Subject: Division by zero caused by CCF

From: [email-masked] (Eduardo Valentin)
Date: Tue, 16 Jul 2013 09:10:55 -0400

Hi,

Adding Mike's correct address.

On 16-07-2013 08:37, Felipe Balbi wrote:
> Hi,
> 
> trying to get USB host verified on OMAP5 uEVM with v3.11-rc1. The
> clk_set_rate() call ends up in a division by zero, which is quite
> interesting provided the driver will only call clk_set_rate() if the
> clock is valid and clk_rate is != 0.
> 
> 
> [   22.009238] Division by zero in kernel.
> [   22.009250] CPU: 0 PID: 1 Comm: swapper/0 Tainted: G        W    3.11.0-rc1-00081-g3310d44-dirty #118
> [   22.009275] [<c001c83c>] (unwind_backtrace+0x0/0xf0) from [<c0018a1c>] (show_stack+0x10/0x14)
> [   22.009289] [<c0018a1c>] (show_stack+0x10/0x14) from [<c057403c>] (dump_stack+0x70/0x8c)
> [   22.009304] [<c057403c>] (dump_stack+0x70/0x8c) from [<c02e4154>] (Ldiv0+0x8/0x10)
> [   22.009319] [<c02e4154>] (Ldiv0+0x8/0x10) from [<c048d460>] (clk_divider_set_rate+0x10/0xdc)
> [   22.009331] [<c048d460>] (clk_divider_set_rate+0x10/0xdc) from [<c048c124>] (clk_change_rate+0x38/0xb0)
> [   22.009341] [<c048c124>] (clk_change_rate+0x38/0xb0) from [<c048c20c>] (clk_set_rate+0x70/0xa8)
> [   22.009354] [<c048c20c>] (clk_set_rate+0x70/0xa8) from [<c042b244>] (nop_usb_xceiv_probe+0x1fc/0x2f8)
> [   22.009369] [<c042b244>] (nop_usb_xceiv_probe+0x1fc/0x2f8) from [<c036b47c>] (platform_drv_probe+0x18/0x1c)
> [   22.009380] [<c036b47c>] (platform_drv_probe+0x18/0x1c) from [<c0369f44>] (really_probe+0x70/0x1f4)
> [   22.009390] [<c0369f44>] (really_probe+0x70/0x1f4) from [<c036a1dc>] (driver_probe_device+0x30/0x48)
> [   22.009401] [<c036a1dc>] (driver_probe_device+0x30/0x48) from [<c036a288>] (__driver_attach+0x94/0x98)
> [   22.009411] [<c036a288>] (__driver_attach+0x94/0x98) from [<c0368748>] (bus_for_each_dev+0x54/0x88)
> [   22.009420] [<c0368748>] (bus_for_each_dev+0x54/0x88) from [<c036972c>] (bus_add_driver+0xdc/0x29c)
> [   22.009430] [<c036972c>] (bus_add_driver+0xdc/0x29c) from [<c036a760>] (driver_register+0x78/0x190)
> [   22.009440] [<c036a760>] (driver_register+0x78/0x190) from [<c00087b0>] (do_one_initcall+0x34/0x164)
> [   22.009453] [<c00087b0>] (do_one_initcall+0x34/0x164) from [<c07b18f4>] (do_basic_setup+0x90/0xc4)
> [   22.009466] [<c07b18f4>] (do_basic_setup+0x90/0xc4) from [<c07b199c>] (kernel_init_freeable+0x74/0x110)
> [   22.009478] [<c07b199c>] (kernel_init_freeable+0x74/0x110) from [<c05676c4>] (kernel_init+0x8/0xe4)
> [   22.009491] [<c05676c4>] (kernel_init+0x8/0xe4) from [<c0014648>] (ret_from_fork+0x14/0x2c)
> 
> I believe the problem is the actual division reaching
> clk_divider_set_rate().
> 
> drivers/clk/clk-divider.c::clk_divider_set_rate()
> 
> | static int clk_divider_set_rate(struct clk_hw *hw, unsigned long rate,
> |                                 unsigned long parent_rate)
> | {
> |         struct clk_divider *divider = to_clk_divider(hw);
> |         unsigned int div, value;
> |         unsigned long flags = 0;
> |         u32 val;
> | 
> |         div = parent_rate / rate;
> 
> right here, but how come rate would zero provided driver checks for it
> as below.
> 
> drivers/usb/phy/phy-nop.c::nop_usb_xceiv_probe()
> 
> |         if (!IS_ERR(nop->clk) && clk_rate) {
> |                 err = clk_set_rate(nop->clk, clk_rate);
> |                 if (err) {
> |                         dev_err(&pdev->dev, "Error setting clock rate\n");
> |                         return err;
> |                 }
> |         }
> 
> I've added a few prints around CCF to try and track what's going on:
> 
> [   21.592690] ====> nop_usb_xceiv_probe rate 19200000
> [   21.592700] ====> clk_set_rate rate 19200000
> [   21.592707] ====> clk_calc_new_rates rate 19200000
> [   21.592713] ====> clk_divider_round_rate rate 19200000
> [   21.592719] ====> clk_divider_bestdiv rate 19200000
> [   21.592726] ====> clk_change_rate best_parent_rate 0

