i3c_master_add_i3c_dev_locked() accept PID, BCR, DCR

[bbrezillon@kernel.org (Boris Brezillon)]

On Tue, 11 Dec 2018 13:21:03 +0100
Boris Brezillon <bbrezillon@kernel.org> wrote:

> 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.

Sorry, small mistake, I counted bytes instead of bits.

max_time_to_query_pid_bcr_dcr_per_bus = 20 * 8 * 11 / 12500000 = 141usec.

> Let's assume we have a huge overhead caused by the OS (and all the
> scheduling involved), and make it 100usec.

And let's make it 500usec with the OS overhead, which is still not
particularly worrisome in term of boot-time.