i3c_master_add_i3c_dev_locked() accept PID, BCR, DCR

[bbrezillon@kernel.org (Boris Brezillon)]

On Tue, 11 Dec 2018 12:07:16 +0000
vitor <vitor.soares@synopsys.com> wrote:

> On 11/12/18 11:58, Boris Brezillon wrote:
> >> I just want to skip GETBCR/DCR/PID operations since those data are
> >> already available in the controller. There's advantages but they
> >> only have relevance depending of use case.  
> > Can you detail those use cases please?  
> 
> If the bus has a lot of devices and you want to decrease the
> initialization time this could be the first step.
> 

Did you quantify the boot-time/perf improvement? The clk should be
running at 12.5MHz, GETPID takes 9bytes and GETBCR/DCR take 4 bytes
each. Let's add one byte per command to account for the start/stop
bits. That makes a total of 20bytes per device, with a maximum of 11
devices per bus:

max_time_to_query_pid_bcr_dcr_per_bus = 20 * 10 / 12500000 = 17.6 usec.
Let's assume we have a huge overhead caused by the OS (and all the
scheduling involved), and make it 100usec.

So we're talking about saving a maximum of ~100 usec per bus. Sorry, but
I think you'll have plenty of other things to optimize before this
becomes a boot-time issue.