linux-kernel - Re: [PATCH v3 17/33] nds32: VDSO support

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Message-ID: <CAJsyPhzQvZ1QTPcE=p3cTv2c9BbjRFCnB1ROoEBqZuRzMraeug@mail.gmail.com>
Date:   Tue, 12 Dec 2017 09:58:31 +0800
From:   Vincent Chen <deanbo422@...il.com>
To:     Mark Rutland <mark.rutland@....com>
Cc:     Greentime Hu <green.hu@...il.com>,
        Greentime <greentime@...estech.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Arnd Bergmann <arnd@...db.de>,
        linux-arch <linux-arch@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Jason Cooper <jason@...edaemon.net>,
        Marc Zyngier <marc.zyngier@....com>,
        Rob Herring <robh+dt@...nel.org>,
        netdev <netdev@...r.kernel.org>,
        DTML <devicetree@...r.kernel.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        David Howells <dhowells@...hat.com>,
        Will Deacon <will.deacon@....com>,
        Daniel Lezcano <daniel.lezcano@...aro.org>,
        linux-serial@...r.kernel.org,
        Geert Uytterhoeven <geert.uytterhoeven@...il.com>,
        Linus Walleij <linus.walleij@...aro.org>,
        Greg KH <greg@...ah.com>, Vincent Chen <vincentc@...estech.com>
Subject: Re: [PATCH v3 17/33] nds32: VDSO support

2017-12-08 20:14 GMT+08:00 Mark Rutland <mark.rutland@....com>:
> On Fri, Dec 08, 2017 at 07:54:42PM +0800, Greentime Hu wrote:
>> 2017-12-08 18:21 GMT+08:00 Mark Rutland <mark.rutland@....com>:
>> > On Fri, Dec 08, 2017 at 05:12:00PM +0800, Greentime Hu wrote:
>> >> +static int grab_timer_node_info(void)
>> >> +{
>> >> +     struct device_node *timer_node;
>> >> +
>> >> +     timer_node = of_find_node_by_name(NULL, "timer");
>> >
>> > Please use a compatible string, rather than matching the timer by name.
>> >
>> > It's plausible that you have multiple nodes called "timer" in the DT,
>> > under different parent nodes, and this might not be the device you
>> > think it is. I see your dt in patch 24 has two timer nodes.
>> >
>> > It would be best if your clocksource driver exposed some stuct that you
>> > looked at here, so that you're guaranteed to user the same device.
>>
>> We'd like to use "timer" here because there are 2 different timer IPs
>> and we are sure that they won't be in the same SoC.
>> We think this implementation in VDSO should be platform independent to
>> get cycle-count register.
>> Our customer or other SoC provider who can use "timer" and define
>> cycle-count-offset or cycle-count-down then we can get the correct
>> cycle-count.
>
> This is not the right way to do things.
>
> So from a DT perspective, NAK.
>
> You should not add properties to arbitrary DT bindings to handle a Linux
> implementation detail.
>
> Please remove this DT code, and have the drivers for those timer blocks
> export this information to your vdso code somehow.
>

Hi, Mark:
Based on your suggestion, we define a new sturct timer_info to let
timer driver record the value
of cycle-count-offset and cycle-count-down in timer_init function. The
above code in timer driver
is validate only when CONFIG_NDS32 is defined.

>> We sent atcpit100 patch last time along with our arch, however we'd
>> like to send it to its sub system this time and my colleague is still
>> working on it.
>> He may send the timer patch next week.
>
> I think that it would make sense for that patch to be part of the arch
> port, especially given that (AFAICT) there is no dirver for the other
> timer IP that you mention.
>
> [...]
>
>> >> +int arch_setup_additional_pages(struct linux_binprm *bprm, int uses_interp)
>> >> +{
>> >
>> >> +     /*Map timer to user space */
>> >> +     vdso_base += PAGE_SIZE;
>> >> +     prot = __pgprot(_PAGE_V | _PAGE_M_UR_KR | _PAGE_D |
>> >> +                     _PAGE_G | _PAGE_C_DEV);
>> >> +     ret = io_remap_pfn_range(vma, vdso_base, timer_res.start >> PAGE_SHIFT,
>> >> +                              PAGE_SIZE, prot);
>> >> +     if (ret)
>> >> +             goto up_fail;
>> >
>> > Maybe this is fine, but it looks a bit suspicious.
>> >
>> > Is it safe to map IO memory to a userspace process like this?
>> >
>> > In general that isn't safe, since userspace could access other registers
>> > (if those exist), perform accesses that change the state of hardware, or
>> > make unsupported access types (e.g. unaligned, atomic) that result in
>> > errors the kernel can't handle.
>> >
>> > Does none of that apply here?
>>
>> We only provide read permission to this page so hareware state won't
>> be chagned. It will trigger exception if we try to write.
>> We will check about the alignment/atomic issue of this region.
>

For alignment issue, we intentionally make an un-alignment read to
access this region and we
got "Segmentation fault" as expected.


Thanks,
Vincent

> Ok, thanks.
>
> This is another reason to only do this for devices/drivers that we have
> drivers for, since we can't know that this is safe in general.
>
> Thanks,
> Mark.