sdhci: runtime suspend/resume on card insert/removal

[Russell King - ARM Linux]

On Mon, Sep 14, 2015 at 11:50:14AM +0100, Russell King - ARM Linux wrote:
> On Mon, Sep 14, 2015 at 03:45:43PM +0530, Vaibhav Hiremath wrote:
> > Came across below lines in the datasheet,
> > 
> > ========= Copy-n-paste from datasheet============
> > 
> > All SDH interfaces share the same clock which is enabled when any of the SDH
> > clock enables are
> > set (from PMUA_SDH1_CLK_RES_CTRL, PMUA_SDH2_CLK_RES_CTRL,
> > PMUA_SDH3_CLK_RES_CTRL, PMUA_SDH4_CLK_RES_CTRL,
> > PMUA_SDH5_CLK_RES_CTRL), with clock source select and divider ratio
> > controlled by
> > PMUA_SDH1_CLK_RES_CTRL.
> > 
> > ==================================================
> > 
> > 
> > And I can confirm that after disabling AXI interface clock for all the
> > SDH modules (1-5) I see I get an abort.
> > 
> > This clearly explains/justifies/proves that the existing code is
> > working as expected. I have eMMC mounted on the board, which makes
> > clock to always stay ON on SDH3.
> > 
> > So there is an OR gate implemented inside which takes input from
> > SDHx_AXI_EN and feeds back to all SDHx instances. Don't ask me why it
> > has been designed that way :)
> > 
> > And I did some experiment as well, so what I have observed is,
> > SDH_AXI_CLOCK is required to generate card detection, without that I do
> > not see card detection working.
> 
> What that means is that if DT configures the interface to use its
> internal card detection, the AXI clock must never shut off when entering
> runtime-PM.
> 
> Yes, it means you don't get the same savings as you would by turning
> off that clock, but that's the choice between using the internal card
> detection and a GPIO for this.  The code shouldn't force you to use a
> GPIO just because the Linux driver implementation dumbly disables the
> AXI clock.

Note that should also happen if a SDIO card is inserted, and the SDIO IRQ
is enabled - so the SDIO card can signal an interrupt back to the host.

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