Le lundi 15 octobre 2018 à 19:13 +0900, Tomasz Figa a écrit : > On Wed, Aug 8, 2018 at 11:55 AM Tomasz Figa <tfiga@xxxxxxxxxxxx> wrote: > > > > On Tue, Aug 7, 2018 at 4:37 PM Hans Verkuil <hverkuil@xxxxxxxxx> wrote: > > > > > > On 08/07/2018 09:05 AM, Tomasz Figa wrote: > > > > On Thu, Jul 26, 2018 at 7:57 PM Hans Verkuil <hverkuil@xxxxxxxxx> wrote: > > > > > > > I wonder if we should make these min buffer controls required. It might be easier > > > > > > > that way. > > > > > > > > > > > > Agreed. Although userspace is still free to ignore it, because REQBUFS > > > > > > would do the right thing anyway. > > > > > > > > > > It's never been entirely clear to me what the purpose of those min buffers controls > > > > > is. REQBUFS ensures that the number of buffers is at least the minimum needed to > > > > > make the HW work. So why would you need these controls? It only makes sense if they > > > > > return something different from REQBUFS. > > > > > > > > > > > > > The purpose of those controls is to let the client allocate a number > > > > of buffers bigger than minimum, without the need to allocate the > > > > minimum number of buffers first (to just learn the number), free them > > > > and then allocate a bigger number again. > > > > > > I don't feel this is particularly useful. One problem with the minimum number > > > of buffers as used in the kernel is that it is often the minimum number of > > > buffers required to make the hardware work, but it may not be optimal. E.g. > > > quite a few capture drivers set the minimum to 2, which is enough for the > > > hardware, but it will likely lead to dropped frames. You really need 3 > > > (one is being DMAed, one is queued and linked into the DMA engine and one is > > > being processed by userspace). > > > > > > I would actually prefer this to be the recommended minimum number of buffers, > > > which is >= the minimum REQBUFS uses. > > > > > > I.e., if you use this number and you have no special requirements, then you'll > > > get good performance. > > > > I guess we could make it so. It would make existing user space request > > more buffers than it used to with the original meaning, but I guess it > > shouldn't be a big problem. > > I gave it a bit more thought and I feel like kernel is not the right > place to put any assumptions on what the userspace expects "good > performance" to be. Actually, having these controls return the minimum > number of buffers as REQBUFS would allocate makes it very well > specified - with this number you can only process frame by frame and > the number of buffers added by userspace defines exactly the queue > depth. It leaves no space for driver-specific quirks, because the > driver doesn't decide what's "good performance" anymore. I agree on that and I would add that the driver making any assumption would lead to memory waste in context where less buffer will still work (think of fence based operation as an example). > > Best regards, > Tomasz
