Re: [RESEND RFC/PATCH 3/8] media: platform: mtk-vpu: Support Mediatek VPU

[Daniel Thompson <daniel.thompson@linaro.org>]

On 30 November 2015 at 11:43, andrew-ct chen
<andrew-ct.chen@mediatek.com> wrote:
> On Fri, 2015-11-27 at 12:21 +0000, Daniel Thompson wrote:
>> On 27/11/15 12:10, andrew-ct chen wrote:
>> >>> +
>> >>> > >+      memcpy((void *)send_obj->share_buf, buf, len);
>> >>> > >+      send_obj->len = len;
>> >>> > >+      send_obj->id = id;
>> >>> > >+      vpu_cfg_writel(vpu, 0x1, HOST_TO_VPU);
>> >>> > >+
>> >>> > >+      /* Wait until VPU receives the command */
>> >>> > >+      timeout = jiffies + msecs_to_jiffies(IPI_TIMEOUT_MS);
>> >>> > >+      do {
>> >>> > >+              if (time_after(jiffies, timeout)) {
>> >>> > >+                      dev_err(vpu->dev, "vpu_ipi_send: IPI timeout!\n");
>> >>> > >+                      return -EIO;
>> >>> > >+              }
>> >>> > >+      } while (vpu_cfg_readl(vpu, HOST_TO_VPU));
>> >> >
>> >> >Do we need to busy wait every time we communicate with the co-processor?
>> >> >Couldn't we put this wait*before*  we write to HOST_TO_VPU above.
>> >> >
>> >> >That way we only spin when there is a need to.
>> >> >
>> > Since the hardware VPU only allows that one client sends the command to
>> > it each time.
>> > We need the wait to make sure VPU accepted the command and cleared the
>> > interrupt and then the next command would be served.
>>
>> I understand that the VPU  can only have on message outstanding at once.
>>
>> I just wonder why we busy wait *after* sending the first command rather
>> than *before* sending the second one.
>
> No other special reasons. Just send one command and wait until VPU gets
> the command. Then, I think this wait also can be put before we write to
> HOST_TO_VPU.Is this better than former? May I know the reason?

Busy waiting is bad; it is a waste of host CPU processor time and/or power.

When the busy wait occurs after queuing then we will busy wait for
every command we send.

If busy wait occurs before next queuing then we will wait for a
shorter time in total because we have the chance to do something
useful on the host before we have to wait.


>> Streamed decode/encode typically ends up being rate controlled by
>> capture or display meaning that in these cases we don't need to busy
>> wait at all (because by the time we send the next frame the VPU has
>> already accepted the previous message).
>
> For now, only one device "encoder" exists, it is true.
> But, we'll have encoder and decoder devices, the decode and encode
> requested to VPU are simultaneous.

Sure, I accept that lock and busy-wait is an appropriate way to ensure
safety (assuming the VPU is fairly quick clearing the HOST_TO_VPU
flag).


> Is this supposed to be removed for this patches and we can add it back
> if the another device(decoder) is ready for review?

No I'm not suggesting that.

I only recommend moving the busy wait *before* end sending of command
(for efficiency reasons mentioned above).


Daniel.