[PATCH 0/4] mmc: core: Add support for MMC power sequences

[Ulf Hansson]

On 2 January 2015 at 19:14, Russell King - ARM Linux
<linux at arm.linux.org.uk> wrote:
> On Fri, Jan 02, 2015 at 05:14:04PM +0100, Ulf Hansson wrote:
>> To be able to handle these SOC specific power sequences, we add a MMC power
>> sequence interface, which helps the mmc core to deal with such.
>
> I think this should be done differently - part of that is with hind sight
> given that we now have a range of interface types.
>
> One of my early design mistakes with MMC was to incorporate the power up
> and initialisation protocol (the 74 clocks business) into the core code.
> This was wrong, because it is a detail which only applies to dumb host
> interfaces.  More inteligent interfaces do not need that complexity as
> they handle that in hardware.
>
> Rather than MMC ending up with more layers and more infrastructure like
> this - turning it more into the turd which is sdhci - I would like to see
> some proper thought put into design, specifically addressing the above
> design issue.
>
> What should be done is to move away from the opaque "set_ios" method into
> something more appropriate - a set of callbacks which describe what we
> want to achieve.
>
> In the case of power up, there should be a single call into the host
> controller code which is responsible for initiating the power up
> sequence, and waits for the power up sequence to complete.  In other
> words, most of mmc_power_up() should be moved to a library function,
> which dumb host adapters hook into the "power up" method.  More
> inteligent adapters (eg, PXA, sdhci) can add their own hook which poke
> the hardware and return when the hardware has completed its power up
> sequence.
>
> That avoids the "mmc_delay(10)" for inteligent adapters where these
> delays are not required - which presumably also are not required if some
> special power up sequence for eMMC is required.
>
> The same thing goes for this "powerseq" stuff - all you're doing is
> propagating the same design mistake (with the POWER_UP/POWER_ON etc
> details) into it.  That shouldn't be the case - think about the dumb
> powerup method I described above vs the inteligent method - these are
> two separate powerup methods, and should be implemented as such.

Hi Russell,

Thanks for your comments!

You certainly have a point around how to improve the use the existing
->set_ios() callbacks. Though, as Hans also pointed out in his reply,
I believe this patchset is not related to that.

Regarding you concern, about me propagating the same design mistake as
for the ->set_ios() callback, I am not sure I fully understand why you
think that's the case?

I am suggesting is to add a set of mmc-pwrseq callbacks, which I think
is needed for a mmc-pwrseq provider to be able handle it's SOC
specific mmc power sequence. There are no MMC/SD/SDIO protocol
specific constraints that should be handled from them, but instead
only SOC specific power resources and sequences.

If the _simple_ mmc power sequence provider, turns out to be too
complex while trying to support several SOCs, we shall instead add yet
another provider. In this case, we should of course share as much code
as possible between the different providers.

Kind regards
Uffe

>
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