On 7/18/2019 7:06 AM, Keyon Jie wrote:
On 2019/7/18 上午11:35, Pierre-Louis Bossart wrote:
On 7/17/19 10:11 PM, Keyon Jie wrote:
From: Marcin Rajwa <marcin.rajwa@xxxxxxxxxxxxxxx>
In some cases, FW might need to use the host_period_bytes field to
synchronize the DMA copying (with host side) but the driver does not
it's your right to edit my suggested wording, but the notion of
'synchronization' is far from clear. it's my understanding that the
host_period_bytes defines the DMA transfer size requested by the
firmware, which isn't a value that matters to the host except for
rewind usages.
Hi Pierre, here the host_period_bytes is requested by host, FW has its
own period size, and DMA will transfer data in FW buffer period size.
It works like this:
FW buffer[period 0, period 1, ...] <==> DMA <==> host/alsa
buffer[host_period 0, host_priod 1, ...]
We need this host_preiod_bytes information in FW to do fast
draining(e.g. record 2 seconds data within 10ms) in mmap capture, we
are slowing down the draining in smaller host_period_bytes cases,
otherwise, arecord can't read the buffer in time and overrun will happen.
Maybe the wording "synchronize" here is inaccurate, how about
something like this:
"FW might need to use the host_period_bytes field to configure and
control the DMA copying speed but the driver does not..."
Hi,
we need *host_period_bytes *in FW to properly drain data - by properly I
mean to no override host buffer but in the same time to avoid situation
when host is waiting for data and doesn't get it. The former is known as
*overrun *while the later *underrun**.
*
So that's why I originally used the word *"to synchronize" *because it
best reflects the use of this variable in FW.
*The problem *- host establishes the *period_size/**host_period_bytes
*and uses it as a "data copy tempo controller" meaning it will copy data
buffered in its own buffer (copied there by FW) in *period_size *time
intervals. Now, in regular copy (real time stream) firmware doesn't
need to care about how fast host copy because the host buffer is big
enough to store even several FW periods (each one or few milliseconds)
without overriding it. That is why we did not need this *period_size
*variable before. However the draining task is very
different ball game - it copies *2,1 seconds* of data as fast as
possible to the host. Therefore we are very prone to *XRUNs*
*The solution* - in FW we need to know how often host copies data out
from its own buffer and this information is stored in
*host_period_bytes, *lets send this information down to firmware. Now,
the FW knowing this can fill the host buffer and wait the time
calculated by *host_period_bytes*
before next copy. This way we copy as much/fast as we can and in the
same time we are safe that host will handle this and no XRUN will ever
happen.
Hope it helped to understand the need of *host_period_bytes *in the
firmware.
**
Marcin
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