linux-kernel - Re: [PATCH 6/6] firmware: qcom_scm: Only compile legacy calls on ARM

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Date:   Fri, 5 Mar 2021 10:18:09 -0800
From:   Elliot Berman <eberman@...eaurora.org>
To:     Stephen Boyd <swboyd@...omium.org>, Andy Gross <agross@...nel.org>,
        Bjorn Andersson <bjorn.andersson@...aro.org>
Cc:     linux-kernel@...r.kernel.org, linux-arm-msm@...r.kernel.org,
        Brian Masney <masneyb@...tation.org>,
        Stephan Gerhold <stephan@...hold.net>,
        Jeffrey Hugo <jhugo@...eaurora.org>,
        Douglas Anderson <dianders@...omium.org>
Subject: Re: [PATCH 6/6] firmware: qcom_scm: Only compile legacy calls on ARM

On 3/3/2021 10:14 PM, Stephen Boyd wrote:
> Quoting Elliot Berman (2021-03-03 19:35:08)
>>
>> On 2/23/2021 1:45 PM, Stephen Boyd wrote:
>>> These scm calls are never used outside of legacy ARMv7 based platforms.
>>> That's because PSCI, mandated on arm64, implements them for modern SoCs
>>> via the PSCI spec. Let's move them to the legacy file and only compile
>>> the legacy file into the kernel when CONFIG_ARM=y. Otherwise provide
>>> stubs and fail the calls. This saves a little bit of space in an
>>> arm64 allmodconfig >
>>>    $ ./scripts/bloat-o-meter vmlinux.before vmlinux.after
>>>    add/remove: 0/8 grow/shrink: 5/7 up/down: 509/-4405 (-3896)
>>>    Function                                     old     new   delta
>>>    __qcom_scm_set_dload_mode.constprop          312     452    +140
>>>    qcom_scm_qsmmu500_wait_safe_toggle           288     416    +128
>>>    qcom_scm_io_writel                           288     408    +120
>>>    qcom_scm_io_readl                            376     492    +116
>>>    __param_str_download_mode                     23      28      +5
>>>    __warned                                    4327    4326      -1
>>>    qcom_iommu_init                              272     268      -4
>>>    e843419@...f_00010432_324                      8       -      -8
>>>    qcom_scm_call                                228     208     -20
>>>    CSWTCH                                      5925    5877     -48
>>>    _sub_I_65535_1                            163100  163040     -60
>>>    _sub_D_65535_0                            163100  163040     -60
>>>    qcom_scm_wb                                   64       -     -64
>>>    qcom_scm_lock                                320     160    -160
>>>    qcom_scm_call_atomic                         212       -    -212
>>>    qcom_scm_cpu_power_down                      308       -    -308
>>>    scm_legacy_call_atomic                       520       -    -520
>>>    qcom_scm_set_warm_boot_addr                  720       -    -720
>>>    qcom_scm_set_cold_boot_addr                  728       -    -728
>>>    scm_legacy_call                             1492       -   -1492
>>>    Total: Before=66737642, After=66733746, chg -0.01%
>>>
>>> Commit 9a434cee773a ("firmware: qcom_scm: Dynamically support SMCCC and
>>> legacy conventions") didn't mention any motivating factors for keeping
>>> the legacy code around on arm64 kernels, i.e. presumably that commit
>>> wasn't trying to support these legacy APIs on arm64 kernels.
>>
>> There are arm targets which support SMCCC convention and use some of
>> these removed functions. Can these functions be kept in qcom-scm.c and
>> wrapped with #if IS_ENABLED(CONFIG_ARM)?
>>
> 
> It can be wrapped in qcom-scm.c, but why? It's all the same object file
> so I'm lost why it matters. I suppose it would make it so the struct
> doesn't have to be moved around and declared in the header? Any other
> reason? I moved it to the legacy file so that it was very obvious that
> the API wasn't to be used except for "legacy" platforms that don't use
> PSCI.
> 

There are "legacy" arm platforms that use the SMCCC (scm_smc_call) and 
use the qcom_scm_set_{warm,cold}_boot_addr and qcom_scm_cpu_power_down 
functions.

 > +	desc.args[0] = flags;
 > +	desc.args[1] = virt_to_phys(entry);
 > +
 > +	return scm_legacy_call_atomic(NULL, &desc, NULL);
 > +}
 > +EXPORT_SYMBOL(qcom_scm_set_cold_boot_addr);

This should still be qcom_scm_call.