Re: [PATCH] net: bcmgenet: Use stronger register read/writes to assure ordering

[Florian Fainelli <f.fainelli@gmail.com>]

On 3/10/2022 5:09 PM, Jeremy Linton wrote:
> On 3/10/22 12:59, Florian Fainelli wrote:
>> On 3/9/22 8:53 PM, Jeremy Linton wrote:
>>> GCC12 appears to be much smarter about its dependency tracking and is
>>> aware that the relaxed variants are just normal loads and stores and
>>> this is causing problems like:
>>>
>>> [  210.074549] ------------[ cut here ]------------
>>> [  210.079223] NETDEV WATCHDOG: enabcm6e4ei0 (bcmgenet): transmit 
>>> queue 1 timed out
>>> [  210.086717] WARNING: CPU: 1 PID: 0 at net/sched/sch_generic.c:529 
>>> dev_watchdog+0x234/0x240
>>> [  210.095044] Modules linked in: genet(E) nft_fib_inet nft_fib_ipv4 
>>> nft_fib_ipv6 nft_fib nft_reject_inet nf_reject_ipv4 nf_reject_ipv6 
>>> nft_reject nft_ct nft_chain_nat]
>>> [  210.146561] ACPI CPPC: PCC check channel failed for ss: 0. ret=-110
>>> [  210.146927] CPU: 1 PID: 0 Comm: swapper/1 Tainted: G            
>>> E     5.17.0-rc7G12+ #58
>>> [  210.153226] CPPC Cpufreq:cppc_scale_freq_workfn: failed to read 
>>> perf counters
>>> [  210.161349] Hardware name: Raspberry Pi Foundation Raspberry Pi 4 
>>> Model B/Raspberry Pi 4 Model B, BIOS EDK2-DEV 02/08/2022
>>> [  210.161353] pstate: 80400005 (Nzcv daif +PAN -UAO -TCO -DIT -SSBS 
>>> BTYPE=--)
>>> [  210.161358] pc : dev_watchdog+0x234/0x240
>>> [  210.161364] lr : dev_watchdog+0x234/0x240
>>> [  210.161368] sp : ffff8000080a3a40
>>> [  210.161370] x29: ffff8000080a3a40 x28: ffffcd425af87000 x27: 
>>> ffff8000080a3b20
>>> [  210.205150] x26: ffffcd425aa00000 x25: 0000000000000001 x24: 
>>> ffffcd425af8ec08
>>> [  210.212321] x23: 0000000000000100 x22: ffffcd425af87000 x21: 
>>> ffff55b142688000
>>> [  210.219491] x20: 0000000000000001 x19: ffff55b1426884c8 x18: 
>>> ffffffffffffffff
>>> [  210.226661] x17: 64656d6974203120 x16: 0000000000000001 x15: 
>>> 6d736e617274203a
>>> [  210.233831] x14: 2974656e65676d63 x13: ffffcd4259c300d8 x12: 
>>> ffffcd425b07d5f0
>>> [  210.241001] x11: 00000000ffffffff x10: ffffcd425b07d5f0 x9 : 
>>> ffffcd4258bdad9c
>>> [  210.248171] x8 : 00000000ffffdfff x7 : 000000000000003f x6 : 
>>> 0000000000000000
>>> [  210.255341] x5 : 0000000000000000 x4 : 0000000000000000 x3 : 
>>> 0000000000001000
>>> [  210.262511] x2 : 0000000000001000 x1 : 0000000000000005 x0 : 
>>> 0000000000000044
>>> [  210.269682] Call trace:
>>> [  210.272133]  dev_watchdog+0x234/0x240
>>> [  210.275811]  call_timer_fn+0x3c/0x15c
>>> [  210.279489]  __run_timers.part.0+0x288/0x310
>>> [  210.283777]  run_timer_softirq+0x48/0x80
>>> [  210.287716]  __do_softirq+0x128/0x360
>>> [  210.291392]  __irq_exit_rcu+0x138/0x140
>>> [  210.295243]  irq_exit_rcu+0x1c/0x30
>>> [  210.298745]  el1_interrupt+0x38/0x54
>>> [  210.302334]  el1h_64_irq_handler+0x18/0x24
>>> [  210.306445]  el1h_64_irq+0x7c/0x80
>>> [  210.309857]  arch_cpu_idle+0x18/0x2c
>>> [  210.313445]  default_idle_call+0x4c/0x140
>>> [  210.317470]  cpuidle_idle_call+0x14c/0x1a0
>>> [  210.321584]  do_idle+0xb0/0x100
>>> [  210.324737]  cpu_startup_entry+0x30/0x8c
>>> [  210.328675]  secondary_start_kernel+0xe4/0x110
