On Thu, 16 Jan 2020 12:25:38 +0100, Keyon Jie wrote:
On Thu, 2020-01-16 at 11:27 +0100, Takashi Iwai wrote:
On Thu, 16 Jan 2020 10:50:33 +0100,
Oh, you're right, and I completely misread the patch.
Now I took a coffee and can tell you the story behind the scene.
I believe the current code is intentionally limiting the size to the preallocated size. This limitation was brought for not trying to allocate a larger buffer when the buffer has been preallocated. In the past, most hardware allocated the continuous pages for a buffer and the allocation of a large buffer fails quite likely. This was the reason of the buffer preallocation. So, the driver wanted to tell the user-space the limit. If user needs to have an extra large buffer, they are supposed to fiddle with prealloc procfs (either setting zero to clear the preallocation or setting a large enough buffer beforehand).
Thank you for the sharing, it is interesting and knowledge learned to me.
For SG-buffers, though, limitation makes less sense than continuous pages. e.g. a patch below removes the limitation for SG-buffers. But changing this would definitely cause the behavior difference, and I don't know whether it's a reasonable move -- I'm afraid that apps would start hogging too much memory if the limitation is gone.
I just went through all invoking to snd_pcm_lib_preallocate_pages*(), for those SNDRV_DMA_TYPE_DEV, some of them set the *size* equal to the *max*, some set the *max* several times to the *size*, IMHO, the *max*s are matched to those hardware's limiatation, comparing to the *size*s, aren't they?
In this case, I still think my patch hanle all TYPE_DEV/SNDRV_DMA_TYPE_DEV/TYPE_SG/SNDRV_DMA_TYPE_DEV cases more gracefully, we will still take the limitation from the specific driver set, from the *max* param, and the test results looks very nice here, we will take what the user space wanted for buffer-bytes via aply exactly, as long as it is suitable for the interval and constraints.
Well, I have a mixed feeling. Certainly we'd need some better way to allow a larger buffer allocation, especially for HDA. OTOH, if the buffer was preallocated, it's meant to be used actually. That's the point of the hw_constraint setup.
And now thinking again after another cup of coffee, I wonder why we do preallocate for HDA at all. For HD-audio, the allocation of any large buffer would succeed very likely because of SG-buffer.
So, just setting 0 to the preallocation size (but keeping else) would work, e.g. something like below? The help text needs adjustment, but you can see the rough idea.
thanks,
Takashi
--- a/sound/hda/Kconfig +++ b/sound/hda/Kconfig @@ -21,9 +21,10 @@ config SND_HDA_EXT_CORE select SND_HDA_CORE
config SND_HDA_PREALLOC_SIZE - int "Pre-allocated buffer size for HD-audio driver" + int "Pre-allocated buffer size for HD-audio driver" if !SND_DMA_SGBUF range 0 32768 - default 64 + default 64 if !SND_DMA_SGBUF + default 0 if SND_DMA_SGBUF help Specifies the default pre-allocated buffer-size in kB for the HD-audio driver. A larger buffer (e.g. 2048) is preferred