Re: Sense of soc bus? (was: [PATCH] base: soc: Export soc_device_to_device() helper)

[Neil Armstrong <narmstrong@baylibre.com>]

On 12/11/2019 11:47, Andreas Färber wrote:
> Am 12.11.19 um 08:29 schrieb Uwe Kleine-König:
>> On Tue, Nov 12, 2019 at 06:23:47AM +0100, Greg Kroah-Hartman wrote:
>>> On Mon, Nov 11, 2019 at 09:10:41PM +0100, Andreas Färber wrote:
>>>> Am 11.11.19 um 07:40 schrieb Greg Kroah-Hartman:
>>>>> On Mon, Nov 11, 2019 at 06:42:05AM +0100, Andreas Färber wrote:
>>>>>> Am 11.11.19 um 06:27 schrieb Greg Kroah-Hartman:
>>>>>>> On Mon, Nov 11, 2019 at 05:56:09AM +0100, Andreas Färber wrote:
>>>>>>>> Use of soc_device_to_device() in driver modules causes a build failure.
>>>>>>>> Given that the helper is nicely documented in include/linux/sys_soc.h,
>>>>>>>> let's export it as GPL symbol.
>>>>>>>
>>>>>>> I thought we were fixing the soc drivers to not need this.  What
>>>>>>> happened to that effort?  I thought I had patches in my tree (or
>>>>>>> someone's tree) that did some of this work already, such that this
>>>>>>> symbol isn't needed anymore.
>>>>>>
>>>>>> I do still see this function used in next-20191108 in drivers/soc/.
>>>>>>
>>>>>> I'll be happy to adjust my RFC driver if someone points me to how!
>>>>>
>>>>> Look at c31e73121f4c ("base: soc: Handle custom soc information sysfs
>>>>> entries") for how you can just use the default attributes for the soc to
>>>>> create the needed sysfs files, instead of having to do it "by hand"
>>>>> which is racy and incorrect.
>>>>
>>>> Unrelated.
>>>>
>>>>>> Given the current struct layout, a type cast might work (but ugly).
>>>>>> Or if we stay with my current RFC driver design, we could use the
>>>>>> platform_device instead of the soc_device (which would clutter the
>>>>>> screen more than "soc soc0:") or resort to pr_info() w/o device.
>>>>>
>>>>> Ick, no, don't cast blindly.  What do you need the pointer for?  Is this
>>>>> for in-tree code?
>>>>
>>>> No, an RFC patchset: https://patchwork.kernel.org/cover/11224261/
>>>>
>>>> As I indicated above, I used it for a dev_info(), which I can easily
>>>> avoid by using pr_info() instead:
>>>>
>>>> diff --git a/drivers/soc/realtek/chip.c b/drivers/soc/realtek/chip.c
>>>> index e5078c6731fd..f9380e831659 100644
>>>> --- a/drivers/soc/realtek/chip.c
>>>> +++ b/drivers/soc/realtek/chip.c
>>>> @@ -178,8 +178,7 @@ static int rtd_soc_probe(struct platform_device *pdev)
>>>>
>>>>         platform_set_drvdata(pdev, soc_dev);
>>>>
>>>> -       dev_info(soc_device_to_device(soc_dev),
>>>> -               "%s %s (0x%08x) rev %s (0x%08x) detected\n",
>>>> +       pr_info("%s %s (0x%08x) rev %s (0x%08x) detected\n",
>>>>                 soc_dev_attr->family, soc_dev_attr->soc_id, chip_id,
>>>>                 soc_dev_attr->revision, chip_rev);
>>>
>>> First off, the driver should not be spitting out noise for when all goes
>>> well like this :)
>>
>> I didn't follow the discussion closely, but I think I want to object
>> here a bit. While I agree that each driver emitting some stuff to the
>> log buffer is hardly helpful, seeing the exact SoC details is indeed
>> useful at times. With my Debian kernel team member hat on, I'd say
>> keep this information. This way the SoC details make it into kernel bug
>> reports without effort on our side.
> 
> Seconded. For example, RTD1295 will support LSADC only from revision B00
> on (and it's not the first time I'm seeing such things in the industry).
> So if a user complains, it will be helpful to see that information.
> 
> Referencing your Amlogic review, with all due respect for its authors,
> the common framework here just lets that information evaporate into the
> deeps of sysfs. 

Hopefully we never had the case where needed to use the soc info in drivers,
but now we have one and having such infrastructure already in-place will help.

Renesas platforms makes a extensive usage of the soc info infrastructure to
figure out plenty of HW parameters at runtime and lower their DT changes.

