[PATCH] mmc: dw_mmc: rockchip: Keep controller working for card detect

[Heiko Stübner]

Am Freitag, 4. Oktober 2024, 19:34:33 CEST schrieb Robin Murphy:
> On 02/10/2024 10:55 pm, Ulf Hansson wrote:
> > On Sat, 14 Sept 2024 at 13:52, Heiko Stübner <heiko at sntech.de> wrote:
> >>
> >> Am Donnerstag, 12. September 2024, 09:26:14 CEST schrieb Kever Yang:
> >>> In order to make the SD card hotplug working we need the card detect
> >>> function logic inside the controller always working. The runtime PM will
> >>> gate the clock and the power domain, which stops controller working when
> >>> no data transfer happen.
> >>>
> >>> So lets skip enable runtime PM when the card needs to detected by the
> >>> controller and the card is removable.
> >>>
> >>> Signed-off-by: Kever Yang <kever.yang at rock-chips.com>
> >>
> >> So for the change itself this looks good, i.e. it fixes an issue for baords relying
> >> on the on-chip-card-detect.
> >>
> >>
> >> But for boards doing that, the controller will be running _all the time_
> >> even if there is never any card inserted.
> >>
> >> So relying on the on-soc card-detect will effectively increase the power-
> >> consumption of the board - even it it'll never use any sd-card?
> > 
> > Good point! A better option is to use a polling based mechanism - and
> > we have MMC_CAP_NEEDS_POLL for exactly that.
> > 
> > Moreover, on DT based platforms one can even use the "broken-cd"
> > property to indicate this.
> 
> Except that goes further than is needed here, since it would fall back 
> entirely to software-based polling for card presence. In this case the 
> CD function is not broken in terms of actually detecting a card, it just 
> doesn't work to wake the controller up from suspend because it can't 
> fire its own interrupt while powered off. In principle all we should 
> require here is to periodically resume/suspend the device, to provide a 
> window for the interrupt to work and normal operation to take over if 
> appropriate.
> 
> Of course the really clever way would be for suspend to switch the pin 
> into GPIO mode, and set the GPIO interrupt as a wakeup to trigger resume 
> and switch it back again, but perhaps that's a bit tricky without 
> explicit pinctrl states in the DT :/

and then the question really becomes, why move away from cd-gpios at all.