Re: [PATCH v1 3/5] spi: Added way to check for pm support for flash devices

[Cyrille Pitchen <cyrille.pitchen-AIFe0yeh4nAAvxtiuMwx3w@public.gmane.org>]

Hi all,

Le 04/02/2017 à 21:47, Kamal Dasu a écrit :
> On Sat, Feb 4, 2017 at 6:25 AM, Mark Brown <broonie-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org> wrote:
>> On Fri, Feb 03, 2017 at 06:31:14PM -0500, Kamal Dasu wrote:
>>
>>> Device drivers can check if the master controller driver has to support
>>> flash pm. The controller driver indicates this using flash_pm_supported()
>>> spi master interface.
>>
>> What is "flash pm" and how would a client use it given that no API for
>> actually managing power is being added here?  Someone looking at the API
>> needs to be able to figure these things out and right now I can't see
>> how they'd do that...o
> 
> The flash_pm function just indicates if  m25p80 resume op that does a
> spi_nor_pm_rescan() is needed, and by default will do nothing for
> other platforms. So the basic idea of the patches was to execute the
> only necessary spi_nor_pm_rescan() on resume when flash parts are
> reset on suspend. So from controller perspective there is no
> additional pm activity to be done. Patches 1/5-2/5 is what is needed
> actually. And Patches 3/3 - 5/5 only add a check if m25p80 resume
> should call spi_nor_pm_rescan(). And the controller driver shall
> implement the flash_pm function and return true if it does need a
> rescan to be done on resume.
> 

I don't understand why we extend in the SPI framework API to add power
management features but only for SPI flashes. I guess concerning the power
management, there is nothing special about SPI flashes: they are just SPI
devices like others. So why not extend the SPI framework, if needed, with
something working for all SPI devices, not just for SPI flashes.

There was a good reason to create a flash specific API in the SPI
framework: spi_flash_read(), flash_read_supported().
The reason is that the SPI controller needs protocol info (op code,
address, dummy cycles, SPI x-y-z protocol, ...) to handle the read
operation correctly and those pieces of information were lost in m25p80.c
when it used the regular SPI API with spi_transfer and spi_message
structures. Hence spi_flash_read() is a mean to bypass the regular SPI API
and provide the SPI controller driver with all the protocol info it needs.

Back to the power management use case, I don't see any technical reason to
create a SPI flash oriented solution.

Best regards,

Cyrille
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