Neil

> People who know can check /sys/bus/soc/devices/soc0, but
> ordinary users will at most upload dmesg output to a Bugzilla ticket.
> 
> And it was precisely info-level boot output from the Amlogic GX driver
> that made me aware of this common framework and inspired me to later
> contribute such a driver elsewhere myself. That's not a bad effect. ;)
> 
> So if anything, we should standardize that output and move it into
> soc_device_register(): "<family> <soc_id> [rev <revision>] detected"
> with suitable NULL checks? (what I did above for Realtek, minus hex)
> 
> The info level seems correct to me - it allows people to use a different
> log_level if they only care about warnings or errors on the console;
> it's not debug info for that driver, except in my case the accompanying
> hex numbers that I'd be happy to drop in favor of standardization.
> 
> Another aspect here is that the overall amount of soc_device_register()
> users is just an estimated one third above the analysis shared before.
> In particular server platforms, be it arm64 or x86_64, that potentially
> have more than one SoC integrated in a multi-socket configuration, don't
> feed into this soc bus at all! Therefore my comment that
> dev_info()-printed "soc soc0:" is kind of useless if there's only one
> SoC on those boards. I'm not aware of any tool or a more common file
> aggregating this information, certainly not /proc/cpuinfo and I'm not
> aware of any special "lssoc" tool either. And if there's no ACPI/DMI
> driver feeding support-relevant information into this framework to be
> generally useful, I don't expect the big distros to spend time on
> improving its usability.
> 
> So if we move info output into base/soc.c, we could continue using
> dev_info() with "soc"-grep'able prefix in the hopes that someday we'll
> have more than one soc device on the bus and will need to distinguish;
> otherwise yes, pr_info() would change the output format for Amlogic (and
> so would a harmonization of the text), but does anyone really care in
> practice? Tools shouldn't be relying on its output format, and humans
> will understand equally either way.
> 
> Finally, arch/arm seems unique in that it has the machine_desc mechanism
> that allows individual SoCs to force creating their soc_device early and
> using it as parent for further DT-created platform_devices. With arm64
> we've lost that ability, and nios2 is not using it either.
> A bad side effect (with SUSE hat on) is that this parent design pattern
> does not allow to build such drivers as modules, which means that distro
> configs using arm's multi-platform, e.g., CONFIG_ARCH_MULTI_V7 will get
> unnecessary code creep as new platforms get added over time (beyond the
> basic clk, pinctrl, tty and maybe timer).
> Even if it were possible to call into a driver module that early, using
> it as parent would seem to imply that all the references by its children
> would not allow to unload the module, which I'd consider a flawed design
> for such an "optional" read-once driver. If we want the device hierarchy
> to have a soc root then most DT based platforms will have a /soc DT node
> anyway (although no device_type = "soc") that we probably could have a
> device registered for in common code rather than each SoC platform
> handling that differently? That might then make soc_register_device()
> not the creator of the device (if pre-existent) but the supplier of data
> to that core device, which should then allow to unload the data provider
> with just the sysfs data disappearing until re-inserted (speeding up the
> develop-and-test cycle on say not-so-resource-constrained platforms).
> 
> On the other hand, one might argue that such information should just be
> parsed by EBBR-conformant bootloaders and be passed to the kernel via
> standard UEFI interfaces and DMI tables. But I'm not aware of Barebox
> having implemented any of that yet, and even for U-Boot (e.g., Realtek
> based consumer devices...) not everyone has the GPL sources or tools to
> update their bootloader. So, having the kernel we control gather
> information relevant to kernel developers does make some sense to me.
> 
> Thoughts?
> 
> Regards,
> Andreas
> 
> P.S. Sorry that a seemingly trivial one-line 0-day fix derailed into
> this fundamental use case discussion.
> 
> arch/arm/mach-ep93xx/core.c:    soc_dev = soc_device_register(soc_dev_attr);
> arch/arm/mach-imx/cpu.c:        soc_dev = soc_device_register(soc_dev_attr);
> arch/arm/mach-mvebu/mvebu-soc-id.c:     soc_dev =
> soc_device_register(soc_dev_attr);
> arch/arm/mach-mxs/mach-mxs.c:   soc_dev = soc_device_register(soc_dev_attr);
> arch/arm/mach-omap2/id.c:       soc_dev = soc_device_register(soc_dev_attr);
> arch/arm/mach-tegra/tegra.c:    struct device *parent =
> tegra_soc_device_register();
> arch/arm/mach-zynq/common.c:    soc_dev = soc_device_register(soc_dev_attr);
> arch/nios2/platform/platform.c:         soc_dev =
> soc_device_register(soc_dev_attr);
> drivers/soc/amlogic/meson-gx-socinfo.c: soc_dev =
> soc_device_register(soc_dev_attr);
> drivers/soc/amlogic/meson-mx-socinfo.c: soc_dev =
> soc_device_register(soc_dev_attr);
> drivers/soc/atmel/soc.c:        soc_dev = soc_device_register(soc_dev_attr);
> drivers/soc/bcm/brcmstb/common.c:       soc_dev =
> soc_device_register(soc_dev_attr);
> drivers/soc/fsl/guts.c: soc_dev = soc_device_register(&soc_dev_attr);
> drivers/soc/imx/soc-imx-scu.c:  soc_dev = soc_device_register(soc_dev_attr);
> drivers/soc/imx/soc-imx8.c:     soc_dev = soc_device_register(soc_dev_attr);
> drivers/soc/qcom/socinfo.c:     qs->soc_dev =
> soc_device_register(&qs->attr);
> drivers/soc/realtek/chip.c:     soc_dev = soc_device_register(soc_dev_attr);
> drivers/soc/renesas/renesas-soc.c:      soc_dev =
> soc_device_register(soc_dev_attr);
> drivers/soc/samsung/exynos-chipid.c:    soc_dev =
> soc_device_register(soc_dev_attr);
> drivers/soc/tegra/fuse/fuse-tegra.c:    dev = soc_device_register(attr);
> drivers/soc/ux500/ux500-soc-id.c:       soc_dev =
> soc_device_register(soc_dev_attr);
> drivers/soc/versatile/soc-integrator.c: soc_dev =
> soc_device_register(soc_dev_attr);
> drivers/soc/versatile/soc-realview.c:   soc_dev =
> soc_device_register(soc_dev_attr);
>