Re: [GIT PULL] ARM: SoC fixes for v5.10, part 3

[Geert Uytterhoeven]

Hi Doug,

On Mon, Dec 7, 2020 at 11:15 PM Doug Anderson <dianders@xxxxxxxxxxxx> wrote:
> On Mon, Dec 7, 2020 at 1:55 PM Arnd Bergmann <arnd@xxxxxxxxxx> wrote:
> > On Mon, Dec 7, 2020 at 9:23 PM Geert Uytterhoeven <geert@xxxxxxxxxxxxxx> wrote:
> > > On Tue, Dec 1, 2020 at 3:06 PM Arnd Bergmann <arnd@xxxxxxxxxx> wrote:
> > > > On Tue, Dec 1, 2020 at 12:39 PM Ulf Hansson <ulf.hansson@xxxxxxxxxx> wrote:
> > > > > So, I think we have two options. If people are willing to move to
> > > > > "disk labels" or to patch their DTBs with mmc aliases, things can stay
> > > > > as is. Otherwise, we can revert the async probe parts of the mmc host
> > > > > drivers, but that would still leave us in a fragile situation.
> > > >
> > > > Can you reliably detect whether the mmc aliases in the dt exist?
> > > > If that's possible, maybe the async flag could be masked out to only have
> > > > an effect when the device number is known.
> > >
> > > IMHO DT aliases are not a proper solution for this.
> > >
> > > Yes, you can detect reliably if an alias exists in the DT.
> > > The problems start when having multiple devices, some with aliases,
> > > some without.  And when devices can appear dynamically (without
> > > aliases, as there is no support for dynamically updating the aliases
> > > list).
> >
> > Actually you hit a problem earlier than that: the async probe is a
> > property of the host controller driver, which may be a pci_driver,
> > platform_driver, usb_driver, or anything else really. To figure out
> > whether to probe it asynchronously, it would have to be the driver
> > core, or each bus type that can host these to understand which
> > device driver is responsible for probing an eMMC device attached
> > to the host.
>
> From what I've seen so far, my current thought on this issue is that
> it's up to Ulf as the MMC maintainer what the next steps are.  For me,
> at least, his argument that MMC block numbers have already shuffled
> around several times in the last several years is at least some
> evidence that they weren't exactly stable to begin with.  While we
> could go back to the numbers that happened to be chosen as of kernel
> v5.9, if someone was updating from a much older kernel then they may
> have different expectations of what numbers are good / bad I think.
>
> I will also offer one possible suggestion: what about a KConfig option
> here?  In theory we could add a KConfig option like
> "CONFIG_MMC_LEGACY_PROBE" or something that.  One can argue about what
> the default ought to be, but maybe that would satisfy folks?  If you
> were happy giving up a little bit of boot speed to get the v5.9 block
> numbers then you could set this.

This is not limited to MMC.
The same is true for sdX, ethX (e* these days), ttyS*, i2cX, spiX, ...
The rule has always been to handle it by udev, disklabels, ...

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@xxxxxxxxxxxxxx

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds