linux-kernel - Re: [PATCH] asm-generic/io: Add _RET_IP_ to MMIO trace for more accurate debug info

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Message-ID: <2f19ea9c-10e6-d0f7-2fc9-fb0f896bfc64@quicinc.com>
Date:   Wed, 26 Oct 2022 19:17:49 +0530
From:   Sai Prakash Ranjan <quic_saipraka@...cinc.com>
To:     Steven Rostedt <rostedt@...dmis.org>, Arnd Bergmann <arnd@...db.de>
CC:     Masami Hiramatsu <mhiramat@...nel.org>,
        <linux-arm-kernel@...ts.infradead.org>,
        <linux-kernel@...r.kernel.org>, <linux-arm-msm@...r.kernel.org>,
        <linux-arch@...r.kernel.org>, <quic_satyap@...cinc.com>
Subject: Re: [PATCH] asm-generic/io: Add _RET_IP_ to MMIO trace for more
 accurate debug info

Hi Steve,

On 10/24/2022 9:39 PM, Steven Rostedt wrote:
> On Mon, 17 Oct 2022 20:04:50 +0530
> Sai Prakash Ranjan <quic_saipraka@...cinc.com> wrote:
> 
>> Due to compiler optimizations like inlining, there are cases where
>> MMIO traces using _THIS_IP_ for caller information might not be
>> sufficient to provide accurate debug traces.
>>
>> 1) With optimizations (Seen with GCC):
>>
>> In this case, _THIS_IP_ works fine and prints the caller information
>> since it will be inlined into the caller and we get the debug traces
>> on who made the MMIO access, for ex:
>>
>> rwmmio_read: qcom_smmu_tlb_sync+0xe0/0x1b0 width=32 addr=0xffff8000087447f4
>> rwmmio_post_read: qcom_smmu_tlb_sync+0xe0/0x1b0 width=32 val=0x0 addr=0xffff8000087447f4
>>
>> 2) Without optimizations (Seen with Clang):
>>
>> _THIS_IP_ will not be sufficient in this case as it will print only
>> the MMIO accessors itself which is of not much use since it is not
>> inlined as below for example:
>>
>> rwmmio_read: readl+0x4/0x80 width=32 addr=0xffff8000087447f4
>> rwmmio_post_read: readl+0x48/0x80 width=32 val=0x4 addr=0xffff8000087447f4
>>
>> So in order to handle this second case as well irrespective of the compiler
>> optimizations, add _RET_IP_ to MMIO trace to make it provide more accurate
>> debug information in all these scenarios.
>>
>> Before:
>>
>> rwmmio_read: readl+0x4/0x80 width=32 addr=0xffff8000087447f4
>> rwmmio_post_read: readl+0x48/0x80 width=32 val=0x4 addr=0xffff8000087447f4
>>
>> After:
>>
>> rwmmio_read: qcom_smmu_tlb_sync+0xe0/0x1b0 -> readl+0x4/0x80 width=32 addr=0xffff8000087447f4
>> rwmmio_post_read: qcom_smmu_tlb_sync+0xe0/0x1b0 -> readl+0x4/0x80 width=32 val=0x0 addr=0xffff8000087447f4
>>
>> Fixes: 210031971cdd ("asm-generic/io: Add logging support for MMIO accessors")
>> Signed-off-by: Sai Prakash Ranjan <quic_saipraka@...cinc.com>
> 
> 
> Acked-by: Steven Rostedt (Google) <rostedt@...dmis.org>
> 
> What tree should this go through?
> 
> -- Steve


Thanks for the ack, with this I believe Arnd can take it through his 
tree like last time.

Thanks,
Sai