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Subject: Re: [PATCH] clk: vc5: Fix div-by-0 when rounding a rate of zero
To:     Steve Longerbeam <steve_longerbeam@mentor.com>,
        Steve Longerbeam <slongerbeam@gmail.com>,
        Michael Turquette <mturquette@baylibre.com>,
        Stephen Boyd <sboyd@kernel.org>
Cc:     linux-clk@vger.kernel.org, linux-kernel@vger.kernel.org,
        Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>,
        Kieran Bingham <kieran.bingham@ideasonboard.com>
References: <1527791036-11251-1-git-send-email-steve_longerbeam@mentor.com>
 <1084deae-ca55-aba1-ec08-7e1d976de7b7@gmail.com>
 <e6d84bd1-5ad9-1a01-1cdf-0356fea82c16@gmail.com>
 <f7a05f4b-35c4-4900-24bd-44aab1c669cb@gmail.com>
 <59591484-b76d-6764-73c3-637bb4e5178f@mentor.com>
From:   Marek Vasut <marek.vasut@gmail.com>
Message-ID: <06fd7d17-5c31-4ac5-d923-276afee056b3@gmail.com>
Date:   Thu, 31 May 2018 21:37:08 +0200
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On 05/31/2018 08:52 PM, Steve Longerbeam wrote:
> 
> 
> On 05/31/2018 11:35 AM, Marek Vasut wrote:
>> On 05/31/2018 08:32 PM, Steve Longerbeam wrote:
>>> Hi Marek,
>>>
>>>
>>> On 05/31/2018 11:25 AM, Marek Vasut wrote:
>>>> On 05/31/2018 08:23 PM, Steve Longerbeam wrote:
>>>>> Just return zero for a rounded rate if requested rate is zero.
>>>>>
>>>>> This was caught by CONFIG_UBSAN:
>>>>>
>>>>> [  192.266748] UBSAN: Undefined behaviour in
>>>>> drivers/clk/clk-versaclock5.c:513:17
>>>>> [  192.274050] division by zero
>>>>> [  192.276976] CPU: 0 PID: 2579 Comm: vsp-unit-test-0 Tainted: G
>>>>> B   WC      4.14.17-02752-g13fb96f #1
>>>>> [  192.286378] Hardware name: Renesas Salvator-X board based on
>>>>> r8a7795 ES2.0+ (DT)
>>>>> [  192.293852] Call trace:
>>>>> [  192.296343] [<ffff2000080900dc>] dump_backtrace+0x0/0x390
>>>>> [  192.301807] [<ffff200008090480>] show_stack+0x14/0x1c
>>>>> [  192.306920] [<ffff200008f66574>] dump_stack+0x134/0x1a8
>>>>> [  192.312213] [<ffff2000087aaa30>] ubsan_epilogue+0x14/0x60
>>>>> [  192.317677] [<ffff2000087ab4d0>]
>>>>> __ubsan_handle_divrem_overflow+0x11c/0x170
>>>>> [  192.324720] [<ffff200008852120>] vc5_fod_round_rate+0x68/0x148
>>>>> [  192.330620] [<ffff20000884567c>] clk_calc_new_rates+0x238/0x3fc
>>>>> [  192.336607] [<ffff2000088456e0>] clk_calc_new_rates+0x29c/0x3fc
>>>>> [  192.342595] [<ffff2000088483ac>]
>>>>> clk_core_set_rate_nolock+0x48/0x11c
>>>>> [  192.349019] [<ffff2000088484b4>] clk_set_rate+0x34/0x4c
>>>>> [  192.354307] [<ffff20000895e304>] rcar_du_pm_suspend+0x274/0x2f4
>>>>> [  192.360297] [<ffff20000898feac>] platform_pm_suspend+0x78/0xb8
>>>>> [  192.366198] [<ffff2000089a5604>] dpm_run_callback+0x584/0xa18
>>>>> [  192.372010] [<ffff2000089a69e0>] __device_suspend+0x1a8/0x534
>>>>> [  192.377822] [<ffff2000089adc48>] dpm_suspend+0x130/0xea0
>>>>> [  192.383197] [<ffff2000089b0344>] dpm_suspend_start+0x130/0x138
>>>>> [  192.389099] [<ffff20000817f584>]
>>>>> suspend_devices_and_enter+0xf0/0x1778
>>>>> [  192.395700] [<ffff200008183014>] pm_suspend+0x2408/0x245c
>>>>> [  192.401162] [<ffff20000817c0a4>] state_store+0xf0/0x130
>>>>> [  192.406451] [<ffff200008f6f19c>] kobj_attr_store+0x5c/0x6c
>>>>> [  192.412002] [<ffff2000084f4c94>] sysfs_kf_write+0xe8/0xfc
>>>>> [  192.417466] [<ffff2000084f30b0>] kernfs_fop_write+0x22c/0x2e4
>>>>> [  192.423281] [<ffff2000083e46d4>] __vfs_write+0x104/0x34c
>>>>> [  192.428656] [<ffff2000083e4cc4>] vfs_write+0x134/0x2d8
