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

[Tudor Ambarus <tudor.ambarus@xxxxxxxxxx>]

On 2/14/24 08:51, Théo Lebrun wrote:
> Hello,

Hi, Théo,
> 
> 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.

I took a random SPI NOR flash [1], its page program typical time is 64µs
according to its SFDP data. We'll have to add here the delay the
software handling takes.

If you want to play a bit more, you can write the entire flash then
compare the ftrace numbers of spi_mem_add_op_stats() with spi_nor_write().

Cheers,
ta

[1] https://ww1.microchip.com/downloads/en/DeviceDoc/20005119G.pdf