Re: [PATCH] spi: spi-mem: add statistics support to ->exec_op() calls

[Théo Lebrun <theo.lebrun@xxxxxxxxxxx>]

Hello,

On Wed Feb 14, 2024 at 9:00 AM CET, Tudor Ambarus wrote:
> On 2/13/24 15:00, Théo Lebrun wrote:
> > On Tue Feb 13, 2024 at 1:39 PM CET, Tudor Ambarus wrote:
> >>>  /**
> >>>   * spi_mem_exec_op() - Execute a memory operation
> >>>   * @mem: the SPI memory
> >>> @@ -339,8 +383,12 @@ int spi_mem_exec_op(struct spi_mem *mem, const struct spi_mem_op *op)
> >>>  		 * read path) and expect the core to use the regular SPI
> >>>  		 * interface in other cases.
> >>>  		 */
> >>> -		if (!ret || ret != -ENOTSUPP || ret != -EOPNOTSUPP)
> >>> +		if (!ret || ret != -ENOTSUPP || ret != -EOPNOTSUPP) {
> >>> +			spi_mem_add_op_stats(ctlr->pcpu_statistics, op, ret);
> >>> +			spi_mem_add_op_stats(mem->spi->pcpu_statistics, op, ret);
> >>> +
> >>
> >> Would be good to be able to opt out the statistics if one wants it.
> >>
> >> SPI NORs can write with a single write op maximum page_size bytes, which
> >> is typically 256 bytes. And since there are SPI NORs that can run at 400
> >> MHz, I guess some performance penalty shouldn't be excluded.
> > 
> > I did my testing on a 40 MHz octal SPI NOR with most reads being much
> > bigger than 256 bytes, so I probably didn't have the fastest setup
> > indeed.
>
> yeah, reads are bigger, the entire flash can be read with a single read op.
>
> > 
> > What shape would that take? A spi-mem DT prop? New field in the SPI
> > statistics sysfs directory?
> > 
>
> I think I'd go with a sysfs entry, it provides flexibility. But I guess
> we can worry about this if we have some numbers, and I don't have, so
> you're fine even without the opt-out option.

Some ftrace numbers:
- 48002 calls to spi_mem_add_op_stats();
- min 1.053000µs;
- avg 1.175652µs;
- max 16.272000µs.

Platform is Mobileye EyeQ5. Cores are Imagine Technologies I6500-F. I
don't know the precision of our timer but we might be getting close to
what is measurable.

Thanks,

--
Théo Lebrun, Bootlin
Embedded Linux and Kernel engineering
https://bootlin.com