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Subject: Re: [PATCH] clk: vc5: Fix div-by-0 when rounding a rate of zero
To:     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,
        Steve Longerbeam <steve_longerbeam@mentor.com>,
        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>
From:   Marek Vasut <marek.vasut@gmail.com>
Message-ID: <f7a05f4b-35c4-4900-24bd-44aab1c669cb@gmail.com>
Date:   Thu, 31 May 2018 20:35:07 +0200
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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 ? Anyway, this looks sane, although maybe the
whole driver could use a once-over to see if there could be more of this.

Or should the clock framework even let us set clock to 0 Hz ?

-- 
Best regards,
Marek Vasut