or because we reach:
	if (clk->ops->set_rate)
		clk->ops->set_rate(clk->hw, clk->new_rate, best_parent_rate);

with clk->new_rate == 0.

> [   21.592732] ====> clk_divider_set_rate rate 0
> [   21.592737] Division by zero in kernel.
> [   21.592747] CPU: 0 PID: 1 Comm: swapper/0 Tainted: G        W    3.11.0-rc1-00081-g3310d44-dirty #121
> [   21.592773] [<c001c83c>] (unwind_backtrace+0x0/0xf0) from [<c0018a1c>] (show_stack+0x10/0x14)
> [   21.592787] [<c0018a1c>] (show_stack+0x10/0x14) from [<c057400c>] (dump_stack+0x70/0x8c)
> [   21.592803] [<c057400c>] (dump_stack+0x70/0x8c) from [<c02e4154>] (Ldiv0+0x8/0x10)
> [   21.592819] [<c02e4154>] (Ldiv0+0x8/0x10) from [<c048d3e0>] (clk_divider_set_rate+0x2c/0x100)
> [   21.592831] [<c048d3e0>] (clk_divider_set_rate+0x2c/0x100) from [<c048c050>] (clk_change_rate+0x48/0xe0)
> [   21.592841] [<c048c050>] (clk_change_rate+0x48/0xe0) from [<c048c174>] (clk_set_rate+0x8c/0xc0)
> [   21.592855] [<c048c174>] (clk_set_rate+0x8c/0xc0) from [<c042b254>] (nop_usb_xceiv_probe+0x20c/0x304)
> [   21.592869] [<c042b254>] (nop_usb_xceiv_probe+0x20c/0x304) from [<c036b47c>] (platform_drv_probe+0x18/0x1c)
> [   21.592880] [<c036b47c>] (platform_drv_probe+0x18/0x1c) from [<c0369f44>] (really_probe+0x70/0x1f4)
> [   21.592891] [<c0369f44>] (really_probe+0x70/0x1f4) from [<c036a1dc>] (driver_probe_device+0x30/0x48)
> [   21.592901] [<c036a1dc>] (driver_probe_device+0x30/0x48) from [<c036a288>] (__driver_attach+0x94/0x98)
> [   21.592911] [<c036a288>] (__driver_attach+0x94/0x98) from [<c0368748>] (bus_for_each_dev+0x54/0x88)
> [   21.592921] [<c0368748>] (bus_for_each_dev+0x54/0x88) from [<c036972c>] (bus_add_driver+0xdc/0x29c)
> [   21.592930] [<c036972c>] (bus_add_driver+0xdc/0x29c) from [<c036a760>] (driver_register+0x78/0x190)
> [   21.592941] [<c036a760>] (driver_register+0x78/0x190) from [<c00087b0>] (do_one_initcall+0x34/0x164)
> [   21.592954] [<c00087b0>] (do_one_initcall+0x34/0x164) from [<c07b18f4>] (do_basic_setup+0x90/0xc4)
> [   21.592966] [<c07b18f4>] (do_basic_setup+0x90/0xc4) from [<c07b199c>] (kernel_init_freeable+0x74/0x110)
> [   21.592980] [<c07b199c>] (kernel_init_freeable+0x74/0x110) from [<c0567694>] (kernel_init+0x8/0xe4)
> [   21.592992] [<c0567694>] (kernel_init+0x8/0xe4) from [<c0014648>] (ret_from_fork+0x14/0x2c)
> 
> even though driver passed 19.2MHz, best_parent_rate ends up being zero
> which triggers the division by zero above.
> 
> cheers
> 

- -- 
You have got to be excited about what you are doing. (L. Lamport)

Eduardo Valentin