>>> [  210.333138]  __secondary_switched+0x94/0x98
>>>
>>> The assumption when these were relaxed seems to be that device memory
>>> would be mapped non reordering, and that other constructs
>>> (spinlocks/etc) would provide the barriers to assure that packet data
>>> and in memory rings/queues were ordered with respect to device
>>> register reads/writes. This itself seems a bit sketchy, but the real
>>> problem with GCC12 is that it is moving the actual reads/writes around
>>> at will as though they were independent operations when in truth they
>>> are not, but the compiler can't know that. When looking at the
>>> assembly dumps for many of these routines its possible to see very
>>> clean, but not strictly in program order operations occurring as the
>>> compiler would be free to do if these weren't actually register
>>> reads/write operations.
>>>
>>> Its possible to suppress the timeout with a liberal bit of dma_mb()'s
>>> sprinkled around but the device still seems unable to reliably
>>> send/receive data. A better plan is to use the safer readl/writel
>>> everywhere.
>>>
>>> Since this partially reverts an older commit, which notes the use of
>>> the relaxed variants for performance reasons. I would suggest that
>>> any performance problems with this commit are targeted at relaxing only
>>> the performance critical code paths after assuring proper barriers.
>>>
>>> Fixes: 69d2ea9c79898 ("net: bcmgenet: Use correct I/O accessors")
>>> Reported-by: Peter Robinson <pbrobinson@gmail.com>
>>> Signed-off-by: Jeremy Linton <jeremy.linton@arm.com>
>>
>> I think this is the correct approach in that it favors correctness over
>> speed, however there is an opportunity for maintaining the speed and
>> correctness on non-2711 and non-7712 chips where the GENET core is
>> interfaced to a system bus (GISB) that guarantees no re-ordering and no
>> buffering. I suppose that until we prove that the extra barrier is
>> harmful to performance on those chips, we should go with your patch.
>>
>> It seems like we missed the GENET_IO_MACRO() in bcmgenet.h, while most
>> of them deal with the control path which likely does not have any
>> re-ordering problem, there is an exception to that which are the
>> intrl2_0 and intrl2_1 macros, which I believe *have* to be ordered as
>> well in order to avoid spurious or missed interrupts, or maybe there is
>> enough barriers in the interrupt processing code that this is moot?
> 
> 
> Ok, so I spent some time and tracked down exactly which barrier "fixes" 
> this immediate problem on the rpi4.
> 
> static void bcmgenet_enable_dma(struct bcmgenet_priv *priv, u32 dma_ctrl)
>   {
>          u32 reg;
> 
> +       dma_mb(); //timeout fix
>          reg = bcmgenet_rdma_readl(priv, DMA_CTRL);
>          reg |= dma_ctrl;
> 
> 
> fixes it as well, and keeps all the existing code. Although, granted I 
> didn't stress the adapter beyond a couple interactive ssh sessions. And 
> as you mention there are a fair number of other accessors that I didn't 
> touch which are still relaxed.

Thanks! This is really helpful. Doug told me earlier today that he 
wanted to take a closer look since your initial approach while correct 
appears a bit heavy handed.
-- 
Florian