RE: [PATCH v5 10/14] clk: imx: Add generic blk-ctl driver

[Jacky Bai]

> -----Original Message-----
> From: Frieder Schrempf [mailto:frieder.schrempf@xxxxxxxxxx]
> Sent: Thursday, February 25, 2021 4:23 PM
> To: Abel Vesa <abel.vesa@xxxxxxx>; Dong Aisheng <dongas86@xxxxxxxxx>
> Cc: Aisheng Dong <aisheng.dong@xxxxxxx>; Rob Herring <robh@xxxxxxxxxx>;
> Peng Fan <peng.fan@xxxxxxx>; Jacky Bai <ping.bai@xxxxxxx>; Anson Huang
> <anson.huang@xxxxxxx>; devicetree <devicetree@xxxxxxxxxxxxxxx>;
> Stephen Boyd <sboyd@xxxxxxxxxx>; Shawn Guo <shawnguo@xxxxxxxxxx>;
> Mike Turquette <mturquette@xxxxxxxxxxxx>; Linux Kernel Mailing List
> <linux-kernel@xxxxxxxxxxxxxxx>; Marek Vasut <marek.vasut@xxxxxxxxx>;
> dl-linux-imx <linux-imx@xxxxxxx>; Sascha Hauer <kernel@xxxxxxxxxxxxxx>;
> Fabio Estevam <fabio.estevam@xxxxxxx>; Philipp Zabel
> <p.zabel@xxxxxxxxxxxxxx>; Adam Ford <aford173@xxxxxxxxx>; linux-clk
> <linux-clk@xxxxxxxxxxxxxxx>; moderated list:ARM/FREESCALE IMX / MXC
> ARM ARCHITECTURE <linux-arm-kernel@xxxxxxxxxxxxxxxxxxx>; Lucas Stach
> <l.stach@xxxxxxxxxxxxxx>
> Subject: Re: [PATCH v5 10/14] clk: imx: Add generic blk-ctl driver
> 
> Hi Abel,
> 
> On 17.11.20 15:48, Abel Vesa wrote:
> > On 20-11-11 17:13:25, Dong Aisheng wrote:
> >> On Tue, Nov 3, 2020 at 7:22 PM Abel Vesa <abel.vesa@xxxxxxx> wrote:
> >> ...
> >>> +static int imx_blk_ctl_reset_set(struct reset_controller_dev *rcdev,
> >>> +                                 unsigned long id, bool assert) {
> >>> +       struct imx_blk_ctl_drvdata *drvdata = container_of(rcdev,
> >>> +                       struct imx_blk_ctl_drvdata, rcdev);
> >>> +       unsigned int offset = drvdata->rst_hws[id].offset;
> >>> +       unsigned int shift = drvdata->rst_hws[id].shift;
> >>> +       unsigned int mask = drvdata->rst_hws[id].mask;
> >>> +       void __iomem *reg_addr = drvdata->base + offset;
> >>> +       unsigned long flags;
> >>> +       u32 reg;
> >>> +
> >>> +       if (!assert && !test_bit(1, &drvdata->rst_hws[id].asserted))
> >>> +               return -ENODEV;
> >>
> >> What if consumers call deassert first in probe which seems common in
> kernel?
> >> It seems will fail.
> >> e.g.
> >> probe() {
> >>      reset_control_get()
> >>      reset_control_deassert()
> >> }
> >>
> >> Regards
> >> Aisheng
> >>
> >
> > OK, I'm trying to explain here how I know the resets are supposed to
> > be working and how the BLK_CTL IP is working.
> >
> >
> > First of, the BLK_CTL bits (resets and clocks) all have the HW init
> > (default) values as 0. Basically, after the blk_ctl PD is powered on,
> > the resets are deasserted and clocks are gated by default. Since the
> > blk_ctl is not the parent of any of the consumers in devicetree (the
> > reg maps are entirely different anyway), there is no way of ordering
> > the runtime callbacks between the consumer and the blk_ctl. So we
> > might end up having the runtime resume callback after the one from
> > EARC (consumer), for example, which will basically overwrite the value
> written by EARC driver with whatever was saved on suspend.
> >
> > Now, about the usage of the reset bits. AFAICT, it would make more
> > sense to assert the reset, then enable the clock, then deassert. This
> > way, you're keeping the EARC (consumer) in reset (with the clocks on)
> > until you eventually release it out of reset by deasserting. This is
> > how the runtime resume should deal with the reset and the clock. As
> > for the runtime suspend, the reset can be entirely ignored as long as you're
> disabling the clock.
> >
> > This last part will allow the blk_ctl to make the following assumption:
> > if all the clocks are disabled and none of the reset bits are asserted, I can
> power off.
> >
> > Now, I know there are drivers outthere that do assert on suspend, but
> > as long as the clocks are disabled, the assert will have no impact.
> > But maybe in their case the reset controller cannot power down itself.
> >
> > As for the safekeeping of the register, I'll just drop it due to the following
> arguments:
> > 1. all the clocks are gated by default 2. all resets are deasserted by
> > default 3. when blk_ctl goes down, all the consumers go down. (all
> > have the same PD)
> >
> >  From 1 and 2 results the IP will not be running and from 3 results
> > the HW state of every IP becomes HW init state.
> 
> Are there any plans to continue this work? As BLK-CTL it is not only relevant
> for the i.MX8MP, but also for i.MX8MM and i.MX8MN, it would be nice to get
> this ready in order to prepare for proper graphics/display support.
> 

Before continuing this work, we need to find out a way to resolve the cycling dependency issue between power domain and blk-ctrl.
it is indeed introduced some troubles in NXP latest internal release when the blk-ctrl driver is added.

BR
Jacky Bai

> Thanks
> Frieder