprecate flush & invalidate entirely >> (usage of this deprecated >> API will fail, resulting in a guiding error message). >> >> We get a sane and versatile (and platform-independent) implementation >> that always work, >> but breaks user space. If anyone would need to work with current user space, >> the deprecating patch can always be reverted locally to get back a >> flush & invalidate >> implementations that (somehow) work. > > User-space API is being broken all the time in dspbridge. The > difference is that this one might require nontrivial changes. But it > must be done. > > So, I tried your patches, and a simple test app worked fine without > modification, but a real video decoding hanged the device > completely... some spinlock was stuck. I don't know if it's because of > your patches, or because of the state of the bridge at that point. > I'll try first to rebase to the latest to have a better idea of what's > happening. I read at your latest posts that after rebasing to newest dspbridge code the issue doesn't reproduce anymore ? Please tell me if it's back. > > Also, I noticed an important problem. Your code assumes that we would > always have a bunch of scattered pages, however you are not taking > into account bridge_brd_mem_map() where vm_flags have VM_IO... in that > case get_user_pages() is skipped completely. This code-path is > important in order to mmap framebuffer memory, which is contiguous. > So, in this case there are no pages too keep track at all. > > This use-case is very important since the dspbridge mmap operation is > very expensive, and due to GStreamer design, we must do it > constantly... if the memory is contiguous (VM_IO), the mmap operation > is really fast (or at least should be... in theory). Thanks for catching this VM_IO code path. I'll take care of this and resubmit the patches. > > Reading your patches I see the ioctl to start the dmap operations > would simply error out, but from the looks of it, they would be > failing already, which is weird, because we are already using > framebuffer memory for video decoding + rendering. In gst-dsp we are > not checking the success of clean/invalidate ioctls so it might be > that it has been failing all this time and seems to work by pure luck. > > Anyway, I'll keep investigating and let you know if I find anything. Again thanks for your review, your comments are very helpful. Ohad. > > Cheers. > > -- > Felipe Contreras > -- To unsubscribe from this list: send the line "unsubscribe linux-omap" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