>>>>> [  192.433857] [<ffff2000083e5150>] SyS_write+0xbc/0x12c
>>>>> [  192.438967] Exception stack(0xffff8006cd1cfec0 to
>>>>> 0xffff8006cd1d0000)
>>>>> [  192.445480] fec0: 0000000000000001 000000001e303f00
>>>>> 0000000000000004 0000ffff959a5000
>>>>> [  192.453397] fee0: 0000000000000000 0000000155510004
>>>>> 0000000000000003 000000000000006d
>>>>> [  192.461314] ff00: 0000000000000040 0000000000000000
>>>>> 0000ffffcc304800 0000000000000020
>>>>> [  192.469230] ff20: 0000000000000000 0000000000000000
>>>>> 0000000000000001 0000000000000008
>>>>> [  192.477148] ff40: 00000000004eb3b8 0000ffff958bb840
>>>>> 000000000000003d 0000000000000001
>>>>> [  192.485065] ff60: 000000001e303f00 0000ffff959a1508
>>>>> 0000000000000004 000000001e303f00
>>>>> [  192.492982] ff80: 0000000000000004 00000000004d4c68
>>>>> 0000000000000001 0000000000000000
>>>>> [  192.500899] ffa0: 000000001e30d5c0 0000ffffcc304820
>>>>> 0000ffff958bec64 0000ffffcc304820
>>>>> [  192.508816] ffc0: 0000ffff95912898 0000000020000000
>>>>> 0000000000000001 0000000000000040
>>>>> [  192.516733] ffe0: 0000000000000000 0000000000000000
>>>>> 0000000000000000 0000000000000000
>>>>> [  192.524650] [<ffff200008083ef0>] el0_svc_naked+0x24/0x28
>>>>>
>>>>> Signed-off-by: Steve Longerbeam <steve_longerbeam@mentor.com>
>>>>> ---
>>>>>    drivers/clk/clk-versaclock5.c | 4 ++++
>>>>>    1 file changed, 4 insertions(+)
>>>>>
>>>>> diff --git a/drivers/clk/clk-versaclock5.c
>>>>> b/drivers/clk/clk-versaclock5.c
>>>>> index decffb3..113523d 100644
>>>>> --- a/drivers/clk/clk-versaclock5.c
>>>>> +++ b/drivers/clk/clk-versaclock5.c
>>>>> @@ -509,6 +509,10 @@ static long vc5_fod_round_rate(struct clk_hw
>>>>> *hw, unsigned long rate,
>>>>>        u32 div_int;
>>>>>        u64 div_frc;
>>>>>    +    /* prevent div-by-0 */
>>>>> +    if (rate == 0)
>>>>> +        return 0;
>>>>> +
>>>>>        /* Determine integer part, which is 12 bit wide */
>>>>>        div_int = f_in / rate;
>>>>>        /*
>>>>>
>>>> Can this actually happen ?
>>> We caught this using the Renesas 3.6.0 BSP release, when performing
>>> a suspend of rcar-du driver. The rcar_du_pm_suspend() in 3.6.0 BSP is
>>> modified
>>> from mainline version, including calling clk_set_rate() on the crtc
>>> clocks with a
>>> rate of zero. So this is not actually reproducible (yet) in mainline.
>> So it sets clock to 0 ?
> 
> Yep, see
> 
> https://kernel.googlesource.com/pub/scm/linux/kernel/git/horms/renesas-bsp/+/rcar-3.6.0/drivers/gpu/drm/rcar-du/rcar_du_drv.c#359
> 
> 
>>   Anyway, this looks sane, although maybe the
>> whole driver could use a once-over to see if there could be more of this.
> 
> Actually I do see more potential divide-by-zeros due to a passed rate
> of zero, including vc5_pfd_round_rate() and vc5_pfd_set_rate().
> 
> I can resubmit this patch fixing all cases in clk-versaclock5.c if you
> like (and probably remove the misleading backtrace in the commit
> message since it is a Renesas 3.6.0 kernel backtrace not a mainline
> backtrace).
> 
> Or perhaps just treat this as a heads-up, I'll leave it up to you.

It'd be nice if you resubmitted it fixing all the cases.

>> Or should the clock framework even let us set clock to 0 Hz ?
> 
> That is a good question, it might make sense for the core clock framework
> to not allow passing a rate of 0 on to the clock ops, and instead treat
> it generically.

Jupp

-- 
Best regards,
Marek Vasut